How pregnancy happens

What happens

1. Sperm enter the vagina. Each ejaculation releases tens of millions of sperm.¹ They swim through the cervix and the uterus into the fallopian tubes.
2. An egg is released. At ovulation, an ovary releases an egg, and the fringed end of a fallopian tube sweeps it inside.²
3. Fertilization. If a sperm meets the egg, usually in the fallopian tube, they join.²
4. Implantation. The fertilized egg travels to the uterus over the next days and attaches to its lining. It then makes hCG, the hormone that keeps progesterone high so no period starts, and the one pregnancy tests detect.³

The fertile window

Two numbers decide when pregnancy can happen:

• Sperm can live up to five days inside the body.¹
• An egg lives about 12 to 24 hours after it's released.⁴

So sex can lead to pregnancy from about five days before ovulation until a day after it. That stretch is the fertile window.⁵ When ovulation happens varies from person to person and cycle to cycle, so the window moves too.⁵

Myth: You can't get pregnant during your period.

This is one of the most common things people are told, and it's partly true: a period is usually the lowest-risk time. But it's still possible.⁶ Sperm can live up to five days, so in a short cycle, or if ovulation comes early, sperm from sex at the end of a period can still be there when an egg is released. And some bleeding isn't a period at all.⁶

Myth: A few days after a period is a "safe" time.

It's an easy rule to pass on, but it doesn't hold. Ovulation can come soon after a period, especially in shorter cycles, and sperm can wait up to five days for it.^5,6 Reliable birth control works much better than timing.⁶

Tracking to avoid pregnancy

Some people track their cycle to avoid sex, or use a barrier, on fertile days. These fertility awareness methods vary a lot in how well they work: between 2 and 23 out of 100 people get pregnant in a year of typical use, depending on the method.⁷ The next lesson compares every method.

How it compares in boys and men

Sperm are made throughout life once puberty starts,¹ while all of a person's eggs are there before birth, as the anatomy lessons explain. Fertility for men changes with age too, which the fertility section covers.