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.


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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. The numeric character references in HTML and XML are "W" and "w" for upper and lower case respectively. Multiple births are common in many animal species, such as cats, sheep, and ferrets. The EBCDIC code for capital W is 230 and for lowercase w is 166. 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. The ASCII code for capital W is 87 and for lowercase w is 119; or in binary 01010111 and 01110111, correspondingly. When monozygotic twins are born with different genders it is because of chromosomal birth defects. In Unicode the capital W is codepoint U+0057 and the lowercase w is U+0077.

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. Bush's nickname of "Dubya". Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). In the Texas dialect of American English, the name is often condensed to two syllables rather than three, resulting in George W. 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. A few speakers therefore shorten the name "double u" into "dub" only, although this is rather rare and nonstandard; for example, University of Washington is known colloquially as "U Dub". This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). This gives the nine-syllable initialism www the irony of being an abbreviation that takes more syllables to say than the unabbreviated form.

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). "Double U" is the only English letter name with more than one syllable. 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. The equivalent representation of the [w] sound in the Cyrillic alphabet is Ў, a letter unique to the Belarusian language. 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. See more (in Hebrew). Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. For example many Israelis say "Hollyvud" rather than "Hollywood" or "Darvin" rather than "Darwin".

Many fertility treatments have no effect on the likelihood of multiple births. In Hebrew the same letter, waw or vav is used to spell both [w] and [v], which can make problems in some cases. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. When a spelling for the [w] sound in a native word is needed, a spelling from the native alphabet, such as U or OU, can be used instead. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. In the alphabets of modern Romance languages, it is not used either, except in foreign names and words recently borrowed (le week-end, il watt, el kiwi). This can vary depending on what types of fertility treatments are used. It is however recognized and maintained in names, like "William".

Women undergoing certain fertility treatments may have a greater chance of multiple births. In the Swedish and Finnish alphabets, "W" is seen as a variant of "V" and not a separate letter. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. There are only four major European languages that use W in native words: English, German, Polish, and Dutch. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. In Dutch, W is a labiodental approximant (with the exception of words with EEUW, which have /eːw/), or other diphthongs containing -UW. 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). In German—as in Romance—the phoneme [w] was lost; this is why German W represents [v] rather than [w].

The cause of monozygotic twinning is unknown. The Latin [w] sound developed into Romance [v]; therefore V no longer adequately represented Germanic [w]. 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. This digraph was not extensively used, the sound usually being represented instead by the runic wynn (Ƿ), but W gained popularity after the Norman Conquest, and by 1300 it had taken wynn's place in common use. 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. The earliest form of the letter W was a doubled V used in the 7th century by the earliest writers of Old English; it is from this <uu> digraph that the modern name "double U" comes. If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. Its name in English is double-u.

Sometimes multiple births may involve more than two fetuses. W is the twenty-third letter of the modern Latin alphabet. 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. w or w/ is an abbreviation for the word "with". Thus, approximately 6% of children born in the US in 2001 were twins. In Final Fantasy XI, Japanese players use "w" to be used in the terms of "lol", also constantly used by English players as well. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. In NetHack, "w" is the command to wield (or unwield) a weapon, and "W" is the command to wear armor.

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". W is the symbol for Wario, a Nintendo character. 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]). In computing and video games,

    . Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. In the Champaign-Decatur-Springfield TV Market (DMA 82)(see Media market), a W is inserted over the picture in an upper corner when the National Weather Service issues a weather watch or warning, etc (see Severe weather terminology#Watches.2C Warnings and Advisories as defined by the NWS. Similar to vanishing twin. On the TV show "Good Eats," W is an ill-tempered kitchen supply salesperson.

    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. W is the name of a Canadian specialty service dedicated to women's issues, formerly called WTN; see W Network. 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 television,

      . 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. W is generally used as the first letter of callsigns allocated to broadcast television or radio stations east of the Mississippi river. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other. In radiocommunication, W is one of the ITU prefixes allocated to the United States.

      One fetus acts as a parasite towards the other. W Hotels is an international chain of boutique hotels in urban centers. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. In publishing, W is the name of an American fashion magazine; see W (magazine). This condition occurs in about 1 in 100,000 pregnancies. In weather forecasting, geography, and weather vanes, W stands for west, one of the four cardinal directions. This occurs where the single zygote of identical twins fails to separate completely. In the United Kingdom, W stands for West London.

