Key concepts
Vocabulary: fertilization, implantation, embryo, fetus, prenatal care, fetal alcohol syndrome
Prenatal Development and Care
The Very Beginning
Main Idea: A single cell, formed from one egg and one sperm, can grow into a complex human being.
The human body begins as one microscopic cell that is formed by the union of a male sperm cell and a female egg cell, called fertilization. This is also known as conception. The cell that results from fertilization is called a zygote.
The zygote begins to divide and travel through the fallopian tube, as shown in Figure 17.1. It divides many times, forming a cluster of cells by the time it reaches the uterus. Within a few days, implantation, the process by which the zygote attaches to the uterine wall, occurs. After about two weeks, the zygote becomes an embryo (EM-bree-oh), a cluster ofcells that develops between the third and eighth week ofpregnancy. This group of developing cells is called a fetus (FEE-tuhs) after about eight weeks.
The Growing Embryo
The cells of an embryo will continue to divide as it grows. Eventually, three layers of tissue are formed. Later, these lay- ers develop into various body systems. One layer becomes the respiratory and digestive systems. A second layer develops into muscles, bones, blood vessels, and skin. The third layer forms the nervous system, sense organs, and mouth. Two important structures form outside the embryo.
During pregnancy, the blood supply of the mother and the developing embryo are kept separate. Oxygen and nutrients are passed from the mother’s blood to the embryo, and waste from the embryo is passed to the mother’s blood. These wastes are excreted from the mother’s body along with her own body wastes.
Substances that are harmful to a developing embryo can also pass through the umbilical cord. If a pregnant female uses tobacco, alcohol, or other drugs, those substances can cross the placenta and harm the developing embryo.
The time from conception to birth usually takes about 280 days, or nine months.The nine months are divided into three trimesters of three months each. Figure 17.2 shows the major changes that occur in each trimester.
A Healthy Pregnancy
Main Idea: A pregnant female can maintain the health of her fetus in many different ways.
When a woman learns that she is pregnant, she should begin prenatal care to ensure her health and that of her growing baby. Prenatal (pree- NAY-tu hl) care refers to the steps that
pregnant female can take to provide for her own health and :he health of her baby. Seeing a doctor regularly throughout the pregnancy will provide a new mother with the care and nutritional advice she needs.
What to Eat While Pregnant
An unborn baby receives nourishment from the mother. Pregnant females are encouraged to take prenatal vitamins to provide a balance of nutrients, such as:
• Protein helps form muscle and other tissue.
• Iron makes red blood cells and supplies oxygen to cells.
• Vitamin A helps in the growth of cells and bones and in eye development.
• Vitamin Bcomplex aids in forming the nervous system.
• Folic acid is critical in development of the neural tube, which contains the central nervous system. It’s recommended that all females of childbearing age consume 400 to 600 micrograms of folic acid daily.
Most pregnant females need to consume only an additional 300 calories per day to achieve a healthy weight gain during pregnancy. This is equivalent to drinking an extra 2 1⁄2 cups of low-fat milk per day. Females at a healthy weight before becoming pregnant can gain between 25 and 35 pounds. Gaining too little weight can result in a small, undeveloped baby. Gaining too much weight can result in an early delivery. Extra weight also increases the mother’s risk of high blood pressure, diabetes, and varicose veins.
A Healthy Fetus
Main Idea: Expectant mothers should avoid tobacco, alcohol, drugs, and environmental hazards.
An expectant mother should avoid substances that can harm her and her fetus. Tobacco, alcohol, and other drugs can be harmful to both mother and baby.
Avoid Tobacco Use
Smoking and using other tobacco products during preg- nancy is harmful to the fetus. It is estimated that smoking accounts for up to 30 percent of low-birth-weight babies, 14 percent of premature births, and 10 percent of all infant deaths. Studies suggest that smoking may also affect growth, mental development, and behavior after a child is born. Research by the American Lung Association shows that pregnant females who are exposed repeatedly to secondhand smoke increase the risk of having a low-birth-weight baby.
Avoid Alcohol Use
When an expectant mother uses alcohol, so does her growing fetus. Alcohol passes through the umbilical cord to the fetus. The fetus, however, breaks down alcohol more slowly than the mother. This means that the alcohol level in the fetus’s blood is higher, and it remains in the bloodstream for a longer period of time. A severe alcohol-related disorders is fetal alcohol syndrome (FAS), a group ofalcohol-related birth defects that includes both physical and mental problems. Infants born with FAS may have learning, memory, and attention problems, as well as visual and hearing impairments.
