Reproduction, Human, is the process by which human beings create more of their own kind. Human beings reproduce sexually. That is, a new individual develops from the joining together of two sex cells, one from a female parent and one from a male parent. The union of these cells is called fertilization.
Biologists refer to sex cells as gametes. Females produce gametes called eggs or ova. Male gametes are called sperm. Fertilization may occur after a male delivers sperm to the female’s egg by means of sexual intercourse. Fertilization begins a remarkable period of development in which the egg develops into a fully formed baby within the body of the female. This period of development, called pregnancy, takes about nine months.
At the beginning of pregnancy, the fertilized egg is smaller than the period at the end of this sentence. The egg develops into a growing mass of cells called an embryo. Gradually, the cells rearrange themselves to form tissues. By the end of the second month of pregnancy, all the major body organs and organ systems have formed and the embryo looks distinctly human. During the rest of pregnancy, the embryo is called a fetus. The fetus grows while its systems prepare for the day when they must function outside the mother’s body. Pregnancy ends when the new baby passes out of the mother’s body at birth.
This article discusses the biological aspects of reproduction in humans. For a discussion of some of the moral and social issues related to human reproductions, see such articles as Abortion, Baby, Birth control, and Family. For information among other living things, see Reproduction.
The human reproductive system
Human beings are born with the body organs needed for reproduction. But reproduction cannot actually occur until these organs mature. This maturation process takes place during puberty, a period in which a boy or girl goes through dramatic physical changes. These changes are regulated by certain hormones (chemicals produced by the body). Puberty begins during or just before the early teen-age years.
The reproductive systems of females and males differ greatly in shape and structure. But both systems are specifically designed to produce, nourish, and transport the eggs or sperm.
In females,
the reproductive system consists primarily of a group of organs located within the pelvis. A woman or girl has external organs called the vulva between her legs. The outer parts of the vulva cover the opening to a narrow canal called the vagina. The vagina leads to the uterus, a hollow, pear-shaped, muscular organ in which a baby develops. Two small, oval organs called ovaries lie to the right and left of the uterus. The ovaries produce, store, and release eggs. These organs also produce two types of hormones—progesterone and estrogens. Eggs from the ovaries reach the uterus through tubes called fallopian tubes or oviducts.
Females produce eggs as part of a monthly process called the menstrual cycle, which begins during puberty. Each menstrual cycle, the female reproductive system undergoes a series of changes that prepares it for fertilization and pregnancy. If the egg is not fertilized, a shedding or loss of tissue in the uterus called menstruation occurs. Bleeding is associated with this process and lasts three to seven days. Menstruation marks the beginning of each menstrual cycle. Each cycle lasts about 28 days. See Menstruation.
Other changes during a menstrual cycle involve cells in the ovaries called oocytes. Eggs develop from these cells. At birth, each ovary has about 400,000 oocytes. These cells remain inactive until the first menstrual cycle. Thereafter, many oocytes grow and begin to mature each month. Normally, only one oocyte in either of the ovaries reaches full maturity. This fully developed cell—the mature egg—is released from the ovary in a process called ovulation. This process occurs at about the midpoint of the menstrual cycle. After ovulation, the egg travels toward the uterus through one of the fallopian tubes by means of wavelike contractions of muscles and the beating of cilia (hairlike structures) located on cells in the walls of the oviduct. Fertilization may occur in one of the tubes. An unfertilized egg lives for about 24 hours after it leaves the ovary.
Important changes also occur in the endometrium (lining of the uterus). During the first half of the menstrual cycle, the ovaries release relatively large amounts of estrogens, which cause the endometrium to thicken. The endometrium reaches its maximum thickness at about the time of ovulation. After ovulation, the ovaries release relatively large amounts of progesterone. This hormone maintains the thickness of the endometrium, so that a fertilized egg can attach to the uterus.
If fertilization occurs, the endometrium continues to develop. If fertilization does not occur, the egg breaks down and the production of progesterone decreases. The thickened endometrium also breaks down and passes out of the body during menstruation.
