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 W8 uses two four-cylinder VR engines mated together, and the W16 uses two eight-cylinder VR banks. Multiple births are common in many animal species, such as cats, sheep, and ferrets. W8 and W16 designs were developed in a similar fashion. 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. For example, two VR6 engines mated together at 72 degrees result in a W12 configuration, which is significantly shorter than a V12 engine but only marginally wider. When monozygotic twins are born with different genders it is because of chromosomal birth defects. Volkswagen has also developed a series of engines which use narrow angle designs mated together at 72 degrees.

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. The Porsche Cayenne, which shares its chassis with the VW Touareg, also uses the 3.2 L VR6 as its base engine. Among monozygotic twins, in extremely rare cases, twins have been born with opposite sexes (one male, one female). The VR6 is also used in other Volkswagen Group products, namely:. 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. The VR6 was used by Volkswagen in:. This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). The 3.2 and 3.6 litre VR6s will also be used to power a new MKV platform R32 (for Europe) and a new R36 model (North America).

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). The introduction of the Passat VR6 also marked the first time a VR6 powered vehicle was made available in North American before Europe. 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. Both the 3.2 and 3.6 feature FSI direct injection. 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. For North American, the Passat receives a new 3.6 L VR6 with a narrower 10.6 degree cylinder angle, producing 280 PS (276 hp/206 kW). 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 2005, the European market version of Volkswagen's fifth generation Passat went on sale with a revised version of the 3.2 L VR6 as its top-spec motor.

Many fertility treatments have no effect on the likelihood of multiple births. The 3.2 is now used as a range-topper in Audi A3 or as an entry level version in the VW Touareg and Porsche Cayenne, although the version used in the Cayenne features modifications to the heads as well as the intake and timing systems. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, allowing the possibility of multiples. This variant produced 250 PS (247 hp/184 kW) and 320 Nm (236 ft•lbf) of torque in TT trim and 241 PS(238 hp/177 kW) in R32 trim. With in vitro fertilisation (IVF), this is primarily due to the insertion of multiple embryos into the uterus. In 2003, a high performance 3.2 L version of the engine was introduced to power VW's limited-production Golf R32 and a new range-topping variant of the Audi TT. This can vary depending on what types of fertility treatments are used. The multivalve V6 was only introduced in North America in 2002 (where it retained the VR6 name).

Women undergoing certain fertility treatments may have a greater chance of multiple births. The corresponding multivalve V5 was only released in 2001, with a 20 PS power increase, to 170 PS (168 hp/125 kW). Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:. The VR6 name was dropped as a commercial designation, and the 4WD system (4Motion) was now standard on the V6 in Europe. Some evidence suggests that the environment of the womb causes the zygote to split in most cases. The new version was not available in the Passat (as it was incompatible with the then-current generation's longitudinal layout), but was introduced as the range topper in the Golf and Bora. 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). This engine produced 204 PS (201 hp/150 kW) and 265 Nm (195 lb.ft) of torque.

The cause of monozygotic twinning is unknown. For 1999, VW added further modifications to the design, with the introduction of the 24-valve 2.8 L VR6. 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. It was introduced in the Passat in 1997, and later in the Golf and Bora in 1999. 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. This version, which had a 2.3 L capacity, was capable of 150 PS (148 hp/110 kW) and had a maximum torque of 209 Nm (154 lb.ft). If there are three, they are called triplets; four, quadruplets; five, quintuplets; six, sextuplets, seven, septuplets, and so on. In 1997, VW removed a cylinder from the VR6, creating the VR5, the first block to use an uneven number of cylinders in a V design.

Sometimes multiple births may involve more than two fetuses. The corresponding Vento/Jetta VR6 versions appeared in the same years. 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. North America only received this engine in 1995, at the same time the European model started to use the 2.9 L in the VR6 Syncro model. Thus, approximately 6% of children born in the US in 2001 were twins. In 1992, with the introduction of the Golf's third generation, a six-cylinder engine was available for the first time in a lower-midsize segment hatchback in Europe. In 2001, for the first time ever in the US, the twinning rate exceeded 3% of all births. This version also had a free flowing 6 cm (2.5 in) catalytic converter, enlarged inlet manifold and larger throttle body.

