Multiple pregnancy is a pregnancy where more than one fetus develops simultaneously in the womb.
Twins happen naturally about one in every 100 births. There are two types of twinning—identical and fraternal. Identical twins represent the splitting of a single fertilized zygote (union of two gametes or male/female sex cells that produce a developing fetus) into two separate individuals. They usually, but not always, have identical genes. When they do not separate completely, the result is Siamese (or conjoined) twins. Fraternal twins are three times as common as identical twins. They occur when two eggs are fertilized by separate sperm. Each has a different selection of its parents' genes. The natural incidence of multiple pregnancy has been upset by advances in fertility treatments, resulting in higher rates of multiple births in the United States. All these children are fraternal; they each arose from a separate egg and a separate sperm. Cloning produces identical twins.
The human female is designed to release one egg every menstrual cycle. A hormone called progesterone, released by the first egg to be produced, prevents any other egg from maturing during that cycle. When this control fails, fertilization of more than one egg is possible. Fertility drugs inhibit these controls, allowing multiple pregnancy to occur. Multiple pregnancy is more difficult and poses more health risks than single pregnancy. Premature birth is greater with each additional fetus.
The problem with multiple births is that there is only so much room in even the most accommodating womb (uterus). Babies need to reach a certain size and gestational age before they can survive outside the uterus. Prematurity is the constant threat of multiple pregnancies. Twins have five times the death rate of single births. Triplets and higher die even more often. The principle threat of prematurity is that the lungs are not fully developed. A disease called hyaline membrane disease afflicts premature infants. Their lungs do not stay open after their first breath because they lack a chemical called surfactant. Survival of premature infants was greatly improved when