WASHINGTON (AP) -- A new birth-control pill named Seasonale promises to reduce the frequency of women's periods, from every month to four times a year.

The contraceptive pills, approved by the Food and Drug Administration on Friday, aren't a new chemical. They contain the same combination of low-dose estrogen and progestin found in many oral contraceptives.

Nor is the idea of menstrual suppression new. For decades, many doctors have told women how to skip a period by continually taking the active birth-control pills in each month's supply and ignoring the week of dummy pills in each packet.

Seasonale promises to make the option a little more convenient, with packaging that gives women 12 straight weeks of active pills and then a week of dummy pills for their period. And the FDA's approval means menstrual suppression could become more common as Seasonale's advertising alerts women to the option.

Seasonale isn't perfect, the FDA cautioned.

While women have fewer scheduled periods, studies show Seasonale users have about twice the risk of unexpected "breakthrough" bleeding between periods as woman taking conventional monthly cycle pills, especially in the first few cycles of use.

Also, 7.7 percent of Seasonale users dropped out of studies citing unacceptable bleeding, compared with 1.8 percent of women taking conventional monthly pills.

While the risk of breakthrough bleeding declines with each cycle of Seasonale use, FDA said some women had it so often that their total days of bleeding over a year were no less than with regular pills.

"Each woman will respond to this product somewhat differently," said FDA's Dr. Scott Monroe. "Some will find they respond entirely as the product was designed to function, and others will have increased intermenstrual bleeding to the extent that they choose not to continue with the product."

The manufacturer, Barr Laboratories, plans to have prescription-only Seasonale in pharmacies by November. Barr wouldn't reveal Seasonale's price but said it will be comparable to other brand-name oral contraceptives, which sell for roughly $1 a pill. Generic versions can cost half that amount.

Having fewer periods "was absolutely fabulous," said Shannon Zaichenko, 27, of Chesapeake, Va., who spent three years in a study of Seasonale.

"It's the convenience, not being bogged down, not having to plan vacations or just lifestyle around seven days of bleeding," she said. A frequent traveler, Zaichenko recalled that before Seasonale her periods "always seemed to happen when I was on a plane for 20 hours."

Seasonale also may be attractive to women who experience severe cramping, heavy bleeding and other menstrual-related symptoms, a number Barr estimates at 2.5 million in the United States.

But the National Women's Health Network says some Seasonale proponents falsely imply that limiting menstruation is generally healthier, a message the consumer group calls particularly unwise for young girls.

"We already have a lot of shame and stigma in this society about menstruation," said the network's Cynthia Pearson, who has asked Barr to ensure that Seasonale ads don't convey that impression.

During the menstrual cycle, fluctuations in estrogen signal the uterine lining, or endometrium, to thicken in preparation for nourishing an embryo. If pregnancy doesn't occur, the excess lining is sloughed off, accompanied by bleeding.

The big safety question is whether four periods a year are enough to allow the uterus to shed any tissue that builds up.

A study by Eastern Virginia Medical School, which developed the three-month pill regimen, shows they are. It tracked 682 women taking either Seasonale or regular monthly pills for a year. Seasonale proved equally effective at preventing pregnancy. Side effects, too, were similar, with the exception of breakthrough bleeding.

"There is no concern in delaying the period for three months," says Dr. Vanessa Cullins of Planned Parenthood.

In fact, she noted, women today have far more periods in their lifetime than their ancestors before the era of contraception, when women spent much more time either pregnant or breast-feeding, both of which block menstruation.

Not every woman will be comfortable with Seasonale, Cullins said -- monthly bleeding gives some contraceptive users added reassurance that they're not pregnant.

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