Washington ? The percentage of babies born prematurely in the United States reached a two-decade high last year, driven by an increase in twins and triplets. The government also found a rise in prenatal care and a drop in smoking during pregnancy.

Births to teenagers fell for the 10th year in a row, with abortion on the decline, too. At the same time, births to women in their 30s and 40s continued a steady climb, according to an annual review of birth statistics that was released Wednesday.

Overall, 4,025,933 babies were born in the United States in 2001, a small drop from 2000.

There were more boys than girls, though only by a tiny margin. There were more births on Tuesdays than any other day; August was the busiest month.

The percentage of Cesarean sections continued to climb, reaching nearly one-quarter of all births, the highest rate since data became available more than a decade ago. Health officials have hoped to reduce the C-section rate to 15 percent, contending that vaginal births carry a lower risk of medical complications for mothers.

Also disturbing: the rise in premature babies, defined as less than 37 full weeks of gestation. The percentage of babies born that early rose to nearly 12 percent, the highest level since officials began tracking this category 20 years ago.