The overall incidence of multiple gestations in the T United States is almost 3%, but these pregnanciesaccount for a disproportionate share of perinatal morbidity and mortality. The natural rate of twinning is approximately 1 in 90, and is slightly higher in blacks than in whites. The rate is rising as a result of an increase in mater-nal age and the more frequent use of assisted reproductive tech-nologies (ART) and ovulation-induction agents. Since 1980,there has been a 65% increase in the frequency of twins, and a 500% increase in triplet and high-order births. It is estimated that 43% of triplet and high-order gestations result from ART procedures and 38% from ovulation induction; spontaneous conception accounts for the re-mainder. Although the exact mechanism is not known, monozygotic twinning is also higher in pregnancies con-ceived using ART.
Twin gestations can be characterized as dizygotic (fra-ternal) or monozygotic (identical). Dizygotic twins occur when two separate ova are fertilized by two separate sperm. Monozygotic twins result from the division of the fertil-ized ovum after conception. There is a marked difference in the incidence of twinning in various populations, almost exclusively the result of the incidence of dizygotic twinning. The incidence of monozygotic twinning is fairly constant around the world, at approximately 1 in 250 pregnancies. Increasing maternal age and increasing parity are independent risk factors for dizygotic twinning, and rates are higher among mothers of families with twins.