WASHINGTON (AP) _ The introduction of the pill coincided with sweeping changes in U.S. birth patterns, but any cause-and-effect relationship between the pill and the number of babies born is unclear a quarter-century later.

Nonetheless, as use of birth control pills was soaring in the 1960s, the nation's total birth rate was plummeting.

By 1965, the pill was the most popular method of birth control, with 23.9 percent of married woman choosing this method of contraception.

The same year, 1,000 typical American women could have been expected to have 2,913 children during their lifetimes, according to figures compiled by the National Center for Health Statistics.

By 1973, that 1,000-women estimate had fallen to 1,879 children, and the figure for 1981 - the most recent available - was 1,815.

Pill use, by comparison, peaked in 1973 at 36.1 percent of married women before declining because of concerns about health risks and other side effects. The 1982 edition of the government's National Survey of Family Growth says 19.8 percent of married women used the pill.

The nation's birth rate has remained relatively constant in the last few years, although there has been some increase in the number of babies born. Officials at the U.S. Census Bureau attribute the increase to the large number of women, born during the post-World War II baby boom, entering the childbearing ages of 15-44.

Demographers, however, are cautious about drawing direct parallels between the use of birth control pills and changes in fertility.

Other factors have also been involved, the experts point out.

For example, intrauterine devices have more or less paralleled the use of birth control pills, although in smaller numbers.

Used by only 1.2 percent of married women in 1965, IUD usage peaked at 9.6 percent in 1973 and declined to 7.1 percent by 1982.

Also, changes in fertility rates have occurred in past years, independently from the development of new methods of birth control or legal abortions.

In the Depression years of the 1930s, for example, the birth rates for Americans struggling with economic hardships fell to historic lows which were only equaled in this decade.

A change in the attitude of young people toward childbearing also occurred during the 1960s and 1970s, sociologists note.

The post-World War II baby boom generation was one in which women sought greater equality in education and careers, and often postponed marriage and childbearing.

Government studies found that between 1970 and 1979, the number of women who had their first child after age 30 increased by 66 percent.

The presence of more effective methods of contraception likely contributed to this in some cases, but not all.

And, since abortions became legal, that also has become an increasingly common method of dealing with unwanted pregnancy - rising from 13.2 abortions per 1,000 women aged 15 to 44 in 1972, to 28.8 per 1,000 in 1982.

Married women recorded 216,000 abortions in 1973, a total which increased to 299,000 in 1981. Meanwhile, abortions to single women climbed from 528,000 to 1,279,000.