Twin

Fraternal twin boys in the tub

The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day. A fetus alone in the womb is called a singleton. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures.

Types of twins

Fraternal twins

Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. Dizygotic twins may be a different sex or the same sex, just as with any other siblings.

Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum.

Identical twins

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). All monoamniotic twins are monochorionic. Also note that any monochorionic or monoamniotic twins are identical twins. This condition does not occur for fraternal twins.

The later in pregnancy that twinning occurs, the more structures will be shared. Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning after 12 days post-fertilization will typically result in conjoined twins.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. About 50% of mono-mono twins die from umbilical cord entanglement.

Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. Examination of details such as fingerprints can tell them apart. As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars.

While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. The exact cause for the splitting of a zygote or embryo is unknown.

Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). They develop their own individual personalities to enable themselves to be identified as individual persons. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. Twins are unique individuals that establish their own individual likes and dislikes. There are usually obvious signs of differences when the identical twins are observed separately or together.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. This is called epigenetic modification. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The number of differences between identical twins increases with age. 50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. (Fraga, et al., 2005).

Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. The incidence of mirror twinning is comparatively rare. They result from a late split of the fertilized egg at around 9-12 days. One mirror may or may not have situs inversus. This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). Such conditions are usually associated with a higher incidence of other birth defects.

Complications of twin pregnancy

Vanishing twins

Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.

Conjoined twins

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. This occurs where the single zygote of identical twins fails to separate completely. This condition occurs in about 1 in 100,000 pregnancies.

Parasitic twins

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.

Sometimes the parasitic twin just becomes an almost indistinguishable part of the other.

A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body.

Miscarried twin

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. Similar to vanishing twin.

Human twins

Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. Thus, approximately 6% of children born in the US in 2001 were twins.

Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly.

Multiple births

Sometimes multiple births may involve more than two fetuses. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets.

Predisposing factors

The cause of monozygotic twinning is unknown. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). Some evidence suggests that the environment of the womb causes the zygote to split in most cases.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

  • She is of African descent
  • Between the age of 30 and 40 years
  • Greater than average height and weight
  • Several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. Many fertility treatments have no effect on the likelihood of multiple births.

Twin studies

Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature.

Unusual Twinnings

There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case.

Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes.

Twinning in animals

Multiple births are common in many animal species, such as cats, sheep, and ferrets. The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed.


This page about Twin includes information from a Wikipedia article.
Additional articles about Twin
News stories about Twin
External links for Twin
Videos for Twin
Wikis about Twin
Discussion Groups about Twin
Blogs about Twin
Images of Twin

The incidence of twinning among cattle is about 1-4%, and research is underway to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. As a result, the wearing of wrist-watches has become less common among mobile phone users, who are now the majority of the population. Multiple births are common in many animal species, such as cats, sheep, and ferrets. As these phones typically display the time on their screens when not in use, it has become common to rely on them for time-keeping, effectively making the mobile phone serve the function of a pocket watch. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotypes. In the early 2000s, the carrying of mobile telephones has become ubiquitous in many affluent countries. When monozygotic twins are born with different genders it is because of chromosomal birth defects. Wrist_PDA, although many digital watches come with extremely sophisticated data management software built in.

The probability of this is so vanishingly small (only 3 documented cases) that multiples having different genders is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. As of 2005, the only programmable computer watches to have made it to market are the Seiko Ruputer, the Matsucom onHand, and the Fossil, Inc. Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). Several companies have however attempted to develop a computer contained in a WristWatch (see also wearable computer). One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details. Now with the ubiquity of the mobile phone in many countries, which have bigger screens, buttons, and batteries, interest in incorporating extra functionality in watches seems to have declined. This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). Such watches have also had the reputation as ugly and thus mainly geek toys.

Among fraternal twins, in rare cases, the eggs are fertilised at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation) or, even more rarely, later on in the pregnancy (superfetation). As well as awkward user interfaces due to the tiny screens and buttons possible in a wearable package, and in some cases short battery life, the functionality available has not generally proven sufficiently compelling to attract buyers. There are some patterns of twinning that are exceedingly rare: while they have been reported to happen, they are so unusual that most obstetricians or midwives may go their entire careers without encountering a single case. These watches have not had sustained long-term sales success. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been invaluable in the exploration of human nature. In the early 2000s, a self-contained wristwatch television receiver came on the market, with a strong enough power source to provide one hour of viewing. Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. In the early 1980s Seiko marketed a watch with a television receiver in it, although at the time television receivers were too bulky to fit in a wristwatch, and the actual receiver and its power source were in a book-sized box with a cable that ran to the wristwatch.

