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In the past two decades, the number of multiple births in the United States has jumped dramatically.

Between 1980 and 2000, twin births have increased 74 percent, and higher order multiples -- triplets or more -- have increased fivefold, says the National Center for Health Statistics.

Today, about 3 percent of babies in this country are born in sets of two, three or more, and about 95 percent of these multiple births are twins.

The increase in multiple gestations is a concern because women who are expecting more than one baby are at greater risk of certain pregnancy complications, including preterm delivery, which is before 37 weeks of pregnancy. Babies who are born preterm are at risk of serious health problems during the newborn period as well as lasting disabilities and death.

Why are multiple gestations increasing?

About one-third of the increase in multiple gestations is due to the fact that more women over age 30, who are more likely to conceive multiples, are having babies. The remainder of the increase is due to the use of fertility-stimulating drugs and assisted reproductive techniques such as in vitro fertilization, where eggs are removed from the mother, fertilized in a laboratory dish and then transferred to the uterus.

Doctors now realize it is crucial to monitor fertility treatments so that women will have fewer but healthier babies. This involves limiting the number of embryos transferred during in vitro fertilization, or halting treatment with fertility drugs during a cycle if ultrasound shows that a large number of eggs could be released. The rate of higher-order multiple births has declined slightly in the past two years.

-- March of Dimes