Once a woman becomes pregnant, her previous lifestyle and her health habits and medical care during pregnancy play an important part in the development of a healthy fetus (Melhado, 2011).
Fetal Development
The 9-month (40-week) span of pregnancy is customarily divided into three 13-week segments, called trimesters. Characteristic changes occur in each trimester.
First-Trimester Development
As with all mammals, a human begins as a zygote (ZYE-goht), a united sperm cell and ovum. The sperm and egg unite in one of the fallopian tubes, where the egg’s fingerlike microvilli draw the sperm to it. The zygote then develops into the multicelled blastocyst (BLAS-tuh-sist) that implants on the wall of the uterus about 1 week after fertilization (I Figure 11.3). Growth progresses steadily. By 9-10 weeks after a woman’s last menstrual period, the fetal heartbeat can be heard with a special ultrasound stethoscope known as the Doppler. By the beginning of the 2nd month from the time of conception, the fetus is 0.5 to 1 inch long, grayish, and crescent shaped. During the 2nd month, the spinal canal and rudimentary arms and legs form, as do the beginnings of recognizable eyes, fingers, and toes. During the 3rd month, internal organs, such as the liver, kidneys, intestines, and lungs, begin limited functioning in the 3-inch fetus.
(a)
I Figure 11.3 the blastocyst implanted on the uterine wall, shown (a) in diagram and (b) in photo taken by a scanning electron microscope.
Conceiving Children: process and Choice
Second-Trimester Development
The second trimester begins with the 4th month of pregnancy. By now the sex of the fetus can often be distinguished. External body parts, including fingernails, eyebrows, and eyelashes, are clearly formed. The fetus’s skin is covered by fine, downlike hair. Future development consists primarily of growth and refinement of the features that already exist. Fetal movements, or quickening, can be felt by the end of the 4th month. By the end of the 5th month the fetus’s weight has increased to 1 pound. Head hair can appear at this time, and subcutaneous fat develops. By the end of the second trimester, the fetus has opened its eyes.