Anatomy & Physiology

Anatomy & Physiology Study Guide • The uterus undergoes a tremendous increase in size. By the end of a full-term gestation, a typical uterus has grown from 7.5 cm (3 in.) in length and 30–40 g (1–1.4 oz.) in weight to 30 cm (12 in.) in length and 1100 g (2.4 lb.) in weight. • The mammary glands increase in size, and secretory activity begins. Labor and Delivery Labor is divided into three stages. These include the dilation, expulsion, and placental stages. The goal of labor is the expulsion (birth) of the fetus. During true labor, each contraction begins near the top of the uterus and sweeps in a wave toward the cervix. The contractions occur at regular intervals and increase in strength as labor progresses. As parturition (delivery) approaches, the contractions increase in frequency and force. This action changes the position of the fetus and moves it toward the cervical canal. The dilatio stage begins with the onset of true labor. Contractions dilate the cervix, and the fetus begins to shift down towards the cervical canal. The expulsion stage begins as the cervix, pushed open by the approaching fetus, completes its dilation. In this stage, contractions reach maximum intensity, occurring in at least two or three-minute intervals and lasting a full minute. Expulsion continues until the fetus has emerged from the vagina; in most cases, the expulsion stage lasts less than two hours. Delivery, or birth, marks the arrival of the newborn into the outside world. During the placental stage of labor, muscle tension builds in the walls of the partially empty uterus, which gradually decreases in size. This uterine contraction tears the connections between the endometrium and the placenta. In general, within an hour of delivery, the placental stage ends with the ejection of the placenta, or afterbirth. If true labor begins before 37 weeks gestation, premature labor occurs, and the fetus may have complications at birth due to not completing normal development. In 3–4 percent of deliveries, the legs or buttocks of the fetus enter the vaginal canal first. Such deliveries are breech birth , and these births can be a danger to the fetus. Therefore, many are delivered by cesarean section (C-section), a surgical procedure in which the fetus is delivered through incisions in the abdomen and uterus. Multiple Births Multiple births (twins, triplets, quadruplets, and so forth) have varied causes for their occurrence. The ratio of twin births to single births in the U.S. population is roughly 1:89. The most common type are dizygotic twins. “Fraternal,” or dizygotic, twins develop when two separate oocytes were ovulated and subsequently fertilized. Chromosomes are shuffled during meiosis; therefore, the odds against any two zygotes from the same parents having identical genes exceed 1 in 8.4million. Seventy percent of twins are dizygotic. The separation of blastomeres early in cleavage, or from the splitting of the inner cell mass before gastrulation, can lead to monozygotic twins, also known as “identical” twins. In these situations, each set of twins forms from the same pair of gametes, and thus, each pair has the same genetic makeup. Triplets, quadruplets, and larger multiples can result from multiple ovulations, blastomere splitting, or some combination of the two.

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