Periods of Development
The length of pregnancy (about 39 weeks)—called the gestation period—is divided into three 3-month segments called trimesters. A number of terms are used to describe development during these periods known as the first, second, and third trimesters of pregnancy.
During the first trimester, or 3 months, of pregnancy, many terms are used. Zygote describes the ovum just after fertilization by a sperm cell. After about 3 days of constant cell division, the solid mass of cells, identified earlier as the morula, enters the uterus. Continued development transforms the morula into the hollow blastocyst, which then implants into the uterine wall.
The embryonic phase of development extends from the third week after fertilization until the end of week 8 of gestation. During this period in the first trimester, the term embryo is used to describe the developing offspring. By day 35 of gestation (Figure 24-6, A), the heart is beating and, although the embryo is only 8 mm (about 3⁄8 inch) long, the eyes and so-called “limb buds,” which ultimately form the arms and legs, are clearly visible.
FIGURE 24-6Embryos and fetuses. A, At 35 days. B, At 49 days. C, At the end of the first trimester. D, At 4 months.
The period of development extending from week 9 to week 39 is termed the fetal phase. During this period, the term embryo is replaced by fetus.
Figure 24-6, C, shows the stage of development of the fetus at the end of the first trimester of gestation. Body size is about 7 to 8 cm (3.2 inches) long. The facial features of the fetus are apparent, the limbs are complete, and gender can be identified. By month 4 (Figure 24-6, D) all organ systems are complete and in place.
RESEARCH, ISSUES, AND TRENDS
HOW LONG DOES PREGNANCY LAST?
This seems like a silly question to most of us; the answer is 9 months, isn’t it? Actually, the length of gestation (the amount of time one is pregnant) is defined in different ways in different situations and can vary from one pregnancy to another.
The average gestation in humans is 266 days, starting at the day of conception. But physicians instead usually count from the beginning of the woman’s last menstrual period, for an average of 280 days. But these are only averages. What is normal in one case can be different from what is normal in another case. In practice, any pregnancy of less than 37 weeks (259 days) is said to be premature, and any lasting more than 42 weeks (294 days) is said to be postmature.
So as with many statistics regarding human function, what is “normal” can be spoken of only in generalities and averages.