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. New Unimogs can be purchased on all major continents, however North American models are officially limited to the UGN series sold through Freightliner dealerships. Multiple births are common in many animal species, such as cats, sheep, and ferrets. All other Unimog models are driven by a diesel powerplant varying from 80 horsepower to upwards of 280 horsepower (205 kW). 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. One of the more popular Unimog models with the largest single design production run is the Unimog 404, produced in troop carrier and radio box variants and powered by a 2 liter waterproofed Mercedes-Benz petrol engine that produced as much as 80 horsepower (60 kW) in the most common versions of this model. When monozygotic twins are born with different genders it is because of chromosomal birth defects. They are also used extensively by the South African National Defence Force and the New Zealand Army.

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. Unimogs have been used by many different militaries, including the German and Swiss armies. Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). Unimogs also feature a flexible frame that allows the tires a wide range of vertical movement to allow the truck to comfortably drive over extremely uneven terrain, even meter-high boulders. 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. They feature very high ground clearance — greater than the Humvee — made possible by portal gears that allow the axles and transmission to be higher than the tires' centers. This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). The Unimog (from the German "UNIversal-MOtor-Gerät", meaning "universal motor machine/equipment") is a series of trucks from Daimler-Benz designed shortly after WWII, originally to be used as slow-moving tractors for Germany's turnip harvest.

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). reverse: 14.569/10.101/6.660/4.900/3.330/2.309/1.522/1.120. 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. forward: 9.570/6.635/4.375/3.219/2.188/1.517/1.000/0.736. 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. transmission gearing

    . Twin studies are studies that assess identical (monozygotic) twins for medical, genetic, or psychological characteristics to try to isolate genetic influence from environmental influence. Electronic Speed governor.

    Many fertility treatments have no effect on the likelihood of multiple births. engine: 4.8 litre straight 4 Turbocharged OM924LA 218 HP (160 kW)

      . Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. average fuel consumption (diesel): not published. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. descent: not published. This can vary depending on what types of fertility treatments are used. climb: not published.

      Women undergoing certain fertility treatments may have a greater chance of multiple births. departure: 53 degrees. Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. approach: 44 degrees. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. on a side slope: not published. 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). traverse angle: not published

        .

        The cause of monozygotic twinning is unknown. 1200 mm (optional equipment). 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. fording 800 mm (standard)

          . 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. ground clearance: 476 mm (varies with tyre). If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. fuel: 145 litres.

          Sometimes multiple births may involve more than two fetuses. tires: 365/85 R 20.5. 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. gross weight: 11,800 kg. Thus, approximately 6% of children born in the US in 2001 were twins. empty weight: 4,300 kg. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. turning radius: 14.5 m.

          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". wheelbase: 2336 mm. 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]). height: 2740 mm. Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. length: 5410 mm. Similar to vanishing twin. track width: 1920 mm.

          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. width: 2474 mm. 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. discontinued: In Production. 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. introduced: 2002. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other. reverse: 20.12/11.09.

          One fetus acts as a parasite towards the other. forward: 14.93/8.23/4.47/2.46/1.52/1.0. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. transmission gearing

            . This condition occurs in about 1 in 100,000 pregnancies. 2 Barrel Zenith NDIX-32 Carbeurator. This occurs where the single zygote of identical twins fails to separate completely. Shielded Distributor Ignition, 24 Volt Electrical System.

            Conjoined twins are monozygotic twins, whose bodies are joined together at birth. engine: 2.2 litre straight 6 SOHC (Mercedes Benz M180) 80 HP (60 kW)

              . Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. average fuel consumption (gasoline): 10–14 mpg (US) (24–17 L/100 km). Researchers suspect that more pregnancies start out as multiples than come to term that way. descent: 90% grade. Such conditions are usually associated with a higher incidence of other birth defects. climb: 70% grade.

              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). departure: 46 degrees. One mirror may or may not have situs inversus. approach: 45 degrees. They result from a late split of the fertilized egg at around 9-12 days. on a side slope: 42 degrees. The incidence of mirror twinning is comparatively rare. traverse angle

                .

                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. fording (w/o equipment): 800 mm (31.5 in). Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. ground clearance: 400 mm (15.7 in). (Fraga, et al., 2005). fuel: 2 x 60 litre (15.9 US gal). Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. tires: 10.5 x 20 (DOT).

                50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. gross weight: 4400 kg (9700 lb). The number of differences between identical twins increases with age. empty weight: 2900 kg (6390 lb). A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. turning radius: 13 m (42.6 ft). This is called epigenetic modification. wheelbase: 2900 mm (114 in).

                Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. cargo bed: 3000 by 2000 mm (118 by 78 in), 2130 by 2080 mm (84 by 82 in) with double-cab. There are usually obvious signs of differences when the identical twins are observed separately or together. height: 2190 mm (86 in), cab height: 1600 mm (63 in). Twins are unique individuals that establish their own individual likes and dislikes. length: 4900 mm. 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. width: 2130 mm (84 in).

                They develop their own individual personalities to enable themselves to be identified as individual persons. discontinued: 1980. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). introduced: 1955. The exact cause for the splitting of a zygote or embryo is unknown. 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.

                As they mature, identical twins often become less alike because of lifestyle choices or external influences such as scars. Examination of details such as fingerprints can tell them apart. (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. Monozygotic twins are genetically identical unless there has been a mutation in development, and they are almost always the same gender.

                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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