Female Reproductive System


Childbirth consists of three stages—dilation, delivery, and placental—that are controlled by a positive feedback system including the hormones progesterone, estrogen, relaxin, and oxytocin.

Childbirth, also called labor and delivery, is the end of pregnancy. During childbirth, the baby (or babies in a multiple pregnancy) exits the uterus. Birth and delivery of the baby and the placenta is known as parturition. Labor is initiated by a decrease in progesterone, a hormone that maintains pregnancy, and an increase in the estrogen hormone estriol. Childbirth consists of three stages: dilation, delivery, and the placental stage.

Dilation is the first stage of childbirth when the cervix softens and opens. Stage 1, dilation, starts with the beginning of contractions and dilates the cervix from 0 to 10 cm. It is the longest stage and is divided into three phases: early labor phase (beginning of contractions to a cervix dilated 3 cm), active labor phase (cervical dilation from 3 cm to 7 cm), and transition phase (cervical dilation from 7 cm to 10 cm). In stage 2, delivery, powerful uterine contractions push the baby out. In stage 3, the placental stage, the placenta is delivered.

The Stages of Childbirth

Childbirth can be divided into three stages: 1) Dilation-marks the dilation of the cervix from 0 cm to 10 cm; 2) Delivery-stronger uterine contractions force the baby from the uterus; 3) Placental-delivers the placenta and umbilical cord, which has been cut and closed off with a clamp.
This first stage of childbirth, the dilation stage, begins with the onset of labor, in which the uterus contracts. Contractions are initiated by estriol and by relaxin, which additionally causes the cervix to efface, or thin out, and open. This stage can last from a few hours to more than a day. There are three phases to the first stage of childbirth—early labor phase, active labor phase, and transition phase. The early labor phase is the time from onset of labor to when the cervix is dilated 3 cm. Pain during this time is typically not severe, and contractions are infrequent. This is followed by the active labor phase, when the cervix dilates up to 7 cm and contractions increase in frequency to about one every five minutes. The pain of contractions increases during this phase. Sometime in the early or active phase, the "water breaks," meaning that the amniotic sac that surrounds the baby ruptures and the fluid in the sac is discharged. The final phase of the first stage of birth, the transition phase, is when the cervix dilates to 10 cm and contractions shorten to about one every three minutes. The pain of contractions becomes intense, and the baby moves toward the vagina. The hormone oxytocin plays a vital role in this phase. Oxytocin promotes uterine contractions during childbirth and causes milk ejection during breastfeeding.

The second stage of birth involves delivery, in which the baby is delivered from the uterus. This can happen either vaginally or through a Caesarian section (C-section). In a vaginal birth, the mother pushes with contractions, moving the baby through the cervix and out of the body through the birth canal. In a C-section, the baby is delivered surgically through the abdomen. This procedure is used when the baby is in the incorrect position or there would be other complications to either the mother or child to have a vaginal birth.

The final stage of birth is the placental stage. In this stage, the placenta, which provided a connection between the fetus and the uterus, is delivered. This typically happens just a few minutes after birth of the baby. Oxytocin is responsible for continuing the contractions after birth of the baby in order to expel the placenta. Oxytocin also assists in clotting and reducing bleeding. It also has psychological effects, enhancing bonding between the mother and infant. After the placenta has been expelled, progesterone levels begin to rise, ceasing contractions.

In the weeks and months after childbirth, prolactin and oxytocin levels remain high, as both hormones assist with breastfeeding and lactation, the secretion of milk from the mammary tissue of the breast. Eventually, estrogen, progesterone, and ovarian hormones return to typical levels, resuming the woman's ovarian and uterine cycles. The female breast normally grows during pregnancy because of mammary glandular development and fat deposition. During this time, the areolae and nipples also enlarge. The mammary glands are responsible for milk production. For every 15 to 20 lobes of glandular tissue, a lactiferous duct (channel that connects the nipple to the lobules of the mammary gland) drains the milk and directs it toward individual openings on the nipple. A dilated portion of the lactiferous duct, called the lactiferous sinus, is positioned just below the areola and is where the droplets of milk accumulate in the nursing mother. When an infant begins to suckle at the nipple, the areola is compressed and the accumulated milk in the lactiferous sinus is released.

In the first two to three days after delivery, a special type of milk, called colostrum, is produced. It contains white blood cells and antibodies, particularly immunoglobulin A (IgA), in addition to a large percentage of proteins, minerals, and fat-soluble vitamins. This colostrum is very important in that it not only fulfills the infant's nutritional requirements, but helps protect the baby from infectious agents in the environment. Milk is produced in larger amounts on days two through four. By two weeks after birth, milk is considered to be mature.

As the baby suckles, impulses are sent to the mother's hypothalamus. This marks the beginning of a hormone pathway that induces milk production and lactation. The hypothalamus then signals the anterior pituitary gland to secrete prolactin and the posterior pituitary to secrete oxytocin. It is the hormone prolactin that stimulates milk production in the mammary glands. The hormone oxytocin stimulates the release of milk, making it readily available and easier for the infant to feed. This oxytocin reflex is also called the "letdown reflex."