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A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple birth are utilized, depending on the number of offspring. 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 the coequal genetic similarity any other full siblings do. In some multiple births, it is possible for a combination of these (for example, a triplet may have one fraternal baby from one egg, plus two identical twins from the other). This is called a polyzygotic birth.
A very rare third type of twinning is believed to occur in which an unfertilized ovum splits into two identical cells prior to fertilization, creating a second complete ovum, called the "polar body". After splitting, both ova are then fertilized by different sperm. This results in twins who have received identical genes from their mother, but of the genetic material they received from their father, only about half of it is shared. Thus polar body twins have about 75% of their genes in common, less than the 100% shared by identical twins but more than the 50% shared by 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, so there has been no confirmation that such twins actually exist.
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) fetuses produced by the splitting of a single zygote
- Dizygotic — multiple (typically two) fetuses produced by two zygotes
- Polyzygotic — multiple fetuses 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) is the most common form of multiple births to seven offspring (septuplets) being the rarest form of multiple births.
- 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 — quattrodecaplets
- 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 also called 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 fertilized egg split and then one of the resultant cells splits again (producing identical triplets) or, even more rarely, a further split occurs (producing identical quadruplets). Sometimes the original fertilized egg will split twice (to produce four embryos), and all four may survive and produce quadruplets, or one of the embryos may not survive and result 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 pregnancies that started out with ten, eleven, twelve or fifteen fetuses, but no known instances of live births of such high multiples in a single pregnancy. Most of these pregnancies are the result of fertility medications and assisted reproductive technology (ART), 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.
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 fertile eggs being produced in one ovulatory cycle).
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. It is theorized that this is due to the higher follicle-stimulating hormone that older women sometimes have as their ovaries respond more sluggishly to FSH stimulation.
In North America, dizygotic twinning occurs about once in 83 conceptions and triplets about once in 8000 conceptions. A traditional approximation of the incidence of multiple pregnancies is as follows: 
- Twins 1:80
- Triplets 1:80² = 1:6400
- Quadruplets (Etc) 1:80³ = 1:512,000
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. Much of the increase can probably be attributed to the impact of fertility treatments, such as in-vitro fertilization. Younger patients who undergo treatment with fertility medication containing artificial FSH, followed by intrauterine insemination, are particularly at 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. Better nutrition also increases the likelihood of multiple births.
Recent increases over the last few years in the number of multiple births have also provoked concern over the risks to the fetus and to the mother.
The greater the number of babies in a single pregnancy, the more likely they are to have 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. When fertility drugs are used, many reproductive endocrinologists are also attempting to monitor follicle production via ultrasound. When too many viable eggs are detected, the patient is often advised to cancel the cycle to prevent high-order multiple gestations.
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.
About 75% of women who undergo selective reduction will go into premature labor. About 4-5% of women undergoing selective reduction also miscarry one or more of the remaining fetuses. The risks associated with multi-fetal pregnancy are considered higher.source
Fertility Therapy Problems and Selective Reduction
Today many multiple pregnancies are the result of fertility therapy. Elsner et al.  studied 42 months of IVF (in vitro fertilization) procedures, and related the number of embryos transferred and the pregnancy outcome. In this time, they found that of the 2,173 embryo transfers preformed, 734 were successfully delivered pregnancies (33.8%). These were analyzed…the overall multiple pregnancy rate was 31.3% (24.7% twins, 5.8% triplets, and .08% quadruplets)(8). This evidence shows that almost all of the births delivered were multiples rather than singletons, because IVF’s are producing more multiples, a number of efforts are being made to reduce the risk of multiple births specifically triplets or more. Medical practitioners are doing this by limiting the number of embryos per embryo transfer to one or two. That way, the risks for the mother and fetuses are decreased. De Sutter et al.  found by looking at a previous study done in 1997, that by using SET (single embryo transfer) the twin birth rate dropped from 30% to 21%. De Sutter et al. also found that the use of this transfer method increased from 1.5% to 17.5%(9). So by limiting the number of embryos transferred it can reduce the risks of having multiples, therefore reducing the risk factors that are brought about with multiple pregnancies.