      Conjoined twins are monozygotic twins, whose bodies are joined together at birth. As the first letter of a postal code,

        . Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. Dubya is generally used by opponents of Bush, or by those making fun of him. Researchers suspect that more pregnancies start out as multiples than come to term that way. "W" is used by Bush himself, on campaign signs, and by many of his supporters. Such conditions are usually associated with a higher incidence of other birth defects. Bush, sometimes spelled out as "Dubya" (the President's name is sometimes written "George Double-U Bush").

        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). In politics, W is a nickname of American President George W. One mirror may or may not have situs inversus. W is a kind of a subatomic particle, the W boson. They result from a late split of the fertilized egg at around 9-12 days. W is often the variable for energy or work, measured in joules. The incidence of mirror twinning is comparatively rare. In physics,

          .

          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. W is the symbol for the watt, the SI derived unit for power. Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. In the SI system,

            . (Fraga, et al., 2005). In journalism, the Five Ws are who, what, when, where and why. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. Also, as Internet Slang, it can be used as "lol".

            50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. There is also a J-Pop act called W (Double You). The number of differences between identical twins increases with age. In Japan, W is commonly used in slang and advertising to represent the English loanword "double". A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. In Italy, W (which does not appear in the Italian alphabet) stands for Viva ("long live"). This is called epigenetic modification. In the International Phonetic Alphabet, [w] represents a labial-velar approximant.

            Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. In population genetics, w is the symbol for fitness. There are usually obvious signs of differences when the identical twins are observed separately or together. WZ denotes female in the WZ sex-determination system. Twins are unique individuals that establish their own individual likes and dislikes. In genetics W denotes the W chromosome.

              . 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. In film, W is the name of a 1973 American film; see W (film).

              They develop their own individual personalities to enable themselves to be identified as individual persons. W is the name of a graphical windowing system for Unix platforms; see W Window System. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). w is a command on Unix systems that displays information on users who are currently logged in. The exact cause for the splitting of a zygote or embryo is unknown. This is because in some programs (e.g., many Unix shells) the keystroke control-W is used to delete the previous word. 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. ^W is often used jokingly to indicate the intended deletion of the previous word (see also H).

              As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars. In computing,

                . Examination of details such as fingerprints can tell them apart. In chemistry, W is the symbol for tungsten, after its German name, Wolfram. (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 calendars, W is often an abbreviation for Wednesday. Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender. In biochemistry, W is the symbol for tryptophan.

                About 50% of mono-mono twins die from umbilical cord entanglement. 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. Monochorionic twins, sharing one placenta, usually also share the placental blood supply. For example, the umbilical cords of monoamniotic twins can become entangled, reducing or interrupting the blood supply to the developing fetus.

                Sharing the same amnion (or the same amnion and placenta) can cause complications in pregnancy. Twinning after 12 days post-fertilization will typically result in conjoined twins. Twinning between 8 to 12 days after fertilization will usually result in monochorionic-monoamniotic ("mono-mono") twins. Twinning between 4 to 8 days after fertilization typically results in monochorionic-diamniotic ("mono-di") twins.

                Zygotes that twin at the earliest stages will be diamniotic and dichorionic ("di-di"). The later in pregnancy that twinning occurs, the more structures will be shared. This condition does not occur for fraternal twins. Also note that any monochorionic or monoamniotic twins are identical twins.

                All monoamniotic twins are monochorionic. Diamniotic identical twins may share the same placenta (known as monochorionic) or not (dichorionic). 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). The two embryos develop into fetuses sharing the same womb.

                Identical twins occur when a single egg is fertilized to form one zygote (monozygotic) but the zygote then divides into two separate embryos. 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. Dizygotic twins may be a different sex or the same sex, just as with any other siblings.

                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 two eggs form two zygotes, and these twins are therefore also known as dizygotic as well as "biovular" 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. .

                Since some premature births often have health consequence to the babies, twins birth are often handled with special procedures. 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). A fetus alone in the womb is called a singleton. 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.

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