Many fertility treatments have no effect on the likelihood of multiple births. As miniaturized electronics become cheaper, watches have been developed containing calculators, video games, digital cameras, keydrives, GPS receivers and cellular phones. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. A number of functionalities non directly related to time have also been inserted into watches. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. In 2005 for example, a company has put into market an alarm wristwatch with an accelerometer inside that monitors the user's sleep and rings during one of his almost-awake phases. This can vary depending on what types of fertility treatments are used. Other technological enhancements to wristwatches have been explored but most of them remained unnoticed.

Women undergoing certain fertility treatments may have a greater chance of multiple births. Suunto is the only company offering a reasonable-sized watch integrating GPS. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. Early examples are the Casio PRO TREK GPS Satellite Navi and the Garmin Forerunner 201. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. As GPS receivers are significantly more complex, very few wrist-watches integrating GPS are available and most of which are very large compared to regular watches. Fewer than 20 families have been described with an inherited tendency towards monozygotic twinning (people in these families have nearly a 50% chance of delivering monozygotic twins). Similarly watches with GPS time synchronisation use the satellite networks time signals.

The cause of monozygotic twinning is unknown. In recent years, mass production has meant that atomic watches have become as cheap as quartz watches, though market share still remains small as interest from big manufacturers is limited. Multiple pregnancies are usually delivered before the full term of 40 weeks gestation: the average length of pregnancy is around 36 weeks for twins, 34 weeks for triplets and 32 weeks for quadruplets. Similar signals are broadcast from Rugby (MSF time signal), England and Frankfurt, Germany. Before the advent of ovulation-stimulating drugs, triplets were quite rare (approximately 1 in 8000 births) and higher order births so rare as to be almost unheard of. It will also reset itself when daylight saving time changes. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. This radio signal tells the wristwatch exactly what time it is, in theory precise to a fraction of a nanosecond.

Sometimes multiple births may involve more than two fetuses. These wristwatches normally receive a radio signal from one of the national atomic clock facilities around the world, for example the National Institute of Standards and Technology located in Colorado in the United States. Nevertheless, the rate of identical twins remains at about 1 in 250 across the globe, further suggesting that pregnancies resulting in identical twins occur randomly. In 1990 radio controlled wristwatches or as they are sometimes called "atomic watches" reached the market. Thus, approximately 6% of children born in the US in 2001 were twins. This is often used as a case study in design schools to demonstrate the commercial potential of industrial and graphic design. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. They founded the Swiss Watch company (Swatch) and called graphic designers to redesign a new annual collection.

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple births". In fact it was so cheap that if a watch broke it would be cheaper to throw it away and buy a new one than to repair it. The rate of twinning varies greatly among ethnic groups, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see [1]). The result was that they could considerably reduce the pieces and production time of an analog watch. Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. They joined forces with designers from many countries to reinvent the Swiss watch. Similar to vanishing twin. At the end of the 20th century, Swiss watch makers were seeing their sales go down as analog clocks were considered obsolete.

Occasionally, a woman will suffer a miscarriage early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. For others, analog watches are just easier to read. A chimera may arise either from identical twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which could be identified by chromosomal comparisons from various parts of the body. In fact, because digital watches are so cheap, analog watches are often worn as status symbols. A chimera is a person who is a completely normal human with no extra parts, but some of the parts actually came from his or her twin. Digital watches have not replaced analog watches, despite their greater reliability and lower cost. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other. In addition to the function of a timepiece, digital watches can have additional functions like a chronograph, calculator, video game, etc.

One fetus acts as a parasite towards the other. The first LCD watch with a six-digit LCD was the 1973 Seiko 06LC, although various forms of early LCD watches with a four-digit display were marketed as early as 1972 including the 1972 Gruen Teletime LCD Watch [3], [4]. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. LED displays were soon superseded by liquid crystal displays (LCDs), which used less battery power. This condition occurs in about 1 in 100,000 pregnancies. It had a red light-emitting diode (LED) display. This occurs where the single zygote of identical twins fails to separate completely. A retail version of the Pulsar was put on sale in 1972.

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. The first digital watch, a Pulsar prototype in 1970, was developed jointly by Hamilton Watch Company and Electro-Data. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. Douglas Adams, in the introduction of his novel The Hitchhiker's Guide to the Galaxy, would say that humans were 'so amazingly primitive that they still think digital watches are a pretty neat idea'. Researchers suspect that more pregnancies start out as multiples than come to term that way. They were seen as the great new thing. Such conditions are usually associated with a higher incidence of other birth defects. Cheaper electronics permitted the popularization of the digital watch (an electronic watch with a numerical, rather than analog, display) in the second half of the 20th century.

