Bleeding that looks exactly like a period but isn't one

Here's a riddle Toni Weschler poses to readers: what is the difference between a woman who ovulates but never gets her period, and a woman who bleeds right on schedule but never ovulates at all?

The first woman is almost certainly pregnant. The second has had what looks, to the naked eye and to an untrained calendar, exactly like a normal period. It isn't one.

Every cycle, rising estrogen builds up the uterine lining in preparation for a possible pregnancy. Normally, an egg is released, progesterone takes over, and if pregnancy doesn't occur, the sudden drop in progesterone about twelve to sixteen days later triggers the lining to shed as true menstruation. But sometimes the estrogen never climbs high enough to trigger ovulation in the first place. When that happens, one of two things follows. Estrogen may build slowly and then simply drop off, producing what Weschler calls estrogen withdrawal bleeding. More often, the lining keeps thickening over an extended stretch of time until it can no longer sustain itself and sheds anyway, in what she calls estrogen breakthrough bleeding. Either way, blood appears, a woman marks it on her calendar as her period, and nothing about the experience announces that no egg was ever released.

This is not a rare glitch. Weschler describes entire seasons of a woman's life when anovulatory cycles are expected and normal: adolescence, the months after coming off hormonal birth control, breastfeeding, and the years approaching menopause. During these stretches, cervical fluid may appear in confusing patches rather than the clean, predictable buildup of an ovulatory cycle, drying up and returning erratically as the body makes repeated, unsuccessful attempts to reach the hormonal threshold needed to release an egg.

The only reliable way to tell the two situations apart from the inside, according to Weschler, is the temperature record. A true ovulatory cycle shows a sustained thermal shift, a rise in waking temperature that holds for the length of the luteal phase. An anovulatory cycle shows no such shift at all, just a flat, unchanging pattern that gives way, eventually, to bleeding that was never really a period.

The next time you get what looks like a period, resist the urge to simply mark it and move on, and instead check whether it was preceded by a sustained rise in your waking temperature held for something like twelve to sixteen days, because that rise is what actually confirms an egg was released. If there's no such rise, what you're looking at is anovulatory bleeding, not a true period, even though it can look identical on the outside. Expect this to happen more often during certain stretches of life, adolescence, the months after stopping hormonal birth control, breastfeeding, or the years approaching menopause, and don't panic when it does. But if your cycles are consistently longer than about thirty-five days, or the bleeding itself seems unpredictable in amount or timing, treat that as worth a conversation with a doctor, backed by your own charts.