In the one-egg, or identical, type of twinning, a single fertilized ovum divides to form two complete organisms. Such twins are always of the same sex, are usually extraordinarily similar in physical appearance, and have identical blood-group types. Twinning to form one-egg identical twins usually takes place early in pregnancy. If considerable development has taken place before the twinning occurs, there may be an incomplete separation of the two embryos resulting in conjoined twins.
Fraternal twins, which are more common than identical twins, are those that develop from two separate ova, each fertilized by a sperm. Fraternal twins may or may not be of the same sex and need not resemble each other more than do any other two offspring of the same parents. In the United States twins occur once in approximately 40 births. In rare cases, non-identical embryos can fuse in the womb to produce a condition called chimerism, in which some of the individual's cells come from one of the embryos and the rest of the cells come from the other, genetically distinct embryo.
The incidence of multiple-egg births is in part genetically determined, varying according to race and family tendencies; and it is also influenced by external factors, i.e., the incidence increases with increasing age of the mother and the number of children she has already borne. One-egg, or identical, twinning occurs with the same frequency in all women, regardless of race, age, or other factors. There is evidence from comparative biology that deleterious factors in the environment of the newly fertilized ovum, such as a reduction in oxygen, increase the likelihood of one-egg twinning. Fertility drugs such as clomiphene, which are used when the cause of infertility is lack of released ova, sometimes cause several ova to be released and fertilized simultaneously. The use of these drugs has led to a rise in the incidence of multiple births, including sextuplets, septuplets, and octuplets.
There are two common types of multiple births, fraternal (dizygotic) and identical (monozygotic). Identical siblings arise where one egg is fertilized 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 but instead have the coequal genetic similarity any other full siblings have. 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.
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, nine (nonuplets) or more being the rarest.
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 and result when the original fertilized egg splits and then one of the resultant cells splits again (for triplets) or, even more rarely, a further split occurs (for quadruplets). Alternatively the original fertilized egg can split twice (to produce four embryos) and all four may survive, to produce identical quadruplets, or one of the embryos may not survive and result in triplets.
The most common form of human multiple birth is twins (two babies), but cases of higher orders up to octuplets (eight babies) have 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 there are 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.
Quadruplets result from the rare occurrence when four eggs are released and fertilized at once, or when one egg splits into four, or two eggs split into two.
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 often transferred to compensate for lower quality) or fertility drugs (which can cause multiple eggs to mature 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 not yet known, the older a woman is the more likely she is to have a multiple birth naturally. It is theorized that this is due to the higher level of 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. To put this into perspective, in the US in 2003 there were over 136,000 multiple human births. 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 much rarer still. 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 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. Such as in the case of the birth of sextuplets to Kate and Jon Gosselin widely known for their show Jon & Kate plus 8. Better nutrition also increases the likelihood of multiple births.
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.
Another procedure that the medical world is using today is known as selective reduction, i.e. the termination of one or more, but not all, of the fetuses. This is often done in pregnancies with multiple gestations to increase the likelihiood that 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. Many 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.
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.
Mayan culture sees twins as a blessing, and were fascinated by the idea of two bodies looking alike. The Mayans used to believe that twins were one soul that had fragmented.
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.