John and Mary Smith are a typical American couple. Mary is in her first pregnancy. She goes to the hospital for delivery. She comes home with twins.
The Smiths are members of the one percent group of American parents who have twins. Multiple births are more common among mammals, but are unusual for humans. To fully understand the cause of this phenomenon, one must first consider the two types of twins, identical and fraternal.
Twins may be identical, twins are those which look exactly alike. Dizygotic, or fraternal, twins look nothing alike and will have individual characteristics. Identical twins occur when half of the baby forming into an embryo falls away from the other half. Each half will then form into a separate child. If a split occurs late in differentiation, the twins will be mirror images; that is, one will be left-handed, the other will be right-handed. If the split occurs early in cell differentiation, the twins will not be mirror images. Differentiation is the stage when cells are dividing and changing to create the different kinds of cells which form the human body.
Siamese twins occur when the split is not complete. The causal objects to be joined in some fashion. The name “Siamese twins comes from two joined brothers who were made famous in 1874 named Chang and Eng.
Fraternal twins are formed by the fertilization of two eggs which develop into two different children with different characteristics. It is even possible for fraternal twins to have different fathers.
Multiple births can be induced through the taking of hormones which encourage multiple ovulation. Birth control pills do just the opposite, shutting off the ovulation altogether. The book, “Human Genetics”, states that a change from warm weather to cold weather can cause multiple births. The book indicated that Texas doctors have noted an increase in the birth and death rates of humans over both temperatures.
Complications can arise, though. Twins are sometimes premature in birth. This can retardation in intellectual development, smaller weight both time birth and health. This is sometimes coupled with the normal health risk involved in twin pregnancies in which the small nutritional needs of two infants can retard intellectual development in the last ten years.