Подпись: Although you would think a trimester would be a 3-month period, because pregnancies are dated from the woman’s last menstrual period, a full-term pregnancy is actually 40 weeks, and therefore each trimester is approximately 12-15 weeks long. Throughout the pregnancy, physicians can use electronic monitoring and sonography, or ultrasound, to check on the status of the fetus. We will now discuss the physical development of the typical, healthy mother and child in each of these trimesters. First Trimester sonography

Electronic monitoring; also called ultrasound.

I ultrasound

The use of ultrasonic waves to monitor a devel oping fetus; also called sonography.

The first trimester includes the first 13 weeks of pregnancy (1 to 13 weeks). It is the trimester in which the most important embryonic development takes place. When a woman becomes pregnant, her entire system adjusts. Her heart pumps more blood, her weight increases, her lungs and digestive system work harder, and her thyroid gland grows. All of these changes occur to encourage the growth of the developing fetus.

Prenatal Development By the end of the 1st month of pregnancy, the fetal heart is formed and begins to pump blood. In fact, the circulatory system is the first organ sys­tem to function in the embryo (Rischer & Easton, 1992). In addition, many of the other major systems develop, including the digestive system, beginnings of the brain, spinal cord, nervous system, muscles, arms, legs, eyes, fingers, and toes. By 14 weeks the liver,

The Prenatal Period: Three TrimestersПодпись: An embryo at 7 to 8 weeks. This embryo is approximately 1 inch long.kidneys, intestines, and lungs have begun to develop. In addition, the circulatory and uri­nary systems are operating, and the repro­ductive organs have developed. By the end of the first trimester, the fetus weighs У2 ounce and is approximately 3 inches long.

Changes in the Pregnant Mother

During the first few weeks of pregnancy, a woman’s body adjusts to increased levels of es­trogen and progesterone. As we saw in Table 12.1, this can cause fatigue, breast tenderness, constipation, increased urination, and nausea

and/or vomiting. Some women experience nausea and vomiting so severe during pregnancy that they must be hospitalized due to weight loss and malnutrition (S. W. Simpson et al.,

2001) . Specific food cravings are normal, as is an increased sensitivity to smells and odors.

Although some women feel physically uncomfortable because of all these changes, many also feel excited and happy about the life growing within them. The final, confirm­ing sign of pregnancy—a fetal heartbeat—can be a joyous moment that offsets all the discomforts of pregnancy. The fetal heartbeat can usually be heard through ultrasound by the end of the first trimester.

Since its introduction in 1950, ultrasound has become a very useful tool in obstet­rics. It can capture images of the embryo for measurement as early as 5V2 weeks into the pregnancy, and a heartbeat can be seen by 6 weeks. Fetal heartbeat can also be heard through a stethoscope at approximately 9 to 10 weeks and after a heartbeat is either seen or heard, the probability of miscarriage drops significantly. Ultrasounds help to confirm a pregnancy, rule out abnormalities, indicate gestational age, and confirm multiple preg­nancies. Newer three-dimensional and even four-dimensional ultrasounds have been in­troduced and will become more mainstream in the next few years.