Another procedure that the medical world is using today is known as selective reduction. Selective reduction is the termination of one or more, but not all the fetuses, which is usually done in pregnancies with multiple gestations. This procedure is done so that hopefully one child may live a healthy life. Armour et al.  found by looking at a review of a series of 1000 selective reduction cases, it has had a loss rate of 5.4% in pregnancies. A lot of the losses (15%) occurred within 4 weeks of the procedures and more than 50% occurred after 8 weeks. This shows that the reduction was successful at reducing the embryos from multiple gestations to single (9).
Though selective reduction seems to be working, mothers of multiples who undergo this procedure are at a higher risk of miscarrying compared to that of a single pregnancy. A study done by looking at 158 pregnant women who underwent selective reduction from high order multiples to twins showed that the mother had a 10.6% chance of miscarriage. Mothers of single pregnancies only had a 9.5% chance of miscarriage (10). Antsaklis et al.  shows that there is not that significant of a difference, but even so there is still a higher percent chance for mothers of multiples to miscarry.
Birthing Process and Neonatal Intensive Care
When it comes to the birthing process of multiples, mothers are more likely to receive a Caesarean (C-section) delivery than vaginal. Michael Kogan et al.  found that between 1989-1991 and 1995-1997 the cesarean delivery rate for mothers of multiples increased from 21.9% to 27%. Kogan et al. discovered this evidence by looking at the National Center for Health Statistics, births and infant death records for twins in the United States.
Multiple-birth infants are usually admitted to neonatal intensive care immediately after being born. The records for all the triplet pregnancies managed and delivered from 1992-1996 were looked over to see what the neonatal statistics were. Kaufman et al.  found from reviewing these files that during a five year period, 55 triplet pregnancies, which is 165 babies, were delivered. Of the 165 babies 149 were admitted to neonatal intensive care after the delivery. That is 90% of the babies born.
Certain cultures consider multiple births a portent of either good or evil. 
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.
- Biological reproduction
- Conjoined twins
- Dionne Quintuplets
- List of multiple births
- List of twins
- Category:Multiple births
- Melissa Bush, MD, & Martin L. Pernoll, MD (2006). "Multiple Pregnancy". Pregnancy Health Center -Multiple Pregnancy. Retrieved 2007-06-20. Unknown parameter
- Elsner C., Tucker M., Sweitzer C., Brockman W., Morton P., Wright G., and Toledo A. (1997). Multiple pregnancy rate and embryo number transferred during in vitro fertilization. American Journal of Obstetrics and Gynecology, 177(2), 350-355
- De Sutter P., Van der Elst J., Coetsier T., and Dhont M. (2003). Single embryo transfer and multiple pregnancy rate reduction in IVF/ICIS: a 5 year appraisal. Reproductive BioMedicine Online 6(4), 464-469
- Armour K., and Callister L. (2005). Prevention of triplets and high order multiples: Trends in reproductive medicine. Journal of Perinatal and Neonatal Nursing, 19(2), 103-111
- Antsaklis A., Drakakis P., Vlazakis G., and Michalas S. (1998). Reduction of multifetal pregnancies to twins does not increase obstetrics or perinatal risks. Human Reproduction, 14(5) 1338-1340
- Kogan M., Alexander G., Kotelchuck M., Macdorman M., Buckens P., Martin J., and Papiernik E. (2000). Trends in twin birth outcomes and prenatal care utilization in the United States, 1981-1997. The Journal of American Medical Association, 284(3), 335-341
- Kaufamn GE., Malone FD., Harvey-Wilkes KB., Chelmow D., Penzias AD., and D’ Alton ME. (1998). Neonatal morbidity and mortality associated with triplet pregnancy. Obstetrics and Gynecologists, 91, 342-348
- "Korea's 'lucky' triplets seized". Herald Sun. Fairfax. 2003-03-30. Check date values in: