Pregnancy

After implantation, the blastocyst slowly takes on the recognizable features of a human infant. This nine-month period of development is called gestation, or pregnancy. Pregnancy is divided into three equal periods, or trimesters. Significant changes occur during each trimester.

First Trimester

The most dramatic changes in human development take place during the first trimester. For the first eight weeks of pregnancy, the developing human is called an embryo. Throughout the first two to three weeks following fertilization, a developing human embryo resembles the embryos of other animals. The embryo develops from the mass of cells on the inner surface of the blastocyst. At first, all of the cells in the mass look alike. But the cells soon reorganize, first into two and then into three distinct types of cells, forming the primary germ layers: the ectoderm, mesoderm, and endoderm. Different parts of the body develop from each of the primary germ layers.

Four membranes that aid the development of the embryo also form during the first trimester. One of these membranes, called the amnion (AM-nee-uhn), forms the fluid-filled amniotic (AM-nee-AHT-ik) sac, which surrounds the developing embryo.

Summarizing Vocabulary Materials pencil, paper, dictionary Procedure Write and define the following list of words: ovary, ovum, follicle, gestation, morula, blastocyst, amnion, chorion, umbilical, uterus, corpus, and luteum. Identify the roots and meanings of the roots for each word.

Analysis Do any of the meanings of the words surprise you? Explain. How does knowing the roots and meanings of the words help you remember them?

(a) 9-Day Blastocyst (b) 16-Day Embryo (c) 4-Week Embryo figure 51-11

(a) 9-Day Blastocyst (b) 16-Day Embryo (c) 4-Week Embryo figure 51-11

(a) An embryo develops from the mass of cells on one side of a blastocyst.

(b) The primary germ layers develop by the third week of pregnancy, and the four embryonic membranes form.

(c) By the end of the first month of pregnancy, all of the embryonic membranes are formed.

figure 51-12

About two weeks after fertilization, the placenta begins to develop. The mother nourishes the developing embryo through the placenta for the duration of the pregnancy.

The fluid in the amniotic sac cushions the embryo from injury and keeps it moist. A second membrane forms the yolk sac. Although it does not contain yolk, the yolk sac is an important structure because it is where the first blood cells originate. A third membrane, called the allantois (uh-LAN-toh-is), forms near the yolk sac. The fourth membrane, the chorion (KAWR-ee-AHN), surrounds all of the other membranes. As shown in Figure 51-11, one side of the chorion forms small, fingerlike projections called chorionic villi (KAWR-ee-AHN-ik VIL-IE), which extend into the uterine lining. Blood vessels that form within the chorionic villi originate in the allantois.

Together, chorionic villi and the portion of the uterine lining that they invade form a close-knit structure called the placenta. The placenta is the structure through which the mother nourishes the embryo. Nutrients, gases, pathogens, drugs, and other substances can pass from the mother to the embryo through the placenta.

Amniotic sac

Umbilical' cord

Fetal portion Maternal portion Endometrium of placenta of placenta

Amniotic sac

Umbilical' cord

Fetal portion Maternal portion Endometrium of placenta of placenta

Thus, women should abstain from alcohol and avoid all unnecessary drugs throughout pregnancy. Alcohol use by women, especially during early pregnancy, is the leading cause of birth defects, such as fetal alcohol syndrome (FAS). FAS can result in severe mental, behavioral, and physical retardation.

The embryo is attached to the placenta by the umbilical (uhm-BIL-i-kuhl) cord, which contains arteries and veins that carry blood between the embryo and the placenta. As Figure 51-12 shows, blood from the mother and fetus never mixes. Materials such as nutrients and wastes are exchanged across the placenta.

A developing placenta begins to secrete a hormone called human chorionic gonadotropin (HCG) early in the second week after fertilization. In the early stages of pregnancy, HCG stimulates the corpus luteum to continue producing sex hormones, and thus the uterine lining and the embryo are retained. Otherwise, the corpus luteum will stop producing estrogen and progesterone, and menstruation will occur. As the placenta grows, it begins to secrete large amounts of progesterone and estrogen, which take over maintenance of the uterine lining. Production of estrogen and progesterone throughout pregnancy prevents the release of FSH and LH, and eggs are not released.

The brain, spinal cord, and the rest of the nervous system begin to form in the third week. The heart begins to beat at 21 days. By the fifth week, arms, legs, eyes, and ears have begun to develop. At six weeks, the fingers and toes form, and the brain shows signs of activity. The embryo also begins to move, although the mother cannot yet feel it. From eight weeks until birth, the developing child is called a fetus (FEET-uhs). The fetus is only about 5 cm (2 in.) long when the first trimester ends, but all of its organ systems have begun to form, as shown in Figure 51-13.

Second Trimester

During the second trimester, the mother's uterus enlarges. The fetus's heartbeat can be heard, its skeleton begins to form, and a layer of soft hair called lanugo grows over its skin. The fetus also begins to wake and sleep. The mother may feel the fetus move. The fetus swallows and sucks its thumb. It can make a fist, hiccup, kick its feet, and curl its toes. By the end of the second trimester, the fetus is about 34 cm (13.4 in.) long and about 900g (2 lb) in weight.

12 weeks 21 weeks

Word Roots and Origins fetus from the Latin fetus, meaning "offspring"

figure 51-13

By 12 weeks, the fetus's arms and legs are developing, and 20 buds for future teeth appear. By week 21, eyelashes, eyebrows, and fingernails have formed, and the skin is covered with fine hair called lanugo. By eight months, the fetus's bones have hardened, lanugo has disappeared, and body fat is developing.

figure 51-13

By 12 weeks, the fetus's arms and legs are developing, and 20 buds for future teeth appear. By week 21, eyelashes, eyebrows, and fingernails have formed, and the skin is covered with fine hair called lanugo. By eight months, the fetus's bones have hardened, lanugo has disappeared, and body fat is developing.

Third Trimester

In the third trimester, the fetus grows quickly and undergoes changes that will enable it to survive outside the mother. The fetus can see light and darkness through the mother's abdominal wall, and it can react to music and loud sounds. During the last half of this trimester, the fetus develops fat deposits under its skin. These fat deposits, which make the fetus look rounded and less wrinkled, insulate the body so that it can maintain a steady body temperature.

figure 51-14

During childbirth, the fetus passes through the greatly enlarged cervix and vagina.

Placenta

Umbilical cord

Placenta

Umbilical cord

Vagina

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