Pregnancy is a pretty amazing process — it all starts with sperm and an egg. Sperm are tiny, microscopic cells that are made in testicles. Sperm cells mix with other fluids to make semen, which spurts out of your penis when you ejaculate. Eggs live in ovaries, and the hormones that control your menstrual cycle cause eggs to mature every month. These hormones also make the lining of your uterus thick and spongy, to prepare for a possible pregnancy. Every month, one mature egg leaves your ovary — this is called ovulation. That egg travels through your fallopian tube, which takes about 12-24 hours.
If semen gets into your vagina, sperm can swim up through your cervix, uterus, and fallopian tubes, searching for an egg. Sperm cells can live in your body for up to 6 days waiting for an egg to show up. Millions of sperm come out during each ejaculation — but it only takes 1 sperm to meet with an egg, which can lead to pregnancy.
When a sperm cell joins with an egg, it’s called fertilization. After that happens, the fertilized egg begins to divide into more and more cells and moves towards your uterus. The ball of cells gets to your uterus about 3–4 days after fertilization, where it can float around for another few days. If the ball of cells attaches to the spongy uterine lining, pregnancy officially begins — this is called implantation. It usually takes around 34 days to finish implanting. Up to half of all fertilized eggs naturally don’t implant — they pass out of your body during your period.
When the ball of cells implants into your uterine lining, your body starts making pregnancy hormones. These hormones keep your uterine lining in place so it can nourish the pregnancy — that’s why you don’t get your period when you’re pregnant. But if sperm and egg don’t meet up, or a fertilized egg doesn’t implant into your uterus, the lining isn’t needed, and it flows out of your vagina. That’s your period. So it actually takes a lot of steps for pregnancy to happen, and up to 23 weeks.
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