(Reuters) - Pregnancy rates for U.S. women in their early 20s fell nearly 18 percent from 1990 to 2008 and their abortion rate dropped by 32 percent, as those women delayed the decision to have a baby and used more effective birth control, said a government report released on Wednesday.

The findings for women in their prime child bearing years mirror similar studies showing declines in pregnancies and abortions among teenagers.

The report from the National Center for Health Statistics stated that in 2008, the pregnancy rate for the 20 to 24 age group was 163 per 1,000 women. By comparison, in 1990 that demographic had a pregnancy rate of 198.5 per 1,000, which was nearly 18 percent higher than in 2008.

Pregnancy rates for women between the ages of 25 and 29 fell a more modest 6 percent during the same time period, to 167.9 per 1,000, according to statistics in the report.

The abortion rate also declined among women in their early 20s, to 38.4 per 1,000 women in 2008 from 56.7 per 1,000 in 1990, the report said. That represented a drop of 32 percent.

Again, the drop was more modest for women in their late 20s, as their abortion rate fell to 28.6 per 1,000 in 2008, from 33.9 per 1,000 in 1990, the report said.

A report by the Guttmacher Institute released in February, based on government statistics, showed the teen abortion rate was down 59 percent in 2008 compared with 1988, and that in 2008 the teen pregnancy rate had fallen 42 percent compared to 1990.

The new report extends some of those trends to women who are beyond their teenage years.

"It's not just the teens. Abortion rates are down across the board," said Stephanie Ventura, an author of the National Center for Health Statistics report, which is titled "Estimated Pregnancy Rates and Rates of Pregnancy Outcomes for the United States, 1990-2008."

While the pregnancy rates are down for teens and women in their 20s, they are up for women in their 30s and 40s, the report found. That is consistent with previous research.

Women between 40 and 44 had a dramatic increase in pregnancy rates of nearly 65 percent from 1990 to 2008, the report said. There were 18.8 pregnancies per 1,000 women in that age group in 2008, compared with 11.4 per 1,000 in 1990.

Women in their 20s are "postponing pregnancy," Ventura said.

Another reason for the decrease in pregnancies among younger women is more effective birth control methods, including the combined use of condoms and other methods such as contraceptive patches that release hormones, she said.

"If the pregnancy rates are down, including both births and abortion rates, that would show more efforts to prevent unwanted pregnancies," Ventura said.

The report said that overall for all age groups in 2008, 65 percent of pregnancies ended in a live birth in 2008, 18 percent in an abortion and 17 percent in fetal loss. In 1990, 61 percent of pregnancies ended in a live birth and 24 percent were aborted, with 15 percent resulting in fetal loss.