Most women produce eggs until the ages of about 45 to 55, when the menstrual cycles become increasingly infrequent and then stop. This period of a woman’s life is called menopause. The completion of menopause marks the end of a woman’s natural childbearing years.
In males,
the reproductive system includes the testicles, a duct system, accessory glands, and the penis. The testicles, also called testes, are the organs that produce sperm. The duct system, which includes the epididymis and the vas deferens, transports the sperm. The accessory glands, mainly the seminal vesicles and the prostate gland, provide fluids that lubricate the duct system and nourish the sperm. The sperm leave the body through the penis, a cylindrical organ that is located between the legs.
The testicles are contained in the scrotum, a pouch behind the penis. The location of the scrotum keeps the testicles about 4 to 5 Fahrenheit degrees (2.2 to 2.8 Celsius degrees) cooler than the normal body temperature of 98.6 °F (37.0 °C). Unlike other cells of the body, sperm cells cannot develop properly at normal body temperature. In addition to producing sperm, the testicles also produce hormones, particularly testosterone.
Sperm develop in the testicles within a complex system of tubes called seminiferous tubules. At birth, a male baby’s tubules contain only simple round cells. But during puberty, the testicles begin to produce testosterone and other hormones that make the round cells divide, and undergo changes to become slender cells with a tail. A sperm cell uses its tail, called a flagellum, to propel itself forward. Sperm pass from the testicles into the epididymis, where they complete their development in about 12 days and are stored.
A healthy adult male normally produces about 200 million sperm per day. Although sperm production begins to decline gradually at about 45 years of age, it normally continues throughout life.
From the epididymis, sperm move to a long tube that is called the vas deferens. The seminal vesicles and prostate gland produce a whitish fluid called seminal fluid. This fluid mixes with sperm to form semen. The vas deferens leads to the urethra, a tube that runs through the penis.
Semen, which contains the sperm, is expelled from the body through the urethra. This process is called ejaculation. The penis usually hangs limp. But when a male becomes sexually excited, special tissues in the penis fill with blood, and the organ becomes stiff and erect. When the erect penis is stimulated, muscles around the reproductive organs contract. This contraction forces fluid from the glands and propels the semen through the duct system and the urethra. The amount of semen ejaculated varies from 2 to 6 milliliters (0.07 to 0.2 fluid ounce). Each milliliter has about 100 million sperm.
Fertilization
A pregnancy begins when a sperm fertilizes an egg. Fertilization, also called conception, normally occurs by means of sexual intercourse. Sexual intercourse takes place when the man’s erect penis is inserted in the woman’s vagina. When a man ejaculates, semen containing the millions of sperm is deposited in the vagina.
Scientists have developed techniques of achieving fertilization without sexual intercourse. In a process called artificial insemination, sperm are collected from a man and later injected into a woman’s uterus. In another technique, called in vitro fertilization, collected sperm are used to fertilize eggs in a laboratory dish. The fertilized eggs are then inserted into the woman’s uterus. See Infertility (Treatment).
After ejaculation, the sperm pass from the vagina into the uterus and then into the fallopian tubes. Most sperm die along the way. In each tube, only a few thousand sperm reach the ampulla, a section that makes up one-half to two-thirds of the tube’s length. If a sperm fertilizes an egg, it usually does so in the part of the ampulla near the uterus.
Some sperm may reach the fallopian tubes in as little as five minutes. Others take hours. Sperm can survive in the fallopian tubes for up to 48 hours. It takes an egg about 72 hours to pass through a fallopian tube. The egg can be fertilized only during the first 24 hours of this period. Therefore, intercourse must take place near the time of ovulation for fertilization to occur.
The surface of a newly released egg is covered with a jellylike layer of cells called the zona pellucida. A second layer of cells, called the cumulus oophorus, surrounds the zona pellucida. A sperm must pass through both layers to fertilize the egg. The acrosome (tip) of the sperm releases special enzymes that scatter the cells of both layers. Although several sperm may begin to penetrate the zona pellucida, usually only one can fertilize the egg. After the first sperm enters, the egg releases substances that prevent other sperm from entering.