This is where some or all of the organs will be on the opposite side of the body, such as the heart being on the right(Dextrocardia). Watch batteries come in many forms, the most common of which are silver oxide and lithium. One mirror may or may not have situs inversus. The most common power source is the battery. They result from a late split of the fertilized egg at around 9-12 days. A seldom used power source is temperature difference between the wearer's arm and the surrounding environment (as applied in the Citizen Eco Drive Thermo). The incidence of mirror twinning is comparatively rare. Kinetic powered quartz watches make use of the motion of the wearer's arm turning a rotating weight, which in turn, turns a generator to supply power.

These are identical twins with opposite features, that is one may be right handed and the other may be left handed; hair will whorl in the opposite direction, and so on. Solar powered quartz watches are powered by available light. Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. There are solar powered, kinetically powered, battery powered and other less common power sources. (Fraga, et al., 2005). There are also several variations of the quartz watch as to what actually powers the movement. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. The first quartz watch to enter production was the Seiko 35 SQ Astron, which appeared in 1969.

50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. The first prototypes were made by the CEH research laboratory in Switzerland in 1962. The number of differences between identical twins increases with age. The quartz analog watch is an electronic watch that uses a piezoelectric quartz crystal as its timing element, coupled to a mechanical movement that drives the hands. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. The first battery-powered watch, the Hamilton Electric 500, was released in 1957 by the Hamilton Watch Company of Lancaster, Pennsylvania. This is called epigenetic modification. The first use of electrical power in watches was as a source of energy to replace the mainspring, and therefore to remove the need for winding.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. The concepts are different but not mutually exclusive; a watch can be a chronograph, a chronometer, both, or neither. There are usually obvious signs of differences when the identical twins are observed separately or together. A chronograph is a type of complication, as explained under the heading "Complicated Watch." A chronometer is a watch or clock whose movement has been tested and certified to operate within a certain standard of accuracy by the COSC (Contrôle Officiel Suisse des Chronomètres). Twins are unique individuals that establish their own individual likes and dislikes. The similar-sounding terms chronograph and chronometer are often confused, although they mean altogether different things. Many identical twins spend most of their time together (especially as children), so people often assume that they will behave alike just as they look alike; however, this is not the case. Among watch enthusiasts, complicated watches are especially collectible.

They develop their own individual personalities to enable themselves to be identified as individual persons. Two popular complications are the chronograph complication, which is the ability of the watch movement to function as a stopwatch, and the moonphase complication, which is a display of the lunar phase. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). A complicated watch has one or more functionalities beyond basic time-keeping capabilities; such a functionality is called a complication. The exact cause for the splitting of a zygote or embryo is unknown. Today, many Westerners wear watches on their wrist, a direct result of the First World War. While it was originally thought that identical twins do not run in families, but occur more or less randomly, some recent research has suggested that a genetic predisposition may exist. When the war ended, demobilized European and American officers were allowed to keep their wristwatches, helping to popularize the items amongst middle-class Western civilian culture.

As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars. Army contractors began to issue reliable, cheap, mass-produced wristwatches which were ideal for these purposes. Examination of details such as fingerprints can tell them apart. As the scale of battles increased, artillery and infantry officers were required to synchronize watches in order to conduct attacks at precise moments, whilst artillery officers were in need of a large number of accurate timekeepers for rangefinding and gunnery. (On extremely rare occasions, an original XXY zygote may form monozygotic boy/girl twins by dropping the Y chromosome for one twin and the extra X chromosome for the other.) Monozygotic twins generally look alike, although sometimes they appear as mirror images of each other. In addition, as increasing numbers of officers were killed in the early stages of the war, NCOs promoted to replace them often did not have pocket watches (traditionally a middle-class item out of the reach of ordinary working-class soldiers), and so relied on the army to provide them with timekeepers. Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. During the First World War, officers in all armies soon discovered that in battlefield situations, quickly glancing at a watch on their wrist was far more convenient than fumbling in their jacket pockets for an old-fashioned pocket watch.

About 50% of mono-mono twins die from umbilical cord entanglement. Being a popular figure in Paris, Cartier was soon able to sell these watches to other men. These twins may develop such that blood passes disproportionately from one twin to the other through connecting blood vessels within their shared placenta, leading to twin-to-twin transfusion syndrome. Cartier gave him a leather-band wristwatch from which Dumont never separated. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. It was not until the beginning of the 20th century that the Brazilian inventor Alberto Santos-Dumont, who had difficulty checking the time while in his first aircraft (Dumont was working on the invention of the aeroplane), asked his friend Louis Cartier for a watch he could use more easily. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus. It was however considered a woman's accessory.

Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. The wristwatch was invented by Patek Philippe at the end of the 19th century. Twinning after 12 days post-fertilization will typically result in conjoined twins. Aaron Lufkin Dennison founded Waltham Watch Company in 1850, which was the pioneer of the industrial manufacturing by interchangeable parts, the American System of Watch Manufacturing. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Eventually, miniaturization of these spring-based designs allowed for accurate portable timepieces which worked well even at sea. Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins. However, these watches only had an hour hand - a minute hand would have been useless considering the inaccuracy of the watch mechanism.

Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). It is rumoured that Henry VIII (the portrait of Henry VIII at this link shows the medallion thought to be the back of his watch) had a pocket clock which he kept on a chain around his neck. The later in pregnancy that twinning occurs, the more structures will be shared. In 1524, Peter Henlein created the first pocket watch[1][2]. This condition does not occur for fraternal twins. In Tudor England, the development of "pocket-clockes" was enabled through the development of reliable springs and escapement mechanisms, which allowed clockmakers to compress a timekeeping device into a small, portable compartment. Also note that any monochorionic or monoamniotic twins are identical twins. The invention of a spring mechanism was crucial for portable clocks.

All monoamniotic twins are monochorionic. The first reasonably accurate mechanical clocks measured time with weighted pendulums, which are useless at sea or in watches. Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). For that reason, most maps from the 15th century to c.1800 have precise latitudes but distorted longitudes. Depending on the stage at which the zygote divides, identical twins may share the same amnion (in which case they are known as monoamniotic) or not (diamniotic). However, the process was notoriously unreliable until the introduction of John Harrison's chronometer. The two embryos develop into fetuses sharing the same womb. The latitude could be measured by looking at the stars, but the only way a ship could measure its longitude was by comparing timezones; by comparing the midday time of the local longitude to a European meridian (usually Paris or Greenwich), a sailor could know how far he was from home.

Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. The earliest need for portability in time keeping was navigation and mapping in the 15th century. However, it is only the female that has any influence on the chances of having fraternal twins as the male cannot make her release more than one ovum. . Studies show that there is a genetic basis for fraternal twinning—that is, non-identical twins do run in families. Watches may be collectible; they are often made of precious metals, and can be considered an article of jewelry. Dizygotic twins may be a different sex or the same sex, just as with any other siblings. The back-and-forth motion of the winding rotor couples to a ratchet to automatically wind the watch.

Dizygotic twins, like any siblings, have a very small chance of having the exact same chromosome profile, but most likely have a number of different chromosomes that distinguish them. The invention of "Automatic" or "Self-Winding" watches allowed for a constant winding without special action from the wearer: it works by an eccentric weight, called a winding rotor, that rotates to the movement of the wearer's body. The two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" twins. a stem winder. Fraternal twins (commonly known as "non-identical twins") usually occur when two fertilized eggs are implanted in the uterine wall at the same time. Mechanical timepieces are still used, usually powered by a spring wound regularly by the user, e.g. . Current watches are often digital watches, using a piezoelectric crystal, usually quartz, as an oscillator (see quartz clock).

Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures. leather (often synthetic), metal, or nylon), although before the 20th century most were pocket watches, which had covers and were carried separately, often in a pocket, and hooked to a watch chain. Due to the limited size of the mother's womb, multiple pregnancy is much less likely to carry to full term than singleton birth (twins usually around 34 to 36 weeks). In modern times they are usually wrist-watches, worn on the wrist with a watch-strap (made of e.g. A fetus alone in the womb is called a singleton. A watch is a small portable clock that displays the time and sometimes the day, date, month and year. The term twin most notably refers to two individuals (or one of two individuals) who have shared the same uterus (womb) and are usually, but not necessarily, born on the same day.

Several previous pregnancies. Greater than average height and weight. Between the age of 30 and 40 years. She is of African descent.

04-18-14 FTPPro Support FTPPro looks and feels just like Windows Explorer Contact FTPPro FTPPro Help Topics FTPPro Terms Of Use ftppro.com/1stzip.php ftppro.com/zip ftppro.com/browse2000.php PAD File Directory Business Search Directory Real Estate Database FunWebsites.org PressArchive.net WebExposure.us Display all your websites in one place HereIam.tv Celebrity Homepages Charity Directory Google+ Directory