Unit A Reproduction

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4.3 Pregnancy: Fertilization to Birth You have learned that the role of the female reproductive system is to produce eggs and to carry the fertilized egg from fertilization (also called conception) to birth. How exactly does fertilization occur? What changes take place in the embryo during the nine months of pregnancy? You began life as a single cell, a zygote, about 0.1 mm in diameter. One single cell divided into the trillions of very different cells that make up your body. Fertilization Millions of sperm are released into the vagina during sexual intercourse. Most die on the way, and only about 100 make it as far as the oviduct. These 100 or so sperm surround the egg and try to penetrate the egg’s outer coating. As soon as one sperm penetrates the egg (Figure 1), the egg releases a protein that prevents other sperm from penetrating. The cell membranes of the sperm and the egg fuse. The sperm’s nucleus enters the egg, where it will fuse with the egg’s nucleus to produce the zygote. Fertilization is the beginning of a nine-month (38-week) period of pregnancy, which ends with the birth of the offspring. The nine months of pregnancy are divided into three trimesters (three-month periods) for convenience. First Trimester The first trimester starts at fertilization and ends after the third month. The zygote begins dividing in the oviduct and is called an embryo when it implants into the uterus by the end of the first week. Hormones are secreted that prevent menstruation from happening. Soon after implanting, a sac forms around the embryo and is filled with amniotic fluid, which supports, protects, and maintains a warm environment for the embryo until birth. When a mother’s water breaks during birth, it is this amniotic fluid that is released. By the fourth week, the brain and nervous system are developing, and the heart is beating. The embryo is now 500 times bigger than the zygote, although it is still only about 5 mm long. The beginnings of eyes, ears, and nose are noticeable at the end of week five (Figure 2). All the cells of the embryo follow the instructions in their DNA and develop different structures and functions. Limb buds and a tail are visible. The tail will reduce, and all that will be left at birth is the tail bone. LEARNING TIP You can use a table to help you organize information for studying. Make a three-column table with the following headings: First trimester, Second trimester, Third trimester. Under the appropriate heading, record important information in point-form notes. Include the words in bold in your notes. 13 000X Figure 1 A single sperm cell fusing with an egg cell Figure 2 In the late first trimester, rudimentary limbs and eyes are visible. This fetus is about 4 cm long and has a mass of less than 10 g. NEL 4.3 Pregnancy: Fertilization to Birth 117
amniotic fluid placenta umbilical cord fetus uterus Figure 3 The fetus receives nutrition from the mother via the placenta and the umbilical cord. Figure 4 By the end of the second trimester, facial features are present and internal systems are maturing. Figure 5 In the third trimester, the fetus fills the uterus. With so little space, the fetus no longer moves around as much. By the end of the eighth week, the embryo now has the beginnings of all the organs and is called a fetus. The fetus looks human, but it is only the size of a lima bean. It is now receiving nutrition from the mother through the placenta (Figure 3). The placenta is shaped like a disk and grows to 20 to 30 cm in diameter. It contains blood vessels from the mother and blood vessels from the fetus. Nutrients and oxygen diffuse from the mother’s blood into the fetus’s blood. Wastes from the fetus diffuse into the mother’s blood. Because the mother’s and fetus’s blood vessels are so close to each other, diffusion is possible. The mother’s blood does not enter the circulatory system of the fetus. The fetus’s circulatory system is connected to the placenta through the umbilical cord. By the end of the third month, muscles and bones are forming. The fetus can flex arm and leg muscles, and may hiccup. The heart is beating and blood is circulating. Sex organs are forming, and gender can be determined. The head is almost half the size of the body. The fetus is about 12 cm long, with a mass of around 50 g. Since the mother’s gain in mass is about 1 kg, she may not look different. There are usually signs of pregnancy, however, such as no menstruation, enlarged breasts, and sometimes nausea. A blood test done by a health-care provider can confirm a pregnancy. The fetus is most susceptible to birth defects during the first trimester. A birth defect is a structural or physiological problem in the fetus that develops before birth. The Second Trimester Months four, five, and six are a time of intense growth for the fetus. The fetus grows from 8 to 30 cm long by the end of the sixth month. The heartbeat can be detected with a stethoscope. The fetus is sucking and swallowing amniotic fluid, which will be excreted in urine. The mother now looks pregnant, as her abdomen swells. Movement of the fetus can be felt by the mother, first as a flutter but soon as pokes and kicks as the legs and arms develop. At 24 weeks, the fetus resembles a tiny infant (Figure 4). Fingers and toes have elongated. Facial features are more developed, and eyelashes are visible. The fetus is now covered with a fine hair. Practice breathing also occurs, although no gas exchange occurs. Oxygen reaches the fetus through the blood vessels in the umbilical cord. The Third Trimester During the last three months (Figure 5), the fetus puts on most of its mass as it continues to develop in preparation for birth. The organ systems are functioning properly. The fetus has sleeping and waking patterns and reacts to stimuli from the outside world. The brain is developing rapidly. The bones are developed but still soft. The eyelids are open. The fetus fills the uterus and the mother’s belly may undulate as the baby changes position. During the last month, the fetal hair disappears. The fetus turns so that the head is pointing down in preparation for birth. The lungs continue to mature right up to birth. At 37 weeks, the fetus is considered full term. The size of the fetus ranges from 45 to 55 cm and can be up to 4.5 kg in mass, although the average size is 53 cm and 3.4 kg at birth. 118 <strong>Unit</strong> A <strong>Reproduction</strong> NEL
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CHAPTER 2 Cell Growth and Reproduct
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2.1 The Importance of Cell Division
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2.3 From DNA to Proteins You have l
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centrioles nucleus chromosome spind
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Unit A Reproduction

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