Twins Health Article

Twins

Two children born at the same birth.

Identical, or onozygotic, twins are of the same sex and are genetically and physically similar because they both come from one ovum, which, after fertilization, divides in two and develops into two separate individuals. Fraternal, or dizygotic, twins occur when the mother produces two eggs in one monthly cycle and both eggs are fertilized. The conceptions may take place on two separate occasions and could involve different fathers. Fraternal twins, who are no more genetically alike than ordinary siblings, may be of the same or different sex and may bear some similarity of appearance. Fraternal twinning appears to be passed on by the female members of a family: if the mother is a fraternal twin herself, has fraternal twin siblings or fraternal twin relatives on her side of the family, or has already given birth to fraternal twins (one in twenty chance), her chances of giving birth to fraternal twins are approximaly five times as great.

Between 1980 and 1994, the number of twin births in the United States increased by 42%, from 68,339 to 97,064. The twin birth rate (i.e, the number of twin births to total live births) increased 30%, from 18.9 to 24.6 per 1,000 live births. According to data gathered by the Centers for Disease Control, there is considerable variation among the states in number and rate of twin births. In 1994, for example, the twin birth rate ranged from 19.8 in Idaho and New Mexico to 27.7 in Connecticut and Massachusetts. One factor that may influence multiple births in a state is whether the state provides insurance coverage for procedures such as in vitro fertilization (IVF) and other treatments to improve fertility. During 1992-1994, 11 states mandated such benefits.

The CDC is also studying whether maternal age has any correlation with the rate of twins births. The data seems to suggest that mothers in states with rates of twin births higher than the overall rate for the United States are older on average, and mothers in states with rates of twin births lower than the overall rate for the United States are younger.

Ethnicity is another factor that may correlate to the twin birth rate. For 1994, the twin birth rate among non-Hispanic white mothers was 24.3; among non-Hispanic black mothers, 28.3; and among Hispanic mothers, 18.6.

Twin birth rate may also affect other statistics related to infant health. The accompanying table illustrates some key statistics on twin births.

While the rate of identical twin births is stable for all ages of childbearing women, the chance of any mother bearing fraternal twins increases from the age of 15 to 39 and then drops after age 40. For women of all ages, the more children they have had previously, the more likely they are to bear twins. Since the 1960s, fertility drugs have also been linked to the chances of producing twins. The majority of research indicates that fathers' genes have little effect on the chances of producing twins.

There are four types of monozygotic twins, determined by the manner in which the fertilized egg, or zygote, divides and the stage at which this occurs. Two independent embryonic structures may be produced immediately at division, or the zygote may form two inner cell masses, with each developing into an embryo. A late or incomplete division may produce conjoined, or Siamese twins. As the zygote develops, it is encased in membranes, the inner of which is called the amnion, and the outer one the chorion. Among monozygotic twins, either or both of these membranes may be either separate or shared, as may the placenta. Together, the arrangement of these membranes and the placenta occurs in four possible permutations. Among dizygotic twins, each one has separate amnion and chorion membranes, although the placenta may be shared. Ascertaining zygosity, or the genetic make up of twins, can be done by analyzing the placenta(s) to determine if it is a single placenta with a single membrane or a double placenta, which account for one-third of identical twins and all fraternal twins. In the case of same-sex twins with two placentas, a DNA or blood test can determine whether they share the same genes or blood groups.

The scientific study of twins, pioneered by Francis Galton in 1876, is one effective means of determining genetic influences on human behavior. The most widely used method of comparison is comparing monozygotic and dizygotic twins for concordance and discordance of traits. Concordant traits are those possessed by either both or neither of a pair of twins; discordant traits are possessed by only one of the pair. Monozygotic twins who are discordant for a particular trait can be compared with each other with reference to other traits. This type of study has provided valuable information on the causes of schizophrenia.

Another common type of twin research compares monozygotic twins reared together with those reared apart, providing valuable information about the role of environment in determining behavior. In general, monozygotic twins reared apart are found to bear more similarities to each other than to their respective adoptive parents or siblings. This finding demonstrates the interaction between the effects of environment and genetic predispositions on an individual's psychological development.