Planned Parenthood

How Pregnancy Happens

Planned Parenthood: How Pregnancy Happens

How Pregnancy Happens at a Glance

  • Pregnancy begins when a fertilized egg implants in the uterus.
  • Getting pregnant can happen in different ways.
  • Understanding your fertility pattern can help with avoiding pregnancy as well as getting pregnant.

Understanding how pregnancy happens is important. If you want to avoid pregnancy, it helps to know when a woman is most likely to get pregnant, and when she is not. If you are trying to become pregnant, it is important to know when getting pregnant is most likely and the different ways it can happen.

Whether you are trying to avoid pregnancy, trying to get pregnant, or are just curious, you may have lots of questions. Here are some answers about how pregnancy happens.

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When Does Pregnancy Begin?

In order for a woman to become pregnant, she must release an egg from her ovary — ovulation. Next, the egg and sperm must meet and form a single cell — fertilization. Then pregnancy begins when and if the fertilized egg attaches to a woman’s uterus and begins to grow — implantation.

MENSTRUATION
During the first two weeks of a woman’s menstrual cycle she has her period. This usually lasts 3–7 days. After that, hormones make eggs mature in her ovaries, and the lining of her uterus thickens.

OVULATION
Ovulation happens about two weeks before a woman’s next period would take place. The egg enters a fallopian tube and starts moving toward the uterus.

FERTILIZATION
After vaginal intercourse or alternative insemination, several hundred sperm travel up through the uterus and into the fallopian tubes. An egg may be in one of the tubes. One sperm may fertilize the egg. The millions of other sperm seep out of the vagina or are absorbed by the woman’s body.

The joining of egg and sperm is called fertilization. It is most likely to occur from sexual intercourse that happens during the five days before the egg is released or on the day of ovulation.

BOY OR GIRL?
Millions of sperm are released when a man ejaculates. About half of them have a gene that could produce a boy. The other half have one that could produce a girl.

IMPLANTATION
The fertilized egg moves down the fallopian tube and divides into more and more cells, forming a ball. The ball of cells reaches the uterus about 3–4 days after fertilization.

The ball floats in the uterus for another 2–3 days.

Pregnancy begins if the ball of cells attaches to the lining of the uterus. This is called implantation. It usually starts about six days after fertilization and takes about 3–4 days to be complete. The embryo will develop from cells on the inside of the ball. The placenta will develop from the cells on the outside of the ball. 

Up to half of all fertilized eggs never implant. They pass out of women’s bodies during menstruation.

TWINS?
It is possible for the developing ball of cells to split up until about the end of week four. If it splits into two, for example, identical twins can develop. It is also possible for two eggs to be released at ovulation. Fraternal — not identical — twins can develop if both eggs get fertilized by sperm and implant in the uterus.

GESTATIONAL AGE
Pregnancy is measured using “gestational age.” Gestational age starts on the first day of a woman’s last menstrual period (LMP).Gestational age can be confusing. Most people think of pregnancy as lasting nine months. And it’s true that a woman is pregnant for about nine months. But because pregnancy is measured from a woman’s last menstrual period — about 3–4 weeks before she is actually pregnant — a full-term pregnancy usually totals about 40 weeks LMP — roughly 10 months.Many women do not remember the exact date of their last menstrual period — that’s OK. The surest way to tell gestational age early in pregnancy is with ultrasound.

How Do Women Get Pregnant?

Women can get pregnant in a variety of ways.

One common way is through unprotected vaginal intercourse, during which a man ejaculates sperm into the vagina. The sperm can then travel up through the cervix, into the uterus, and, if an egg is present, the sperm may fertilize it.

It is also possible — but less likely — for women to become pregnant through any kind of sex play in which semen, or ejaculate, comes in contact with the vulva. The sperm can travel through the moisture on the vulva into the vagina.

Another way for a woman to get pregnant is through alternative insemination. During alternative insemination, sperm are inserted into a woman’s vagina or uterus using a syringe or other device. The sperm may come from a woman’s male partner, or from a donor.  When a sperm donor is used, the procedure is sometimes called donor insemination.  Alternative insemination is an option for single women, women who have women partners, or for couples who are dealing with infertility.

A woman who is dealing with infertility may also get pregnant through other fertility treatments like in vitro fertilization. To do this, a woman’s eggs are taken out of her body and combined with sperm. Then, one or more of the fertilized eggs are implanted back into the uterus.

When Is Getting Pregnant Most Likely?

A woman's fertile days depend on the life span of the egg and the sperm. Her egg only lives for about a day after ovulation. Sperm can live inside her body for about six days. The egg and sperm are most likely to join when intercourse happens during the five days before the egg is released or on the day of ovulation. It is also possible for a day or two after ovulation — even though it's less likely to happen then.

How Can I Avoid Pregnancy?

Using birth control consistently and correctly is the best way a sexually active woman can avoid pregnancy.  There are many safe, effective birth control options.

How Can I Plan a Pregnancy?

Knowing when your fertile days will happen can help you plan your pregnancy. The key is to figure out when you will ovulate. This will let you figure out the other fertile days that come before you ovulate. Then you can track your fertility pattern — the days of the month when you are fertile and the days of the month when you are not. You must do this carefully. Women don't all have the same fertility pattern. And some women have different patterns from one month to the next.

One way is to chart your menstrual cycle. The average menstrual cycle is 28 days long, but many women have cycles that are shorter or longer, and some women have irregular cycles. If you have an irregular cycle, it can be hard to predict ovulation.

You can also determine your fertile days by tracking your temperature, charting the consistency of your cervical mucus, using beads to keep track of your cycle, or using a combination of these methods to predict ovulation.

Ovulation Predictors

Test kits that attempt to predict ovulation are available for home use. They may be useful for planning pregnancies. But they are not reliable for preventing pregnancy. 


Learning about your fertility pattern is just one step in choosing to become pregnant. There are many other issues to think about when considering pregnancy — including your emotional and financial readiness, and your health. One way to have the best pregnancy you can have is by planning for it ahead of time.

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How Pregnancy Happens