Your chances of “twinning” are actually not bad — about one in ninety births results in twins. If fraternal twins run in your family, your odds are slightly higher. The number of twins and births of multiples has skyrocketed over the past several decades, and the incidence of higher order multiples — which include triplets, quadruplets, or more — grew more than 400 percent between 1980 and 1998 but has stabilized since 1999. Twins are more common than ever, however, with 128,665 twin births occurring in 2003.

Why the bonus baby boom? The CDC attributes approximately two-thirds of all U.S. higher order multiples to the use of fertility treatments, also known as assisted reproductive technology (ART). In 2003 over half of all infants born using ART were part of a multiples' birth.

National statistics also reveal that more women are waiting until their thirties and forties to have children, and the increasing twin rate perhaps reflects that reality. Women over thirty-five, especially those who had a previous multiples' birth, have an increased chance of having multiples.

Several studies comparing birth outcomes in triplet pregnancies have found that moms over age forty have more favorable fetal growth parameters and better birth weight outcomes than younger triplet moms.

Although birth outcomes in a singleton pregnancy involve more risk as maternal age increases, studies indicate that the opposite seems to hold true for pregnancies of multiples among older moms. This could be due in part to the increased use of ART in older women, and the fact that ART multiples rarely share the same amniotic sac — a risk factor for a number of prenatal complications in multiples' pregnancies.