How sex is determined.
Fertilization is complete when the chromosomes of the sperm unite with the chromosomes of the egg. Chromosomes are threadlike structures that contain genes, the units of heredity that determine each person’s unique traits. Most body cells have 46 chromosomes that occur in 23 pairs. However, as each egg or sperm develops, it undergoes a special series of cell divisions called meiosis. As a result, each sperm or egg cell contains only one member of each chromosome pair, or 23 unpaired chromosomes. During fertilization, the chromosomes pair up so that the fertilized egg has the normal number of 46 chromosomes. The fertilized egg is called a zygote.
Special sex chromosomes determine whether the zygote will develop into a boy or a girl. Each body cell contains a pair of sex chromosomes. In females, the two sex chromosomes are identical. Each of the chromosomes is called an X chromosome. The cells of males have one X chromosome and a smaller chromosome called the Y chromosome.
After meiosis, each sperm or egg cell has only one sex chromosome. All egg cells carry one X chromosome. Half the sperm cells carry an X chromosome, and the other half have a Y chromosome. At fertilization, a sperm with an X chromosome uniting with an egg will develop into a girl baby because the fertilized egg will have two X chromosomes. A sperm with a Y chromosome uniting with an egg will form a boy baby because the fertilized egg will have the X and Y combination. See Heredity (Chromosomes and genes).
Multiple birth.
In most cases, a single egg is fertilized and develops into one baby. Occasionally, however, two or more infants develop and are born at the same time. The birth of more than one baby from the same pregnancy is called multiple birth.
Multiple births can result from separate zygotes or from a single zygote. For example, if two eggs are released during ovulation, each may be fertilized by a separate sperm, producing separate zygotes. The two zygotes develop into dizygotic twins, also called fraternal twins. Monozygotic twins develop from a single zygote that divides into separate cells, with each cell developing independently. The infants born have the same genetic makeup and usually resemble each other. Such twins are also called identical twins. See Multiple birth.
Development of the embryo
The zygote goes through a series of changes before it reaches the uterus. In the uterus, the zygote develops into a form called the embryo. The embryo develops rapidly. Within two months, all the tissues and organs of the body have begun to form.
The first days of pregnancy.
After fertilization, the zygote travels through the fallopian tube toward the uterus. Along the way, the zygote begins to divide rapidly into many cells with no increase in overall size. The resulting cell mass is called a morula. By the third or fourth day after fertilization, the morula enters the uterus. At that time, the morula is still surrounded by the zona pellucida and consists of about 12 to 16 cells.
The embryo develops from the central cells of the morula. These cells are called the inner cell mass. The outer cells of the morula are called the outer cell mass. They develop into the placenta, a special organ that enables the embryo to obtain food and oxygen from the mother.
After the morula enters the uterus, it continues to divide. A fluid-filled cavity forms between the inner cell mass and the outer cell mass, and the zona pellucida begins to disintegrate. At this stage, the ball of cells is called a blastocyst or blastula. The cells of the blastocyst divide as it floats in the uterus for one or two days.
About the fifth or sixth day of pregnancy, the blastocyst becomes attached to the internal surface of the uterus. The outer cells of the blastocyst, called the trophoblast, secrete an enzyme that breaks down the lining of the uterus. The trophoblast begins to divide rapidly, invading the uterine tissue. The process of attachment to the uterine wall is called implantation. By the 11th day of the pregnancy, the blastocyst is firmly implanted in the uterus.
Nourishing the embryo.
Various structures develop in the uterus to help the embryo grow. These structures include the placenta and certain membranes.
By the 13th day of pregnancy, a space called the chorionic cavity has formed around the embryo. Two membranes surround the chorionic cavity. The outer membrane is called the chorion, and the inner membrane is called the amnion. The chorion interacts with tissues of the uterus to form the placenta. The chorion pushes into the wall of the uterus with fingerlike projections called chorionic villi. The chorionic villi contain the embryo’s first blood vessels. The chorion is attached to the embryo by a structure called the body stalk. The body stalk develops into the umbilical cord, which joins the embryo to the placenta.
