WASHINGTON - Birth control pills sometimes fail, but setting limits on how often that could happen would put newer, low-dose contraceptives off-limits to women, federal health advisers said yesterday.

The lower-dose pills are less effective at preventing pregnancy than the first oral contraceptives approved beginning in 1960. Yet the newer drugs offer other health benefits or cause fewer side effects. That has split federal health officials on the need to define a pregnancy or failure rate that would be unacceptably high for next-generation pills.

Newer pills contain less estrogen and progestin. Those pills can reduce the risk of blood clots, stroke, and other side effects. But as the hormone content of the pills has dipped, failure rates have climbed.

Over the last decade, the FDA has approved some pills with failure rates that exceed two pregnancies per 100 women taking them for a year, according to agency documents. That is twice the rate considered acceptable in the 1960s.

But allowing the less effective pills on the market can increase the options for women and their doctors, said Yale University's Charles Lockwood, acting chair of FDA's reproductive health drugs advisory committee.

"We don't want an arbitrary number to be ascribed," Lockwood said after the panel declined to define a pregnancy rate that would be considered unacceptable.

While most women take the pill to prevent pregnancy, others rely on hormonal contraceptives to regulate their monthly periods or curb acne.

The FDA is looking at how well studies done prior to approval of new birth control pills reflected their "real-world" use. Typically, that use is less consistent and reliable than it is in clinical studies.

Also, newer versions of the pill are not being tested on women who reflect the broader population. The women in clinical trials are younger, thinner and healthier than are U.S. women on average, panelists and others said, and they don't smoke.

The more that trials can mimic real-world use, the more confident women will be that the pills are safe and effective, said Kirsten Moore, president of the nonprofit Reproductive Health Technologies Project.

"When expectations don't match up to reality," she told panelists, "women are more likely to discontinue their contraceptive use and possibly expose themselves to unwanted pregnancies."

The risk of pregnancy and blood clots both can rise in obese women who take the pill, Amy Allina, program and policy director with the advocacy group National Women's Health Network, told the panel. The pills also appear not to work as well in overweight women, according to the FDA.