WASHINGTON (AP) - Those instant families of five, six or more children born at once get lots of publicity, but a government study shows that multiple births of three or more have actually begun to decline.

The study also found that programs to reduce teen-age births seem to be getting their message across, with the birth rate for mothers ages 15 to 19 now at a record low. At the same time, birth rates for women in their thirties are at their highest level in more than three decades.

The number of triplets-plus births had been growing steadily, doubling in the 1990s, before reversing course in 1999, the report Tuesday from the National Center for Health Statistics said.

The report showed 7,321 triplet-plus births in 1999, down from 7,625 in 1998.

"One possible explanation is that the fertility-enhancing therapies that are the primary contributors to the increase in these births have been refined and improved upon so they are less likely to result in multiple births," said Joyce Martin, an epidemiologist and co-author of the study.

The Society for Reproductive Medicine issued new guidelines nearly two years ago calling for the transfer of fewer embryos, and that may be reflected in this decline, said society spokesman Sean Tipton.

"Over the last couple of years treatments have gotten better, which has enabled physicians to culture embryos longer before transferring. Also, they are better at evaluating which embryo is more likely to implant and go to term, so they are able to transfer fewer embryos," he explained.

As for teen pregnancies, the report said the birth rate for mothers 15-19 declined 3 percent to 49.6 births per 1,000 women in 1999. This rate has fallen 20 percent since 1991.

"Teen sexual activity has leveled off compared to the increases we saw in the previous couple of decades," commented demographer Stephanie Ventura.

"Also, teens who are sexually active are more likely to be using contraception," she said. "Every state has a teen pregnancy prevention program. ... The message has been transmitted."

The previous low teen birth rate, recorded in 1986, was 50.2 births per 1,000 teen-age women. The records go back to 1940.

The report also found an increase in Caesarean births.

After falling steadily from 1989 to 1996, the rate increased again in 1999, up 4 percent from 1998. Twenty-two percent of all births were Caesarean deliveries in 1999. The rate of vaginal birth after previous Caesarean delivery declined 11 percent between 1998 and 1999.

The reasons for the increase in Caesarean births are not clear, said statistician Sally Curtin of the center. But she said it is widespread among women of all ages and across most of the country, which suggests a change in medical practice.

Dr. Frederic Frigoletto of the American College of Obstetricians and Gynecologists said that Caesareans had declined in the 1980s when there was an effort to encourage women to attempt a vaginal birth even if they had previously had a Caesarean.

In the mid-1990s, he explained, "There was a growing concern based on experience, the results from experience, that there were some safety considerations to this highly promoted practice.

"Many of us think that the present turn up (in Caesareans) has been the result of the growing awareness of this risk on the part of the mother and the doctors," Frigoletto said in a telephone interview from his Boston office.

Other findings in the report included:

- The median age for first-time mothers increased to 24.5 years, continuing a slow but steady rise since 1972.

- The birth rate for unmarried women increased slightly to 44.4 births per 1,000 unmarried women aged 15-44.

- Total births in the U.S. increased to 3,959,417 in 1999, up less than 1 percent from 1998.

- More pregnant women are getting timely prenatal care (83.2 percent in 1999 compared to 82.8 percent in 1998) while the percentage of women with late or no care has fallen to 3.8 percent, down from 6.1 in 1990.

- Nearly 12 percent of all births were delivered pre-term (less than 37 completed weeks of gestation). The percentage of babies born pre-term has risen 11 percent since 1990.

Copyright 2001 The Associated Press. All rights reserved.