Sexuality Resource Center for Parents: Tools, Tips, and Tricks for Teaching Children about Human Sexuality

Human Reproduction

At least 40 percent of all females get pregnant before they turn 20 years old, and approximately 70 percent of those pregnancies are unplanned. Among females of all ages, as many as 50 percent of all pregnancies are unplanned. Clearly, many people don’t understand human reproduction.

The Basics

Human reproduction may revolve mostly around a sperm meeting an egg, but it’s not like those two things are floating around in space. Reproduction also involves body fluids and our reproductive organs.

Female Reproductive System

In a typical ejaculation, a male ejaculates anywhere from 300-500 million sperm. Some will seep out of the female’s body, but most will begin the onward journey in search of an egg – from the vagina, through the cervix and uterus, and into the fallopian tubes. Out of that vast number of sperm, only several hundred will reach an egg in one of the fallopian tubes, and one of those sperm can enter the egg – in a process we call fertilization. All the other sperm die – at which point they may seep out of the vagina or be absorbed by the female’s body.

Now you know why the male body produces so many sperm!

The fertilized egg moves down the fallopian tube and divides into more and more cells, forming a ball of cells. The ball reaches the uterus about 3–6 days after fertilization.

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

Pregnancy begins if the ball of cells attaches itself to the lining of the uterus. This is called implantation, and the process takes about 3–4 days to be completed. The embryo will develop into a fetus from cells on the inside of the ball. The placenta will develop from cells on the outside of the ball. The placenta is the organ that will eventually connect the fetus, via the umbilical cord, to the lining of the uterus. The placenta supplies food and oxygen from the mother, and passes back waste from the fetus.

Up to half of all fertilized eggs never implant. When a fertilized egg doesn’t implant, it passes out of the body during menstruation.

The fetus is affected by the mother’s health during the pregnancy. To have a healthy baby, a female should get medical care, eat nutritious foods, and avoid cigarettes, alcohol, and other drugs during her pregnancy.

How Do Females Get Pregnant?

This is not a trick question… females can get pregnant in a variety of ways.

The most common way for a female to get pregnant is when a male ejaculates during vaginal sex. It is also possible – but less likely – for females to become pregnant through any kind of sexual activity in which ejaculate comes in contact with the vulva. The sperm can travel through the moisture on the vulva into the vagina.

Another way for a female to get pregnant is through artificial insemination. During artificial insemination, sperm are inserted into a female’s vagina or uterus using a syringe or other device. The sperm may come from a female’s partner or from a donor.

A female may also get pregnant through treatments like in vitro fertilization. To do this, a female’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.

Gestational Age

Pregnancy is measured using “gestational age.” Gestational age starts on the first day of a female’s last period.

Gestational age can be confusing. Most people think of pregnancy as lasting nine months, and it is true that a female is pregnant for about nine months. But because pregnancy is measured from a female’s last period – about 3-4 weeks before she is actually pregnant – a full-term pregnancy usually totals about 10 months.

Girl or Boy?

The sex of a baby is determined by the two sex chromosomes inherited from the parents. The mother provides the X chromosome and the father provides either the X chromosome or the Y chromosome. If the embryo gets the X chromosome from the father, the XX pairing will result in a baby girl. If the embryo gets the Y chromosome from the father, the XY pairing will result in a baby boy. About half of the sperm in a typical ejaculation contains the X chromosome – the other half contains the Y chromosome.

In some cultures, there is a belief that the mother is to blame if she has a baby that is the “wrong” sex – not the sex that family members wanted. That is completely impossible.

There are chromosomal combinations other than XX and XY, but they are rarer.

Twins and More

There are two types of twins. Fraternal twins happen when there are two eggs, each fertilized by a different sperm. Usually, a female’s ovary releases only one ripe egg during ovulation, but once in a while the ovary may release two ripe eggs. Each of these eggs could then be fertilized by a different sperm. If both fertilized eggs implant themselves in the lining of the uterus, the female will be pregnant with fraternal twins. These twins may not look alike. They may not even be the same sex.

Identical twins are the other type of twins. They develop from a single fertilized egg that splits in two shortly after fertilization. Nobody knows why this happens. Because identical twins come from the same egg and sperm, they look alike. They are always the same sex.

Siamese twins are identical twins who are born with their bodies attached to each other in some way. For some unknown reason, the fertilized egg doesn’t split completely, so the babies develop with parts of their bodies joined together. Siamese twins may be joined in a number of ways. If they are joined at the feet, shoulders, or arms, an operation can separate the babies. In other cases, it’s more difficult to separate them – they may be joined in such a way that cutting them apart would kill one or both of them (for example, if the bodies are joined at the chest and they share one heart). Some parents decide to have the operation done even if one baby must die. Other parents decide against the operation. If they aren’t separated, the twins spend their lives attached.

Okay, there’s actually a third type of twins, semi identical twins, but they are so rare – there are only a few known cases in the world – that we almost didn’t want to mention them. Normally, it’s only possible for one sperm to enter an egg and fertilize it. That’s because the instant a sperm begins to enter, the egg goes through chemical changes – these changes make it practically impossible for another sperm to enter. But in the case of semi identical twins, two sperm manage to enter and fertilize one egg. The egg then splits, and the twins are identical on the mother’s side but share only half the genes on the father’s side.

Triplets (three babies), quadruplets (four babies), quintuplets (five babies), sextuplets (six babies), septuplets (seven babies), and octuplets (eight babies) are pretty rare. Females who give birth to more than two babies have usually had trouble getting pregnant in the past, so their healthcare providers have put them on drugs that stimulate the ovaries. But such drugs often overstimulate the ovaries, and several ripe eggs may be released at the same time.


Adapted with permission from an article by Heather Corinna. The original article, “Where DID I come from? A Refresher Course in Human Reproduction,” can be found on Additional information was obtained from an article by Planned Parenthood Federation of America. The original article, “How Pregnancy Happens,” can be found on Information was also obtained from The “What’s Happening to My Body?” Book for Girls by Lynda Madaras with Area Madaras (Newmarket Press, 2000).

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