Like HowStuffWorks on Facebook!

5 Strange Pregnancies

I Didn't Know I Was Pregnant

When you're trying to get pregnant, it may seem that everyone around you is suddenly in her third trimester while you wait, month to month, for your happy news. Some women, however, are pregnant but unaware of it -- and we don't mean in the first few weeks of the first trimester.

There are a few reasons women might not be clued in to their pregnancies. Take home pregnancy tests, for example. Home pregnancy tests aren't perfect, especially if used too early in the pregnancy when there hasn't been enough time for the pregnancy hormone levels to build. Monthly periods can be misleading, too. Bleeding can be a common occurrence for some women during their pregnancies. And if your periods aren't regular (because of stress, intense exercise and strict diet, or conditions such as polycystic ovarian syndrome), this breakthrough bleeding can make you think you're having monthly cycles.

Constipation, gas, mood swings -- if you have no idea that you might be pregnant, all of these can easily be attributed to something else. What about the big one -- weight gain? If a woman is not trying to get pregnant or thinks she's infertile, the cause of pregnancy weight gain might be attributed to recent slacking off in diet and exercise, especially if the woman is heavy naturally or is carrying a small baby.