Multiple Births: Twins and Twinning
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In most mammals the litter size is fairly constant and is roughly correlated with, among other features, body size, gestation period, life span, type of uterus, and number of teats. For example, a large mammal with a normal pregnancy of more than 150 days, a life span of more than 20 years, a simple (or unicornuate) uterus, and two breasts rarely bears more than one young (a singleton) at a birth.
The great majority of primates, humans included, fit into this category; multiple births among them are exceptional and occur with decreasing frequency from twins on.
Twinning refers to the process that leads to the production of more than one offspring at one birth. In humans the most frequent type of twinning results in the birth of two babies (twins) at once, although repetition of twinning may lead to triplets, quadruplets, or other multiples of one.
The two major types of twins are identical twins and fraternal twins. Identical twins are two individuals that have developed from a single egg fertilized by a single sperm. This fertilized egg is called a zygote.
At a relatively early stage in its growth, the zygote splits into two separate cell masses which go on to become embryos; these embryos are genetically identical to each other and are always of the same sex. Three-fourths of such embryo pairs share a common placenta.
Since they both developed from a single zygote, such twins are called monozygotic (MZ) twins. A zygote's atypical separation into two independent embryonic structures can occur at any of several growth stages. Its incomplete or late division into two cell masses results in conjoined twins, formerly known as Siamese twins. MZ twins usually show a striking physical resemblance to one another.
It should be noted that even though hereditary characteristics such as eye colour and hair colour and texture are the same in MZ twins, these traits as well as the majority of physical characteristics may be modified during embryonic development. Identical twins may therefore not truly be "identical"; the correspondence between such twins is closer to what would be expected between the right and left sides of a single individual, which vary slightly from one another.
Fraternal twins develop from two separate eggs that have been fertilized by two separate sperm. Such egg pairs stem from multiple ovulations in the same cycle. Each egg develops singly, is fertilized singly, and forms its own individual placenta.
Since fraternal twins develop from different zygotes, they are called dizygotic (DZ) twins. DZ twins may be of the same sex or of different sexes, and they are not genetically identical to one another. Fraternal twins bear the resemblance of ordinary siblings.
The occurrence of DZ twinning varies among racial groups: multiple births are most common among people of African descent, less common among those of European descent, and least common among people of Asian descent. DZ twinning is most common in older mothers (up to about 40 years of age) and tends to recur in families having a history of multiple births, indicating that a hereditary factor may be involved. MZ twinning, on the other hand, occurs randomly in all races and follows no discernible genetic pattern; it tends to occur more frequently in older mothers, however.
In the United States, Canada, and Great Britain about 1 out of every 83.4 births is a twin birth. Reported frequencies throughout the world vary from about 1 in 70 to 1 in 145 births; however, source records from many areas are unreliable.
In North America DZ twins are more frequent than MZ twins, the relative proportions being 71.8 to 28.2; similar proportions hold among other populations of European origin. Excluding the use of fertility drugs, approximate figures for the frequency of multiple births are 1 in 80 births for twins, 1 in 6,400 for triplets, 1 in 512,000 for quadruplets, 1 in 40,960,000 for quintuplets, and so on.
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Nelson Ndalila is a recognized infertility expert and health practitioner from Nottingham. Discover more about current issues related to infertility through his site http://infertilityhospital.blogspot.com.