Birth

Birth, or parturition, in mammals is the process in which a fully developed fetus is expelled from the mother's uterus by the force of strong, rhythmic muscle contractions. The birth of live offspring is a reproductive feature shared by mammals, some fishes, and selected

invertebrates (such as scorpions), as well as some reptiles and amphibians. Animals who give birth to live offspring are called viviparous (meaning "live birth").

In contrast to viviparous animals, other animals give birth to eggs; these animals are called oviparous (meaning "egg birth"). Some oviparous species, such as birds, retain their eggs inside their bodies for long periods of time; in these animals, the eggs are laid at an advanced stage of development. Other animals, such as frogs, give birth to less developed eggs, which undergo development outside the mother's body.

At the end of the gestation period, the mother's uterus begins to contract rhythmically, a process called labor. The initiation of labor leading up to birth is the result of a number of hormones, notably oxytocin.

Fetal hormones are also thought to play a role in initiating labor. At the end of gestation, the fetal adrenal glands secrete steroid hormones called cortico steroids, which cause the hormone-like substances known as prostaglandins. Prostaglandins contribute to the contraction of the uterus during labor.

Labor culminating in birth in humans begins with the rhythmic contractions of the uterus, which dilate the cervix. This causes the fetus to move down the birth canal and be expelled together with the placenta, which had supplied the developing fetus with nutrients from the mother. Ususually, the entire birth process takes about 16 hours, but it can range anywhere from less than one hour to 48 hours.

The first stage: dilation of the cervix

In order for the fetus to leave the uterus and to enter the birth canal, it must pass through the cervix, the opening of the uterus. The cervix is normally tightly closed, and is sealed with a plug of mucus during gestation to protect the fetus from invading microorganisms. During the first stages of labor, the contractions of the uterus dilate the cervix, which widens to about 4 in (10 cm), to accommodate the passage of the fetal head.

In the last weeks of pregnancy, before labor begins, the uterus undergoes irregular contractions, which serve to exercise the muscles of the uterus and may even dilate the cervix; it's not unusual for a woman to go into active labor with a cervix that is already dilated to 1 or 2 cm. During the last weeks of pregnancy, the cervix also thins out (or effaces), which makes dilation easier.

In preparation for birth, the fetus moves further down into the mother's pelvis. When labor begins, the fetus is usually positioned with its head engaged with the top of the cervix. This engagement is called "lightening" or "dropping." When labor begins, the contractions loosen the mucus plug in the cervix causing small capillaries in the cervix to break, and the mucus and blood are discharged from the vagina. This discharge is sometimes called "bloody show" and signals the onset of labor.

Another sign that may signal the beginning of labor is the rupturing of the amniotic sac. In the uterus the fetus is encased in a membrane (the amniotic sac) and literally floats in amniotic fluid. When uterine contractions begin, this sac ruptures and the amniotic fluid can leak from the uterus. Not all women experience an abrupt rupturing of the amniotic sac; in some, the amniotic fluid gradually leaks out as labor progresses. Once the amniotic sac has ruptured, or the amniotic fluid begins to leak, labor usually progresses more rapidly. During the first stage of labor, the cervix dilates about 0.5-0.6 in (1.2-1.5 cm) an hour. The uterine contractions are 5-30 minutes apart, and last for 15-40 seconds. The end of the first stage of labor is associated with the strongest uterine contractions. Contractions are two to five minutes apart, and last for 45-60 seconds. The cervix opens rapidly at this point. This period of labor, sometimes called transition, is usually the most difficult for the mother. The contractions are very strong and close together, and nausea and vomiting are common. After the cervix has dilated to its full width of 4 in (10 cm), the contractions slow down somewhat to about three to five minutes apart. The fetus is then ready to be born, and the second stage of labor begins.

The second stage: birth

During the second stage, lasting about one to two hours, the mother uses her abdominal muscles to push the fetus through and out of the birth canal.

The pushing is actually a reflex action, but if a woman can help the reflex by actively using her muscles, Stage 1: Dilation of the cervix. Illustration by Hans & Cassidy. Courtesy of Gale Group. Stage 2: Expulsion of the fetus. Illustration by Hans & Cassidy. Courtesy of Gale Group. Stage 3: Expulsion of the placenta. Illustration by Hans & Cassidy. Courtesy of Gale Group. Natural childbirth. © Jules Perrier/Corbis. Reproduced by permission. birth goes much faster. As the fetus moves down the birth canal to the vaginal opening, the head begins to appear. The appearance of the head at the opening of the vagina is called crowning. After the head is delivered, first one shoulder is delivered, then the other. The rest of the body follows.

After the baby is born, the umbilical cord that has attached the fetus to the placenta is clamped. The clamping cuts off the circulation of the cord, which eventually stops pulsing due to the interruption of its blood supply. The baby now must breathe air through its own lungs.

The third stage: delivery of the placenta

Before delivery, the placenta separates from the wall of the uterus. Since the placenta contains many blood vessels, its separation from the wall of the uterus causes bleeding. This bleeding, if not excessive, is normal. After the placenta separates from the uterine wall, it moves into the birth canal and is expelled from the vagina. The uterus continues to contract even after the placenta is delivered, and it is thought that these contractions serve to control bleeding.