Like HowStuffWorks on Facebook!

10 Reasons You Might Not Know You're Pregnant (Until You're in Labor)

Overconfidence in Contraceptives

Women who don't know they're pregnant until they're in labor are often in shock over their situation. How can this be if they used a contraceptive while having sex?

Although contraceptives such as birth control pills and condoms are highly effective in preventing pregnancy, they aren't 100 percent successful. When contraceptives aren't used correctly, their effectiveness drops significantly. For example, missing a birth control pill or not taking it at the same time every day can increase your chances of getting pregnant if you have unprotected sex. Using expired condoms that are more likely to break can also increase this risk.

You should consult your doctor if you're unsure of whether switching birth control methods compromises your protection against pregnancy. Some women have claimed that switching from oral contraceptives to injected forms left them susceptible to an unexpected pregnancy.

Mistaking pregnancy with a medical condition is possible, too. Read on to learn more.