The frequency of multiple births in the United States has been steadily increasing with advances in reproductive technologies. It is estimated that pregnancies resulting from assisted technologies have a 25-30 percent incidence of twins and a 5 percent incidence of triplets. The frequency of naturally occurring twins is approximately one in 80 births; however the frequency of multiple births in the United States for 2002 was as follows:
twins, one in 32
triplets, one in 583
quadruplets, one in 9,267
quintuplets and up, one in 58,286
Twin birth is by far the most common multiple birth. There are two types of twin pregnancy: fraternal and identical. Fraternal twins develop from two separate ova released at the same time and fertilized by two separate sperm. Fraternal twins are referred to as dizygotic twins, meaning that two unions of two gametes or male/female sex cells occurred to produce two separate embryos.
Characteristically, with fraternal twins, each has its own placenta and amniotic sac. They may be the same or different sex, occur twice as frequently as identical twins, and have a mortality rate of 11.5 percent. Identical twins represent the splitting of a single fertilized zygote (union of two gametes or male/female sex cells to produce a developing embryo) into two separate individuals. Identical twins will have the same DNA, genetic material (genotype), but it may be expressed differently (phenotype). There are three ways identical twins can exist in the uterus: dichorionic-diamniotic twins; monochorionic-diamniotic twins; monochorionic-monoamniotic twins.
In the instance of dichorionic-diamniotic twins, division of the fertilized egg occurs within 72 hours past fertilization, before the inner cell mass has developed. About 30 percent of identical twins have this classification, and each twin has its own chorion, amnion, and placenta. The mortality rate for this type of twinning is 9 percent. With monochorionic-diamniotic twins, division occurs in the range of four to eight days after fertilization, and the inner cell mass divides in two. The placenta has one chorion and two amnions, so each twin has its own amniotic sac.
Approximately 68 percent of identical twins are in this classification, and they have a mortality rate of 25 percent. Thirdly, monochorionic-monoamniotic twins are contained in the same amniotic sac. The division of the fertilized egg in this case occurs nine to 13 days past fertilization or near the time of implantation in the uterus.
Since they share an amniotic sac, they have an increased risk of their umbilical cords becoming entangled or knotted. Only 2 percent of identical twins are in this classification, and they have a mortality rate of greater than 50 percent. If a complete separation does not take place during the division process, the result is Siamese (or conjoined) twins.