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 Wednesday.

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.

Throughout the 1960s, the earliest birth control pills to win Food and Drug Administration approval failed just once per 100 woman-years of use. That is, for every 100 women taking the pills for a year, there was fewer than one pregnancy on average among them.

Today, newer pills contain less estrogen and progestin. Those pills can reduce the risk of blood clots, stroke and other sometimes deadly 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 woman-years of use, 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 Dr. Charles Lockwood of Yale University, acting chairman 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.

The FDA scheduled meetings both Tuesday and Wednesday with its outside experts to gather advice on future guidelines that drug makers could follow in seeking approval for new hormonal contraceptives. It 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 aren’t being tested on women who reflect the broader population. The women in clinical trials are younger, skinnier and healthier than are U.S. women on average, panelists and other experts said.