WASHINGTON - The number of babies arriving in twins, triplets and more has increased in the last two decades, as fertility drugs soar in popularity and women wait longer to have children.

The increase is particularly striking among older women: More twins were born to women in their late 40s in 1997 than in all of the 1980s, according to a report released yesterday by the National Center for Health Statistics.

"Imagine being 48 years old and having quadruplets," said John Kiely, a professor of pediatrics at Children's Hospital Medical Center of Cincinnati. "Up until recently, women that old couldn't even get pregnant."

Overall, the increase in multiple births between 1980 and 1997 is unprecedented, researchers say.

The number of infants born in twins jumped 52 percent to more than 104,000 babies in 1997. The number born in larger groups quadrupled to more than 6,700 births.

The steep climb is attributed largely to older women, who are more likely to have a naturally occurring multiple birth to begin with. They're also likely to encounter trouble getting pregnant and therefore more likely to try fertility drugs.

Twenty years ago, women didn't wait so long to start families. "Now they're in the work force and they want to develop a career before giving birth," said Dr. Louis Keith, president of the Center for the Study of Multiple Births at Northwestern University Medical School.

Also, the science has kept up with the social trends.

In the late 1970s, doctors began using hormonal drugs, which prompt women to produce several eggs at a time. Their use skyrocketed in the late 1980s, as techniques to help fertilize the eggs - both inside and outside the womb - grew in popularity.

Researchers estimate that about one-third of the increase is due to older women having babies, with two-thirds attributable to fertility treatments.

Despite warm and fuzzy publicity surrounding large births, the trend is not all good news.

Babies born in groups - particularly groups of three or more - are more likely to die as infants or to be dangerously small, which can lead to vision and hearing impairments, mental retardation and developmental delays.

Twins are almost five times as likely to die in their first year as single births, and babies from triplets, quadruplets and larger groups are more than 10 times as likely to die as infants.

Twins are about nine times as likely to be under 3.3 pounds, or "very low birth weight," than infants born alone. The risk is about the same for being "low birth weight," or under 5.5 pounds.

For babies in larger groups, chances are even greater. More than one in three of these babies is under 3.3 pounds, compared to just 1 percent of single births. And 93 percent of them weigh less than 5.5 pounds, compared to 6 percent of single births.

The increase is likely to continue unless fertility specialists change aggressive treatments, experts say.

"Some of these women have been trying for years to get pregnant and can't, and it's a great joy to them," said Kiely. But, he suggested, there should be a "national debate about whether fertility drugs and fertility treatments are being overused."

It's not just the babies at risk. Mothers also face significantly greater risk of developing anemia, high blood pressure or convulsions during pregnancy, according to research Keith is presenting at a conference this month.

The report, based on data from birth certificates, also found that women in certain states, generally in the Northeast, were more likely to have multiple births.

The triplet-and-up birth rates in New Jersey and Nebraska were twice the national rate. And the twin birth rates in Massachusetts and Connecticut were 25 percent higher than average. Women in the Southwest generally had lower rates.

Researchers said the variation comes because women in some states tend to be older. They also noted that fertility clinics, while found across the country, are concentrated in the Northeast.