About Human Reproduction

All living things reproduce. Reproduction — the process by which organisms make more organisms like themselves — is one of the things that sets living things apart from nonliving matter. But even though the reproductive system is essential to keeping a species alive, unlike other body systems, it's not essential to keeping an individual alive.

In the human reproductive process, two kinds of sex cells, or gametes, are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system to create a new individual.

Both the male and female reproductive systems are essential for reproduction. The female needs a male to fertilize her egg, even though it is she who carries offspring through pregnancy and childbirth.

Humans, like other organisms, pass certain characteristics of themselves to the next generation through their genes, the special carriers of human traits. The genes that parents pass along are what make their children similar to others in their family, but also what make each child unique. These genes come from the male's sperm and the female's egg.

Most species have two sexes: male and female. Each sex has its own unique reproductive system. They are different in shape and structure, but both are specifically designed to produce, nourish, and transport either the egg or sperm.

Components of the Female Reproductive System

Unlike the male, the human female has a reproductive system located entirely in the pelvis. The external part of the female reproductive organs is called the vulva, which means covering. Located between the legs, the vulva covers the opening to the vagina and other reproductive organs located inside the body.

The fleshy area located just above the top of the vaginal opening is called the mons pubis. Two pairs of skin flaps called the labia (which means lips) surround the vaginal opening. The clitoris, a small sensory organ, is located toward the front of the vulva where the folds of the labia join. Between the labia are openings to the urethra (the canal that carries urine from the bladder to the outside of the body) and vagina. Once girls become sexually mature, the outer labia and the mons pubis are covered by pubic hair.

A female's internal reproductive organs are the vagina, uterus, fallopian tubes, and ovaries.

The vagina is a muscular, hollow tube that extends from the vaginal opening to the uterus. The vagina is about 3 to 5 inches (8 to 12 centimeters) long in a grown woman. Because it has muscular walls, it can expand and contract. This ability to become wider or narrower allows the vagina to accommodate something as slim as a tampon and as wide as a baby. The vagina's muscular walls are lined with mucous membranes, which keep it protected and moist.

The vagina serves three purposes:

1. It's where the penis is inserted during sexual intercourse.
2. It's the pathway that a baby takes out of a woman's body during childbirth, called the birth canal.
3. It provides the route for the menstrual blood (the period) to leave the body from the uterus.

A thin sheet of tissue with one or more holes in it called the hymen partially covers the opening of the vagina. Hymens are often different from female to female. Most women find their hymens have stretched or torn after their first sexual experience, and the hymen may bleed a little (this usually causes little, if any, pain). Some women who have had sex don't have much of a change in their hymens, though.

The vagina connects with the uterus, or womb, at the cervix (which means neck). The cervix has strong, thick walls. The opening of the cervix is very small (no wider than a straw), which is why a tampon can never get lost inside a girl's body. During childbirth, the cervix can expand to allow a baby to pass.

The uterus is shaped like an upside-down pear, with a thick lining and muscular walls — in fact, the uterus contains some of the strongest muscles in the female body. These muscles are able to expand and contract to accommodate a growing fetus and then help push the baby out during labor. When a woman isn't pregnant, the uterus is only about 3 inches (7.5 centimeters) long and 2 inches (5 centimeters) wide.

At the upper corners of the uterus, the fallopian tubes connect the uterus to the ovaries. The ovaries are two oval-shaped organs that lie to the upper right and left of the uterus. They produce, store, and release eggs into the fallopian tubes in the process called ovulation. Each ovary measures about 1½ to 2 inches (4 to 5 centimeters) in a grown woman.

There are two fallopian tubes, each attached to a side of the uterus. The fallopian tubes are about 4 inches (10 centimeters) long and about as wide as a piece of spaghetti. Within each tube is a tiny passageway no wider than a sewing needle. At the other end of each fallopian tube is a fringed area that looks like a funnel. This fringed area wraps around the ovary but doesn't completely attach to it. When an egg pops out of an ovary, it enters the fallopian tube. Once the egg is in the fallopian tube, tiny hairs in the tube's lining help push it down the narrow passageway toward the uterus.

The ovaries are also part of the endocrine system because they produce female sex hormones such as estrogen and progesterone.

What the Female Reproductive System Does

The female reproductive system enables a woman to:

• produce eggs (ova)
• have sexual intercourse
• protect and nourish the fertilized egg until it is fully developed
• give birth

Sexual reproduction couldn't happen without the sexual organs called the gonads. Although most people think of the gonads as the male testicles, both sexes actually have gonads: In females the gonads are the ovaries. The female gonads produce female gametes (eggs); the male gonads produce male gametes (sperm). After an egg is fertilized by the sperm, the fertilized egg is called the zygote.

