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| The term '''twin''' |
The term '''twin''' refers to two individuals who have shared the same ] (womb), usually born on the same day but not necessarily. There are two different kinds of twins: identical and fraternal. | ||
| ] occur when a single ] is fertilized and then divides into two separate ]s. The two embryos develop into fetuses sharing the same womb, and generally the same ]. These twins are identical genetically unless there has been a mutation in development, and are necessarily the same gender. | ] occur when a single ] is fertilized and then divides into two separate ]s. The two embryos develop into fetuses sharing the same womb, and generally the same ]. These monozygotic twins are identical genetically unless there has been a mutation in development, and are necessarily the same gender. | ||
| ] occur when two eggs are |
] occur when two fertilized eggs are implanted in the uterine wall at about the same time, within the same ], or in rare cases within one menstrual cycle of each other. These dizygotic twins are no more similar genetically than any siblings and develop in separate ]s, with separate ]e. They may be of mixed gender or the same gender. | ||
| ] are twins whose bodies are joined together at birth. This occurs where the zygote of identical twins fails to completely separate. This |
] are twins whose bodies are joined together at birth. This occurs where the zygote of identical twins fails to completely separate. This condition occurs in about 1 in 100,000 pregnancies. | ||
| Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasound exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. | |||
| A fetus alone in the womb is caleld a ''singleton''. | |||
| ==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 group, ranging as high as about 6% for the ] or 10% for a tiny Brazilian village (see ). The widespread use of fertility drugs causing hyperovulation 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 percent of all births. Thus, over 6% of children born in the US in 2001 were twins. | |||
| Twin studies refers to the practice of assessing identical twins for medical, genetic, or psychological studies to try to find innate similarities and differences. Twins that have been separated early in life and raised in separate households are the most sought-after for these studies. | Twin studies refers to the practice of assessing identical twins for medical, genetic, or psychological studies to try to find innate similarities and differences. Twins that have been separated early in life and raised in separate households are the most sought-after for these studies. | ||
| Sometimes ]s 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. Historically, triplets have been quite rare and more than triplets so rare as to be almost unheard of. However, modern fertility drugs often stimulate release of multiple eggs by the mother, sometimes resulting in multiple fetuses. | Sometimes ]s 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. Historically, triplets have been quite rare and more than triplets so rare as to be almost unheard of. However, modern fertility drugs often stimulate release of multiple eggs by the mother (hyperovulation), sometimes resulting in multiple fetuses. | ||
| 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 a nearly 50 percent chance of delivering monozygotic twins!) Some evidence suggests that the environment of the womb causes the zygote to split in most cases. | |||
| Dizygotic pregancies are slightly more likely in women of African descent, in women between 30 and 40 years old, in tall and heavy women, in women who conceive soon after stopping birth control pills, and in women who have had several previous pregnancies. Women who have been taking fertility drugs have the greatest chance of multiple births, 10-20%. | |||
| ==Twinning in animals== | |||
| Multiple births are common in many animal species, such as ], ], and ]. 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. | |||
| ==Famous twins== | ==Famous human twins== | ||
| * ], Iranian conjoined twins who died during separation surgery | * ], Iranian conjoined twins who died during separation surgery | ||
| * ], conjoined twins from whom the expression "Siamese twins" is derived | * ], conjoined twins from whom the expression "Siamese twins" is derived | ||
Revision as of 03:58, 30 August 2003
The term twin refers to two individuals who have shared the same uterus (womb), usually born on the same day but not necessarily. There are two different kinds of twins: identical and fraternal.
Identical twins occur when a single egg is fertilized and then divides into two separate embryos. The two embryos develop into fetuses sharing the same womb, and generally the same amnion. These monozygotic twins are identical genetically unless there has been a mutation in development, and are necessarily the same gender.
Fraternal twins occur when two fertilized eggs are implanted in the uterine wall at about the same time, within the same menstrual cycle, or in rare cases within one menstrual cycle of each other. These dizygotic twins are no more similar genetically than any siblings and develop in separate amnions, with separate placentae. They may be of mixed gender or the same gender.
Conjoined twins are twins whose bodies are joined together at birth. This occurs where the zygote of identical twins fails to completely separate. This condition occurs in about 1 in 100,000 pregnancies.
Researchers suspect that more pregnancies start out as multiples than come to term that way. Early obstetric ultrasound exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes.
A fetus alone in the womb is caleld a singleton.
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 group, ranging as high as about 6% for the Yoruba or 10% for a tiny Brazilian village (see Twinning rate at a small village in south Brazil). The widespread use of fertility drugs causing hyperovulation 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 percent of all births. Thus, over 6% of children born in the US in 2001 were twins.
Twin studies refers to the practice of assessing identical twins for medical, genetic, or psychological studies to try to find innate similarities and differences. Twins that have been separated early in life and raised in separate households are the most sought-after for these studies.
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. Historically, triplets have been quite rare and more than triplets so rare as to be almost unheard of. However, modern fertility drugs often stimulate release of multiple eggs by the mother (hyperovulation), sometimes resulting in multiple fetuses.
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 a nearly 50 percent chance of delivering monozygotic twins!) Some evidence suggests that the environment of the womb causes the zygote to split in most cases.
Dizygotic pregancies are slightly more likely in women of African descent, in women between 30 and 40 years old, in tall and heavy women, in women who conceive soon after stopping birth control pills, and in women who have had several previous pregnancies. Women who have been taking fertility drugs have the greatest chance of multiple births, 10-20%.
Twinning in animals
Multiple births are common in many animal species, such as cat, sheep, and ferret. 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.
Famous human twins
- Ladan and Laleh Bijani, Iranian conjoined twins who died during separation surgery
- Chang and Eng Bunker, conjoined twins from whom the expression "Siamese twins" is derived
- Ronald and Reginald Kray, British gangsters
- Tia and Tamara Mowry, American actresses
- Mary-Kate and Ashley Olsen, American actresses
- Dora and Cora Webber, American women boxers
- Lloyd and Paul Waner, American baseball players, Hall of Famers.