Why Was My Period Late If I’m Not Pregnant?

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My period was so late that I took a pregnancy test, but it came back negative. The period arrived a few days later. Why does this happen?

Whether you want to get knocked up or not, a missed period can send you on an emotional roller-coaster until the thing finally decides to show up or you take a pregnancy test, whichever happens first. Why is your uterus playing you like that? Often there’s really no reason at all, says Scott Sullivan, MD, FACOG, associate professor of obstetrics and gynecology and director of maternal-fetal medicine at the Medical University of South Carolina in Charleston.

“Usually, it’s just biological variability,” Dr. Sullivan says. “If you live long enough, you’re going to have an irregular period somewhere.” Great. But there is a chance that you were, briefly, pregnant.

It could have been a very early pregnancy where the embryo either failed to implant in the uterine lining or implanted and then failed shortly after (anywhere from 15 to 30 percent of all pregnancies fail, he says). But since fertilization occurred, your body would have started producing the pregnancy hormone hCG — it’s a very low amount, but enough for a pee stick to catch.

After your period comes, hCG falls back down to zero. This is known as a chemical pregnancy and it’s technically a miscarriage, Dr. Sullivan says. Most chemical pregnancies are the result of genetic or chromosomal problems with the embryo. Though if you didn’t take a pregnancy test, you might be none the wiser. (Quick note: If you took a test very early and got a negative result, you could still be pregnant. Sullivan says you should probably retest in 10 to 14 days.)

There are other extremely common things that can make your period late, like stress, lack of sleep, alcohol, smoking, and weight gain or loss. “A period is balance between your brain, your ovaries, and your uterus. All three work as a team, so anything that can disturb that delicate balance can throw things off,” says Dr. Sullivan, who also serves on the ethics committee of the American Congress of Obstetricians and Gynecologists. “It’s not going to throw it off forever, but a few days? Absolutely.”

He adds that menstrual abnormalities are more common the younger you are and the older you are, so this phenomenon more likely to happen on its own in teenagers and women in their 40s. If you’re in your early-to-mid-40s, you might chalk it up to perimenopause, but Dr. Sullivan says it’s a little too early for that.

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