When a baby girl is born, her ovaries contain hundreds of thousands of eggs, which remain inactive until puberty begins. At puberty, the pituitary gland, located in the central part of the brain, starts making hormones that stimulate the ovaries to produce female sex hormones, including estrogen. The secretion of these hormones causes a girl to develop into a sexually mature woman.

Toward the end of puberty, girls begin to release eggs as part of a monthly period called the menstrual cycle. Approximately once a month, during ovulation, an ovary sends a tiny egg into one of the fallopian tubes.

Unless the egg is fertilized by a sperm while in the fallopian tube, the egg dries up and leaves the body about 2 weeks later through the uterus — this is menstruation. Blood and tissues from the inner lining of the uterus combine to form the menstrual flow, which in most girls lasts from 3 to 5 days. A girl's first period is called menarche.

It's common for women and girls to experience some discomfort in the days leading to their periods. Premenstrual syndrome (PMS) includes both physical and emotional symptoms that many girls and women get right before their periods, such as acne, bloating, fatigue, backaches, sore breasts, headaches, constipation, diarrhea, food cravings, depression, irritability, or difficulty concentrating or handling stress. PMS is usually at its worst during the 7 days before a girl's period starts and disappears once it begins.

Many girls also experience abdominal cramps during the first few days of their periods caused by prostaglandins, chemicals in the body that make the smooth muscle in the uterus contract. These involuntary contractions can be either dull or sharp and intense.

It can take up to 2 years from menarche for a girl's body to develop a regular menstrual cycle. During that time, her body is adjusting to the hormones puberty brings. On average, the monthly cycle for an adult woman is 28 days, but the range is from 23 to 35 days.

Fertilization

If a female and male have sex within several days of the female's ovulation, fertilization can occur. When the male ejaculates (when semen leaves a male's penis), between 0.05 and 0.2 fluid ounces (1.5 to 6.0 milliliters) of semen is deposited into the vagina. Between 75 and 900 million sperm are in this small amount of semen, and they "swim" up from the vagina through the cervix and uterus to meet the egg in the fallopian tube. It takes only one sperm to fertilize the egg.

About a week after the sperm fertilizes the egg, the fertilized egg (zygote) has become a multicelled blastocyst. A blastocyst is about the size of a pinhead, and it's a hollow ball of cells with fluid inside. The blastocyst burrows itself into the lining of the uterus, called the endometrium. The hormone estrogen causes the endometrium to become thick and rich with blood. Progesterone, another hormone released by the ovaries, keeps the endometrium thick with blood so that the blastocyst can attach to the uterus and absorb nutrients from it. This process is called implantation.

As cells from the blastocyst take in nourishment, another stage of development, the embryonic stage, begins. The inner cells form a flattened circular shape called the embryonic disk, which will develop into a baby. The outer cells become thin membranes that form around the baby. The cells multiply thousands of times and move to new positions to eventually become the embryo.

After approximately 8 weeks, the embryo is about the size of an adult's thumb, but almost all of its parts — the brain and nerves, the heart and blood, the stomach and intestines, and the muscles and skin — have formed.

During the fetal stage, which lasts from 9 weeks after fertilization to birth, development continues as cells multiply, move, and change. The fetus floats in amniotic fluid inside the amniotic sac. The fetus receives oxygen and nourishment from the mother's blood via the placenta, a disk-like structure that sticks to the inner lining of the uterus and connects to the fetus via the umbilical cord. The amniotic fluid and membrane cushion the fetus against bumps and jolts to the mother's body.

Pregnancy lasts an average of 280 days — about 9 months. When the baby is ready for birth, its head presses on the cervix, which begins to relax and widen to get ready for the baby to pass into and through the vagina. The mucus that has formed a plug in the cervix loosens, and with amniotic fluid, comes out through the vagina when the mother's water breaks.

When the contractions of labor begin, the walls of the uterus contract as they are stimulated by the pituitary hormone oxytocin. The contractions cause the cervix to widen and begin to open. After several hours of this widening, the cervix is dilated (opened) enough for the baby to come through. The baby is pushed out of the uterus, through the cervix, and along the birth canal. The baby's head usually comes first; the umbilical cord comes out with the baby and is cut after the baby is delivered.