The amnion forms a sac around the embryo and is filled with fluid. The embryo floats in this fluid, called amniotic fluid. The amniotic fluid protects the embryo by absorbing jolts to the uterus. It also allows the embryo to move without damaging the amnion and other tissues.
About the 21st day of pregnancy, blood begins to circulate between the placenta and the embryo. The blood vessels of the mother and those of the embryo exchange substances through a thin layer of cells called the placental barrier. Waste products from the embryo are carried away through the barrier. Likewise, nutrients and oxygen from the mother’s blood pass through the thin walls of the barrier and enter the embryo’s blood. However, such organisms as viruses and bacteria, as well as chemical substances, including drugs, also may cross the placental barrier and harm the embryo.
Origin of tissues and organs.
At about the same time that the placenta begins to form, the inner cell mass flattens and develops into three layers of cells in what is called the embryonic disc. The three types of cell layers are the ectoderm, the mesoderm, and the endoderm. In a process called differentiation, cells from each layer move to certain areas of the embryonic disc and then fold over to form tubes or clusters. These tubes and clusters develop into various tissues and organs of the body.
Cells from the ectoderm form the brain, nerves, skin, hair, nails, and parts of the eyes and ears. Cells from the mesoderm form the heart, muscles, bones, tendons, kidneys, glands, blood vessels, and reproductive organs. The linings of the digestive and respiratory systems develop from cells of the endoderm.
Development of organs and organ systems.
The body’s organs and organ systems grow rapidly from the third through eighth weeks of pregnancy. The major structures include the central nervous system and the circulatory system, as well as such organs as the eyes, ears, and limbs. Defects in the development of these structures often occur during these weeks. Such defects sometimes are caused by substances introduced from the mother’s body through the placental barrier. These substances are called teratogens. They include medications taken by the mother, as well as viruses, bacteria, and other infectious organisms. Other teratogens include nonmedicinal drugs, alcoholic beverages, and cigarette smoke.
The central nervous system,
which consists of the brain and spinal cord, starts to develop in the middle of the third week of pregnancy. It begins as a flattened strip of cells within a long cylinder of cells called the neural tube. At about the 25th day of pregnancy, one end of the neural tube closes. The brain develops from three sacs formed in this end of the tube. The other end of the tube closes two days later. Failure of the tube to close can result in birth defects, especially spina bifida, a disorder of the spine.
The circulatory system
also begins to develop in the third week of pregnancy. Two tubes of cells combine to form a single tube that becomes the heart. By the fourth week, a simple circulatory system is functioning and the heart has begun to pump blood. During the fourth to seventh weeks of pregnancy, the heart tube divides into four chambers. Any irregularity in the normal pattern of development during this period can produce a defect in the heart.
The eyes and ears
begin to develop in the fourth week of pregnancy. Both these organs form rapidly. The external parts of the ears appear by the sixth week. Defects in the eyes or ears often stem from abnormalities that occur during the fourth to sixth weeks.
The arms and legs
appear as buds of tissue during the fifth week of pregnancy. The arms develop a few days ahead of the legs. The fingers and toes become recognizable in the sixth week. They form when certain cells die and leave spaces in the remaining tissue.
The structures of the mouth,
such as the lips and palate, begin to form during the fourth and fifth weeks of pregnancy. The lips and palate form during the sixth to ninth weeks. Each forms from paired structures that gradually move from the sides toward the middle of the face and fuse (join). If anything interferes with normal development during this period, a split in the upper lip or palate may develop. Such a defect is called cleft lip or cleft palate. See Cleft palate.
Growth of the fetus
From the ninth week of pregnancy until birth, the developing baby is called a fetus. In the first three months of this period, the fetus increases rapidly in length. It grows about 2 inches (5 centimeters) in each of these months. In the later months of pregnancy, the most striking change in the fetus is in its weight. Most fetuses gain about 25 ounces (700 grams) in both the eighth and ninth months of pregnancy.
Stages of growth.