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". The Passat, Passat Variant wagon and US-spec Corrado used the original 2.8 L design, while the Euro-spec Corrado and the 4WD Passat Syncro received a 2.9 L version with 190 PS (187 hp/140 kW). 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 VR6 engine was introduced in Europe in 1991 in the Passat and Corrado, and in North America the following year. Historically, about 1 in 80 human births (1.2%) has been the result of a twin pregnancy. These engines produced 174 PS (172 hp/128 kW) and 240 Nm (177 ft·lbf) of torque. Similar to vanishing twin. The original VR6 engine displaced 2.8 L and featured a 12 valve design.

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. There are several different variants of the VR6 engine. 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. This is most similar to a DOHC Inline-6 engine. 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. However, later (24 valve) VR6 engines use one camshaft for all intake valves and one camshaft for all exhaust valves. Sometimes the parasitic twin just becomes an almost indistinguishable part of the other. This is most similar to the operation of a SOHC V6 engine.

One fetus acts as a parasite towards the other. In early (12 valve) VR6 engines, one camshaft is used per bank of cylinders. Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. This simplifies engine construction and reduces costs. This condition occurs in about 1 in 100,000 pregnancies. The narrow angle between cylinder banks also allows just two camshafts to drive all of the valves, and a single cylinder head to be used. This occurs where the single zygote of identical twins fails to separate completely. As a result, it is nearly as smooth as an Inline-6.

Conjoined twins are monozygotic twins, whose bodies are joined together at birth. In addition, the VR6 is able to use the firing interval of an Inline-6 engine. Early obstetric ultrasonography exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. A wider V6 engine of conventional design would have required lengthening existing vehicles to provide enough crumple zone between the front of the vehicle and the engine, and between the engine and the passenger cell. Researchers suspect that more pregnancies start out as multiples than come to term that way. By using the narrow 15° VR6 engine, it was possible to install a six-cylinder engine in existing Volkswagen models. Such conditions are usually associated with a higher incidence of other birth defects. The VR6 was specifically designed for transverse installation in front wheel drive vehicles.

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). The combination of the two can be roughly translated as "in-line Vee.". One mirror may or may not have situs inversus. The name, VR6 comes from a combination of Vee and the German word Reihenmotor (straight engine). They result from a late split of the fertilized egg at around 9-12 days. It is similar to the V engine, but with the cylinders offset from each other and tilted by 15° instead of the usual 60°. The incidence of mirror twinning is comparatively rare. VR6 is an engine configuration developed by the Volkswagen Group.

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. . Some percentage of monozygotic twins are called "mirror twins" or mirror image twins. SEAT Leon Cupra. (Fraga, et al., 2005). Audi TT. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. Audi A3 Mk.II.

50-year-old twins had over 3 times the epigenetic difference that the 3-year-old twins had. VW Sharan/SEAT Alhambra/Ford Galaxy. The number of differences between identical twins increases with age. VW Transporter T4 and T5. A study of 80 pairs of twins ranging in age from 3 to 74 showed that the youngest twins have relatively few epigenetic differences. VW Touareg. This is called epigenetic modification. VW Phaeton.

Identical twins have identical DNA but differing environmental influences throughout their lives affect which genes are switched on or off. VW Corrado. There are usually obvious signs of differences when the identical twins are observed separately or together. VW Bora/VW Jetta Mk.IV. Twins are unique individuals that establish their own individual likes and dislikes. VW Vento/VW Jetta Mk.III. 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. VW Passat (B3, B4, and B6 chassis).

They develop their own individual personalities to enable themselves to be identified as individual persons. Golf R32 MK.IV and Mk.V. Identical twins can behave as differently as any other siblings (a matter of much interest to psychologists). VW Golf Mk.III and Mk.IV. 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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