TRENTON, N.J. — Women looking for a simple way to avoid their menstrual period could soon have access the first birth control pill designed to let women suppress monthly bleeding indefinitely.

The U.S. Food and Drug Administration is expect to announce approval Tuesday for Lybrel, a drug from Wyeth which would be the first pill to be taken continuously.

Lybrel, a name meant to evoke "liberty," would be the fourth new oral contraceptive that doesn't follow the standard schedule of 21 daily active pills, followed by seven sugar pills — a design meant to mimic a woman's monthly cycle. Among the others, Yaz and Loestrin 24 shorten monthly periods to three days or less and Seasonique, an updated version of Seasonale, reduces them to four times a year.

Gynecologists say they've been seeing a slow but steady increase in women asking how to limit and even stop monthly bleeding. Surveys have found up to half of women would prefer not to have any periods, most would prefer them less often and a majority of doctors have prescribed contraception to prevent periods.

"I think it's the beginning of it being very common," said Dr. Leslie Miller, a University of Washington-Seattle obstetrician-gynecologist who runs a Web site focused on suppressing periods. "Lybrel says, 'You don't need a period."'

While that can be done easily — sometimes more cheaply — by skipping the sugar pills or replacing birth-control patches or vaginal rings sooner, doctors say the trend is fueled mainly by advertising for the new options. They expect plenty for Lybrel's July launch, although Madison, N.J.-based Wyeth says it will market to doctors first.

Analysts have estimated Lybrel sales could reach $40 million this year and $235 million by 2010. U.S. sales of Seasonique, launched last August, hit $6.1 million in the first quarter of 2007. Predecessor Seasonale, which got cheaper generic competition in September, peaked at about $100 million. Yaz, launched last August, had first-quarter sales of $35.6 million; Loestrin 24, launched in April 2006, hit $34.4 million in the first quarter.

Still, some women raise concerns about whether blocking periods is safe or natural. Baltimore health psychologist Paula S. Derry wrote in an opinion piece in the British Medical Journal two weeks ago that "menstrual suppression itself is unnatural," and that there's not enough data to determine if it is safe long-term.

Sheldon J. Segal, a scientist at the nonprofit research group Population Council, wrote back that a British study found no harm in taking pills with much higher hormone levels than today's products for up to 10 years.

"Nothing has come up to indicate any unexpected side effects," said Segal, who co-authored the book "Is Menstruation Obsolete?"

Most doctors say there's no medical reason women need monthly bleeding and that it triggers health problems from anemia to epilepsy in many women. They note women have been tinkering with nature since the advent of birth control pills and now endure as many as 450 periods, compared with 50 or so in the days when women spent most of their fertile years pregnant or breast-feeding.