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Identical triplet sisters

A multiple birth occurs where more than one fetus exits the womb in a single pregnancy. Different names for multiple birth are used, depending on the actual multiple. Common multiples are two and three, known as twins and triplets respectively. Twins, triplets and other multiple births occur to varying degrees in most animal species, although the term is most applicable to placental species.

There are two common types of multiple births, fraternal (dizygotic) and identical (monozygotic). Identical siblings arise where one egg is fertilised and the resulting zygote splits into more than one embryo. Identical siblings therefore have the same genetic material. Fraternal siblings result from the fertilisation and implantation of more than one egg, so fraternal siblings are not genetically identical, and instead have an analogous genetic similarity of any brothers and sisters.

A very rare third type of twinning is now believed to occur when an unfertilized ovum splits into two equal parts prior to fertilization, creating a second complete ovum, called the "polar body". After splitting, both ovum are each then fertilized by different sperm. This results in twins who share half their genes in common (from the mother) and the other half different (from the two sperm). Polar body twins share about 75% of their genetic markers, less than identical twins but more than fraternal twins. They share some features of identical twins and some features of fraternal twins and are so-called half-identical twins. However, DNA-based zygosity tests are currently not available to determine if twins are polar-body twins.

The most common form of human multiple birth is twins (two babies), but the typical order of multiple births in other placental species differs dramatically. Some species give birth to multiple offspring as a matter of course and the resulting group of offspring is called a litter.


  • Monozygotic — multiple (typically two) foetuses produced by the splitting of a single zygote
  • Dizygotic — multiple (typically two) foetuses produced by two zygotes
  • Polyzygotic — multiple foetuses produced by two or more zygotes
  • Litter — the offspring produced by a multiple birth in non-human placentals.

Terms used for the order of multiple births are largely derived from the Latin names for numbers.

  • Two offspring — twins
  • Three offspring — triplets
  • Four offspring — quadruplets
  • Five offspring — quintuplets
  • Six offspring — sextuplets
  • Seven offspring — septuplets
  • Eight offspring — octuplets
  • Nine offspring — nonuplets
  • Ten offspring — decaplets
  • Eleven offspring — undecaplets
  • Twelve offspring — duodecaplets
  • Thirteen offspring — tredecaplets
  • Fourteen offspring — quattuordecaplets
  • Fifteen offspring — quindecaplets

Higher order multiples

High orders of multiple births (three or more offspring in one birth) may result in a combination of fraternal (genetically different) and identical (genetically identical) siblings. The latter are occasionally given the term of super twins. For example, a set of quadruplets may consist of two sets of identical twins. In such a case each child has one identical and two fraternal siblings. Identical triplets or quadruplets are very rare, as this means that the original egg split and then either one of those split off sections split again (producing identical triplets) or, even more rare, both of the split off sections split again (producing identical quadruplets). Some times the original egg will split twice (to produce four embryos), and either all four survive and produce quadruplets, or one of the embryos does not survive and results in identical triplets.

Human multiple births

The most common form of human multiple birth is twins (two babies), but cases of higher orders up to octuplets (eight babies) have all been recorded with all siblings being born alive. The largest set in which all members survived more than a few days is septuplets, the first of which was in 1997. The largest set to have even a single member survive is octuplets, in 1998 (with the seven surviving octuplets born in Texas).

There have been a few sets of nonuplets (nine) in which a few babies were born alive, though none lived longer than a few days. There have been cases of human pregnancy which started out with ten, eleven, twelve and fifteen fetuses, but no known instances of live births of such high multiples in a single pregnancy. Most of these pregnancies are a result of fertility medications, though a set of duodecaplets (twelve) was conceived spontaneously (without the aid of fertility treatments) in Argentina in 1992.

Multiple pregnancies in humans 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.

Fraternal twin boys in the tub

Causes and frequency

Human multiple births can occur either naturally (the woman ovulates multiple eggs or the fertilized egg splits into two) or as the result of infertility treatments such as IVF (several embryos are usually implanted to compensate for their lower viability) or fertility drugs (which can induce multiple ovulation).

In general, twins occur naturally at approximately the rate of 1/89 of singleton births, triplets at 1/89 the rate of twin births, and so on (Hellin's Law). However, for reasons that are unknown, the older a woman is, the more likely she is to naturally have a multiple birth.

The number of multiple births has increased over the last decades. For example, in Canada, between 1979 and 1999, the number of multiple birth babies increased 35%. 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.[citation needed] Much of the increase can probably attributed to the impact of fertility treatments, such as in vitro fertilisation. Younger patients who undergo treatment with fertility medication containing FSH followed by intrauterine insemination are at particular risk for multiple births of higher order.

Certain factors appear to increase the likelihood that a woman will naturally conceive multiples. These factors include:

  • mother's age — women over 35 are more likely to have multiples than younger women
  • mother's use of fertility drugs — approximately 35% of pregnancies arising through the use of fertility treatments such as IVF involve more than one child

The increasing use of fertility drugs as well as the increasing life expectancy for women have contributed to the rise in the rate of multiples over the last fifty years.


Recent increases over the last few years in the number of multiple births have also provoked concern over the risks to the fetus and also to the mother.

The greater the number of babies in a single pregnancy, the more likely they are to have a low birth weight, to be born prematurely and to consequently suffer medical problems. For example, in 1999, 53% of babies in multiple births were premature, compared to 7% of singletons. There is also a higher rate of stillbirths for multiples than for singletons.

Virtually all obstetrical risks are increased for the mother during a pregnancy with multiples. As many multiple pregnancies today are the result of the use of fertility therapy, efforts are being made to reduce the risks of multiple pregnancy, specifically triplets or more, by limiting the number of embryos per embryo transfer during IVF to one or two. Sometimes a selective reduction — the abortion of one or more of the multiple fetuses — is performed to give the pregnancy a higher chance of producing at least one healthy or live child.

Cultural aspects

Certain cultures consider multiple births a portent of either good or evil. A Korean tradition stating that triplets are 'lucky' has led to the seizure of all sets of triplets born in North Korea, apparently out of leader Kim Jong-il's fear that one may overthrow him in the future.[1]

In Ancient Rome, the legend of the twin brothers who founded the city (Romulus and Remus) made the birth of identical twin boys a blessing, while identical twin girls were seen as an unlucky burden.

See also


  1. includeonly>"Korea's 'lucky' triplets seized", Herald Sun, Fairfax, 2003-03-30.

External links

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