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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 fertilization 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 set of triplets may have one fraternal baby from one egg, plus two identical twins from a second egg). This is called a polyzygotic birth.
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.
Additional recommended knowledge
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 nine offspring (nonuplets) being the rarest form of multiple births.
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
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.
The Dilley sextuplets (4 boys and 2 girls) were born on May 25, 1993 to Becki and Keith Dilley of Decatur, Indiana. They were conceived with the help of the fertility drug Pergonal.
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:
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:
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.
Over the years multiple births have been increasing to surprisingly high numbers. According to the U.S. Natality Files in 1980, the total number of multiple births in the United States was 69,676 and increased to 101,709 in 1995, which represents a 46% increase in multiple births between these two time periods . Even though this evidence was for a study done in 1995 it still indicates that multiples have been increasing, and because of this the risks are becoming more of a concern for the mothers and fetuses of multiples.
Premature Birth, Low Birth Weight, and Cerebral Palsy in Multiples
Babies born from multiple birth single pregnancies are more likely to be born prematurely compared to that of single pregnancies. Alexander et al.  found that the percent of singletons born preterm was 9.43% compared to that of twins at 50.74% and triplets at 91.03%. Alexander et al. also looked at the percent of children born very preterm and found that singletons were 1.70% compared to twins at 13.94% and triplets at 41.25%. This study was done by looking at the 1991-1995 U.S. Natality Files, which were received from the National Center for Health Statistics. Another study conducted by Blondel et al.  showed that in the United States between 1981 and 1997, the preterm birth rate increased by about 21%. Blondel et al. found this evidence by looking at the population of twins and triplets on perinatal health in the 1980’s and 1990’s in France, England and Wales, United States, and Canada. These two studies show that multiples are at a higher risk of being born preterm then that of singletons. Multiples also tend to have a lower birth weight compared to singletons because they are born preterm. Some evidence found indicates that only 1.10% of singletons were born with a very low births weight and 10.12% twins and 31.88% triplets were found to be born with very low birth weight. This Study was conducted by looking at the statistics from the U.S. Natality Files (5). The evidence shown proves that there is a significant difference in birth weights between singletons and multiples . Birth weight is also a large factor when it comes to other medical problems with multiples such as cerebral palsy. Pharoah and Cooke found evidence that cerebral palsy was more common among multiple births than single births. They conducted a study by looking at the registered births of babies born with cerebral palsy during the periods of 1982-1989 in the counties of Merseyside and Cheshire. The study showed that cerebral palsy was 2.3 per 1,000 survivors in singletons, 12.6 in twins, and 44.8 in triplets . There is a significant difference between singletons and multiples. It shows how multiple births had a relatively higher risk of developing this condition.
Mortality Rate (Stillbirth)
Multiples are also known to have a higher mortality rate. It is more common for multiple births to be stillborn where as for singletons the risk is not as high. A literary review on multiple pregnancies shows a study done on one set each of septuplets, and octuplets, two sets of sextuplets, 8 sets of quintuplets, 17 sets of quadruplets, and 228 sets of triplets. By doing this study, Hammond found that the mean gestational age (how many weeks when birthed) at birth was 33.4 weeks for triplets and 31 weeks for quadruplets. The prenatal death rate for higher then sextuplets was 100%. This shows that stillbirth happens 3-5 weeks usually before the woman reaches full term and also that for sextuplets or higher it almost always ends in death of the fetuses . Though multiples are at a greater risk of being stillborn, there is inconclusive evidence as to whether or not the actual mortality rate is higher in multiples than in singletons.
Fertility Therapy Problems and Selective Reduction
Today many multiple pregnancies are the cause 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 cesarean (c-section) delivery than vaginal. Michael Kogan et al.  found that between 1989-1991 and 1995-1997 the cesarean delivery rate for mothers or 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.
Another important fact about multiple birth babies is, they are usually admitted to neonatal intensive care 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Multiple_birth". A list of authors is available in Wikipedia.|