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How does pregnancy happen?

In order for pregnancy to happen, sperm needs to meet up with an egg. Pregnancy officially starts when a fertilized egg implants in the lining of the uterus.

How do people get pregnant?

Pregnancy is actually a pretty complicated process that has several steps. It all starts with sperm cells and an egg.

Sperm are microscopic cells that are made in testicles. Sperm mixes with other fluids to make semen (cum), which comes out of the penis during ejaculation. Millions and millions of sperm come out every time you ejaculate — but it only takes 1 sperm cell to meet with an egg for pregnancy to happen.

Eggs live in ovaries, and the hormones that control your menstrual cycle cause a few eggs to mature every month. When your egg is mature, it means it’s ready to be fertilized by a sperm cell. These hormones also make the lining of your uterus thick and spongy, which gets your body ready for pregnancy.

About halfway through your menstrual cycle, one mature egg leaves the ovary — called ovulation — and travels through the fallopian tube towards your uterus.

The egg hangs out for about 12-24 hours, slowly moving through the fallopian tube, to see if any sperm are around.

If semen gets in the vagina, the sperm cells can swim up through the cervix and uterus and into the fallopian tubes, looking for an egg. They have up to 6 days to find an egg before they die.

When a sperm cell joins with an egg, it’s called fertilization. Fertilization doesn’t happen right away. Since sperm can hang out in your uterus and fallopian tube for up to 6 days after sex, there’s up to 6 days between sex and fertilization.

If a sperm cell does join up with your egg, the fertilized egg moves down the fallopian tube toward the uterus. It begins to divide into more and more cells, forming a ball as it grows. The ball of cells (called a blastocyst) gets to the uterus about 3–4 days after fertilization.

The ball of cells floats in the uterus for another 2–3 days. If the ball of cells attaches to the lining of your uterus, it’s called implantation — when pregnancy officially begins. 

Implantation usually starts about 6 days after fertilization, and takes about 3-4 days to complete. The embryo develops from cells on the inside of the ball. The placenta develops from the cells on the outside of the ball.

When a fertilized egg implants in the uterus, it releases pregnancy hormones that prevent the lining of your uterus from shedding — that’s why people don’t get periods when they’re pregnant. If your egg doesn’t meet up with sperm, or a fertilized egg doesn’t implant in your uterus, the thick lining of your uterus isn’t needed and it leaves your body during your period. Up to half of all fertilized eggs naturally don’t implant in the uterus — they pass out of your body during your period.

What are early pregnancy symptoms?

Many people notice symptoms early in their pregnancy, but others may not have any symptoms at all.

Common signs and symptoms of pregnancy can include:

  • Missed period

  • Swollen or tender breasts

  • Nausea and/or vomiting

  • Feeling tired

  • Bloating

  • Constipation

  • Peeing more often than usual

Some early pregnancy symptoms can sometimes feel like other common conditions (like PMS). So the only way to know for sure if you’re pregnant is to take a pregnancy test. You can either take a home pregnancy test (the kind you buy at the drug or grocery store), or get a pregnancy test at your doctor’s office or local Planned Parenthood Health Center.

How do people get pregnant with twins?

There are 2 ways that twins can happen. Identical twins are made when 1 already-fertilized egg splits into 2 separate embryos. Because identical twins come from the same sperm and egg, they have the same genetic material (DNA) and look exactly alike.

Non-identical twins (also called “fraternal” twins), are made when two separate eggs are fertilized by two separate sperm, and both fertilized eggs implant in the uterus. This can happen if your ovaries release more than one egg, or during certain kinds of fertility treatments. Non-identical twins have completely different genetic material (DNA), and usually don’t look alike. They’re the most common type of twin.

What is gestational age?

The term “gestational age” basically means how far along into a pregnancy you are. Gestational age is counted by starting with the first day of your last menstrual period (called LMP).

Gestational age can be kind of confusing, since it measures pregnancy from your last period — about 3-4 weeks BEFORE you’re actually pregnant. Common knowledge about pregnancy says it lasts 9 months, and it’s true that you’re usually pregnant for about 9 months. But the way pregnancy is measured makes it a little longer. A typical full-term pregnancy ranges from 38-42 weeks LMP — around 10 months.

Many people can’t remember the exact date of their last menstrual period — that’s totally okay. Your nurse or doctor can find out the gestational age using an ultrasound.

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