Avoid Drug Use
Prescription or over-the-counter medications should be used only with the approval of a doctor or other health care professional. These substances can also harm the fetus.
Any use of illegal drugs poses a health risk to both the mother and the fetus. Drug abuse can harm the mother’s health and make her less able to support the pregnancy. Drugs can also harm the development of the fetus. Infants born to mothers who use drugs may not grow at the same rate as infants born to mothers who do not use drugs. They may have respiratory or cardiovascular problems, mental impairments, or birth defects. In some cases, drug use may lead to the premature birth of the infant, or even a miscarriage. The baby may also be born addicted to the same drugs the mother used during pregnancy.
Avoid Hazards in the Environment
A pregnant female should avoid common hazardous sub- stances in the environment. Family members can help by also being aware of these substances.
Lead. Exposure to lead has been linked to miscarriage, low birth weight, mental disabilities, and behavior problems in children. Lead can be found in the paint ofhouses built before 1978, and in some glassware or dinnerware.
Mercury. Pregnant females should avoid eating certain types of fish that are known to contain higher than average levels of mercury. These include shark, swordfish, and king mackerel.
Smog. Medical studies have linked air pollution with birth defects, low birth weight, premature birth, stillbirth, and infant death. The greatest period of risk is the second month of pregnancy, when organs are developing.
Radiation. Ionizing radiation, such as that found in X-rays, can affect growth and cause mental retardation.
Complications of Pregnancy
Main Idea: A pregnancy may have an unexpected outcome.
Most pregnancies result in the birth ofa healthy baby.About 70 percent of all births occur through a vaginal delivery. Preg- nancy complications, however, can result in a cesarean delivery, made through an incision in the mother’s abdomen.
The complications of pregnancy can also result in a premature birth. This type of birth takes place at least three weeks before the due date. Serious complications may lead to spontaneous expulsion of a fetus occurring before the twentieth week of pregnancy. The delivery of a fetus that has died after the twentieth week of pregnancy is called a stillbirth.
Sometimes, a medical reason causes a miscarriage or stillbirth. Using tobacco or drugs during pregnancy, however, can increase the risk of miscarriage or stillbirth. Receiving prenatal care during pregnancy can reduce the risk, or severity, of problems during pregnancy. During prenatal care, a doctor may be able to identify medical problems with the fetus before the birth. In some cases, medical care, and even surgery, may be performed on the baby before birth.
Other medical complications result from medical condi- tions affecting the pregnant female. Gestational hypertension, or high blood pressure during pregnancy, may occur after the · twentieth week of pregnancy. A severe form of this is pre- eclampsia. Symptoms include high blood pressure, swelling, and large amounts of protein in the urine. Preeclampsia can prevent the placenta from getting enough blood to nourish the fetus. Treatment includes reducing blood pressure through bed rest or medication. Hospitalization may be necessary.
An ectopic pregnancy results when a zygote implants not n the uterus but in the fallopian tube, abdomen, ovary, or cervix. This makes it impossible for the fetus to receive nour- ishment and grow. An ectopic pregnancy cannot lead to the birth of a healthy fetus. It is also the number one cause of death in women in the first trimester of pregnancy.
Childbirth
Main Idea: The birth of a baby takes place in three steps: labor, delivery, and afterbirth.
Expectant parents must decide where the birth will occur. Most births occur in hospital maternity wards staffed by nurses and doctors, with medical equipment to handle complications. Other options include birthing centers or home births, which offer a more comfortable environment. Midwives may attend home births and births at birthing centers.
As the birth approaches, the fetus becomes more crowded in the uterus. Birth occurs in three steps.
Step 1: Labor. Muscle contractions of the uterus become regular, stronger, and closer together. This causes the cervix-the opening to the uterus-to dilate, or widen.
Step 2: Delivery. Once the cervix is fully dilated, the baby passes through the birth canal and emerges from the mother’s body. The baby takes its first breath and cries to clear its lungs of amniotic fluid.
Step 3: Afterbirth. The placenta is still attached to the baby by the umbilical cord. Contractions, although weaker, will continue until the placenta (now called the afterbirth) is pushed from the mother’s body.
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