Physicians commonly divide pregnancy into three, three-month parts called trimesters. At the end of the first trimester, the fetus weighs about 1 ounce (28 grams) and is about 3 inches (7.6 centimeters) long. At the end of the second trimester, the fetus weighs about 30 ounces (850 grams) and measures about 14 inches (36 centimeters) long. At the end of the third trimester, the fetus measures about 20 inches (50 centimeters) and weighs about 7 pounds (3.2 kilograms).
The mother can feel movements of the fetus by the fifth month of pregnancy. By this time, fine hair called lanugo covers the body of the fetus. Hair also appears on the head. Lanugo disappears late in pregnancy or shortly after birth. The eyelids open by the 26th week of pregnancy. By the 28th week, the fingernails and toenails are well developed.
Until the 30th week of pregnancy, the fetus appears reddish and transparent because of the thinness of its skin and a lack of fat beneath the skin. In the last six to eight weeks before birth, fat develops rapidly and the fetus becomes smooth and plump.
The mother also experiences many physical changes during pregnancy. For example, a pregnant woman gains weight and her breasts increase in size. For more information on such changes, see Pregnancy.
Checking the fetus.
Physicians can use several procedures to monitor the development of the fetus in the mother’s uterus. Two of the most commonly used techniques are ultrasonography and amniocentesis.
Ultrasonography, also called ultrasound, involves the use of high-frequency sound waves to produce an image of the fetus on a screen. By viewing the shape and body features of the fetus, a physician can measure its growth and detect malformations. Fetal abnormalities also can be detected through amniocentesis. This technique involves the removal of a sample of the amniotic fluid, which contains cells of the fetus. The fluid and cells are then analyzed and examined. See Amniocentesis; Ultrasound.
Birth
The process of giving birth is called parturition or labor. By this process, the fetus and the placenta are pushed out of the uterus. Scientists believe that labor is triggered by the release of certain hormones from the adrenal glands of the fetus.
A fetus that undergoes the normal period of development before labor begins is considered to have reached term. Labor occurs at term if it begins during the 38th to 41st week of pregnancy. Labor that starts before the 38th week is called preterm labor. Labor that begins after the 41st week is called postterm labor. Babies born at term or postterm have the best chance for survival. Most babies born from the 26th to 36th weeks of pregnancy also will live, but some of these babies may experience serious health problems because their respiratory and central nervous systems are not fully developed at birth. Babies born before the 26th week have a poor chance of surviving.
The stages of labor.
Labor has three stages. The first stage begins with an alternating tensing and relaxing of muscles in the uterus. These muscle contractions are called labor pains. When labor begins, the fetus lies within its protective membranes and is held in place by the cervix (neck of the uterus). During the first stage of labor, the cervix begins to dilate (open). This stage ends when the cervix has fully dilated to a diameter of about 4 inches (10 centimeters). The first stage of labor is the longest, averaging about 14 hours in women giving birth for the first time. In women who have had children before, this stage normally takes 8 hours or less.
The second stage of labor begins at full dilation of the cervix and ends with the delivery of the baby. This stage may last from one to five hours. The muscle contractions of the uterus and abdomen help push the baby through the cervix and out the vagina. Most babies are born headfirst, but some are born with their shoulders or buttocks first. After the head comes out, the rest of the baby follows easily.
The third stage of labor starts after the baby’s delivery and ends when the placenta, now called the afterbirth, is expelled from the uterus. This stage lasts about 30 minutes. A few minutes after the baby is born, the umbilical cord is clamped and cut. The placenta then detaches from the uterus and passes out the vagina.
Sometimes, the smallness of a woman’s pelvis or some other condition makes it difficult to deliver a child through the vagina. In these situations, doctors may perform surgery to remove the baby through the mother’s abdomen. This procedure is called a cesarean section. See Childbirth.
The newborn infant.
At birth, most babies weigh about 7 pounds (3.2 kilograms) and measure about 20 inches (50 centimeters) long. The newborn infant is fed with the mother’s breast milk or with a formula of milk and other nutrients. The baby can now survive outside its mother’s body but needs constant care.