The last stage of the birth process involves the delivery of the placenta, which at that point is called the afterbirth. After it has separated from the inner lining of the uterus, contractions of the uterus push it out, along with its membranes and fluids.

Problems of the Female Reproductive System

Some girls might experience reproductive system problems, such as:

Problems of the Vulva and Vagina

• Vulvovaginitis is an inflammation of the vulva and vagina. It may be caused by irritating substances (such as laundry soaps or bubble baths) or poor personal hygiene (such as wiping from back to front after a bowel movement). Symptoms include redness and itching in the vaginal and vulvar areas and sometimes vaginal discharge. Vulvovaginitis also can be caused by an overgrowth of Candida, a fungus normally present in the vagina.
• Nonmenstrual vaginal bleeding is most commonly due to the presence of a vaginal foreign body, often wadded-up toilet paper. It may also be due to urethral prolapse, in which the mucous membranes of the urethra protrude into the vagina and form a tiny, doughnut-shaped mass of tissue that bleeds easily. It also can be due to a straddle injury (such as when falling onto a gymnastics beam or bicycle frame) or vaginal trauma from sexual abuse.
• Labial adhesions, the sticking together or adherence of the labia in the midline, usually appear in infants and young girls. Although there are usually no symptoms associated with this condition, labial adhesions can lead to an increased risk of urinary tract infection. Sometimes topical estrogen cream is used to help separate the labia.

Problems of the Ovaries and Fallopian Tubes

• Ectopic pregnancy occurs when a fertilized egg, or zygote, doesn't travel into the uterus, but instead grows rapidly in the fallopian tube. A woman with this condition can develop severe abdominal pain and should see a doctor because surgery may be necessary.
• Endometriosis occurs when tissue normally found only in the uterus starts to grow outside the uterus — in the ovaries, fallopian tubes, or other parts of the pelvic cavity. It can cause abnormal bleeding, painful periods, and general pelvic pain.
• Ovarian tumors, although they're rare, can occur. Girls with ovarian tumors may have abdominal pain and masses that can be felt in the abdomen. Surgery may be needed to remove the tumor.
• Ovarian cysts are noncancerous sacs filled with fluid or semisolid material. Although they are common and generally harmless, they can become a problem if they grow very large. Large cysts may push on surrounding organs, causing abdominal pain. In most cases, cysts will disappear on their own and treatment is unnecessary. If the cysts are painful, a doctor may prescribe birth control pills to alter their growth or they may be removed by a surgeon.
• Polycystic ovary syndrome is a hormone disorder in which too many male hormones (androgens) are produced by the ovaries. This condition causes the ovaries to become enlarged and develop many fluid-filled sacs, or cysts. It often first appears during the teen years. Depending on the type and severity of the condition, it may be treated with drugs to regulate hormone balance and menstruation.
• Ovarian torsion, or the twisting of the ovary, can occur when an ovary becomes twisted because of a disease or a developmental abnormality. The torsion blocks blood from flowing through the blood vessels that supply and nourish the ovaries. The most common symptom is lower abdominal pain. Surgery is usually necessary to correct it.

Menstrual Problems

A variety of menstrual problems can affect girls, including:

• Dysmenorrhea is when a girl has painful periods.
• Menorrhagia is when a girl has a very heavy periods with excess bleeding.
• Oligomenorrhea is when a girl misses or has infrequent periods, even though she's been menstruating for a while and isn't pregnant.
• Amenorrhea is when a girl has not started her period by the time she is 16 years old or 3 years after starting puberty, has not developed signs of puberty by age 14, or has had normal periods but has stopped menstruating for some reason other than pregnancy.

Infections of the Female Reproductive System

• Sexually transmitted infections (STIs). These include infections and diseases such as pelvic inflammatory disease (PID), human immunodeficiency virus/acquired immunodeficiency syndrome (HIV/AIDS), human papillomavirus (HPV, or genital warts), syphilis, chlamydia, gonorrhea, and genital herpes (HSV). Most are spread from one person to another by sexual contact.
• Toxic shock syndrome. This uncommon illness is caused by toxins released into the body during a type of bacterial infection that is more likely to develop if a tampon is left in too long. It can produce high fever, diarrhea, vomiting, and shock.

If you think your daughter may have symptoms of a problem with her reproductive system or if you have questions about her growth and development, talk to your doctor — many problems with the female reproductive system can be treated.

Reviewed by: Steven Dowshen, MD
Date reviewed: May 2010