Knowing zilch about your menstrual cycle isn’t just inconvenient at the doctor’s office (or when you realize you need a tampon like right now). Changes to your cycle, or abnormalities—like irregular or heavy bleeding, or severe pain—can all signal issues that benefit from medical evaluation.

If something suddenly changes with your cycle, or your period is late, you may not realize it right away if you’re not keeping track.

Pregnancy happens when sperm enters a vagina, travels through the cervix and womb to the fallopian tube, and fertilizes an egg. It’s important to know when your ovulation date so you can more accurately time intercourse to try to get pregnant. This is when an egg becomes ready and you are at your most fertile.

If you are under 40 and have regular sex without using contraception, there is an 8 in 10 chance that you will get pregnant within 1 year.

Fertility awareness method (FAM) or “natural family planning” and “the rhythm method” is a way to determine the days to obtain from unprotected sex to help prevent pregnancy. As a method of reliable birth control, it can take several months of recording your monthly cycle to figure out your fertile window.

The calendar-based contraceptive method prevents pregnancy by monitoring the fertile periods during the menstrual cycle to determine which days to avoid unprotected sex.