system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are there so many accidental pregnancies but so many women try so hard to get pregnant, even though they aren't infertile?
gpt
Because a lot of women are having sex, which means statistically improbable things will happen.

A fertile couple who have sex regularly have about an 80% chance per year of conceiving.  That means 1 in 5 won't get pregnant after a year of trying, 1 in 25 after two years, and 1 in 125 after three, even if nothing is wrong.

Similarly, most birth control methods have between 1 and 10% effectiveness, depending on how diligently they are used.  That means we should see about as many women getting pregnant unexpectedly (1 in 20ish) per year as we would see women who are unable to get pregnant after 2 years of trying (1 in 25).