Contemporary obstetrical research focuses on the physiology and pathophysiology of the fetus, its development, and its environment. An important result is that the status of the fetus has been elevated to that of a patient who, in large measure, can be given the same meticulous care that obstetricians provide for pregnant women. In the course of these studies, it has become apparent that the conceptus is a dynamic force in the pregnancy unit. Normal fetal development is considered in this chapter. Anomalies, injuries, and diseases that affect the fetus and newborn are addressed in Chapter 29.
Several terms are used to define the duration of pregnancy, and thus fetal age, but these are somewhat confusing. They are shown schematically in Figure 4-1. Gestational age or menstrual age is the time elapsed since the first day of the last menstrual period, a time that actually precedes conception. This starting time, which is usually about 2 weeks before ovulation and fertilization and nearly 3 weeks before implantation of the blastocyst, has traditionally been used because most women know their last period. Embryologists describe embryofetal development in ovulation age, or the time in days or weeks from ovulation. Another term is postconceptional age, nearly identical to ovulation age.
Terminology used to describe the duration of pregnancy.
Clinicians customarily calculate gestational age as menstrual age. About 280 days, or 40 weeks, elapse on average between the first day of the last menstrual period and the birth of the fetus. This corresponds to 9 and 1/3 calendar months. A quick estimate of the due date of a pregnancy based on menstrual data can be made as follows: add 7 days to the first day of the last period and subtract 3 months. For example, if the first day of the last menses was July 5, the due date is 07-05 minus 3 (months) plus 7 (days) = 04-12, or April 12 of the following year. This calculation has been termed Naegele's rule. Many women undergo first- or early second-trimester sonographic examination to confirm gestational age. In these cases, the sonographic estimate is usually a few days later than that determined by the last period. To rectify this inconsistency—and to reduce the number of pregnancies diagnosed as postterm—some have suggested assuming that the average pregnancy is actually 283 days long and that 10 days be added to the last menses instead of 7 (Olsen and Clausen, 1998).
The period of gestation can also be divided into three units of three calendar months (13 weeks) each. These three trimesters have become important obstetrical milestones.
Ovum, Zygote, and Blastocyst
During the first 2 weeks after ovulation, development phases include: (1) fertilization, (2) blastocyst formation, and (3) blastocyst implantation. Primitive chorionic villi are formed soon after implantation. With the development of chorionic villi, it is conventional to refer to the products of conception as an embryo. The early stages of preplacental development and formation of the placenta are described in Chapter 3, Implantation, Placental Formation, and Fetal Membrane Development.
The embryonic period commences at the beginning of the third week after ovulation and fertilization, which coincides in time with the expected day that the next menstruation would have started. The embryonic period lasts 8 weeks and is when organogenesis takes place (see Fig. 4-1). The embryonic disc is well defined, and most pregnancy tests that measure human chorionic gonadotropin (hCG) become positive by this time (see Chap. 8, Pregnancy Tests). As shown in Figure 4-2, the body stalk is now differentiated, and the chorionic sac is approximately 1 cm in diameter. There is a true intervillous space that contains maternal blood and villous cores in which angioblastic chorionic mesoderm can be distinguished.
Early human embryos. Ovulation ages: A. 19 days (presomite). B. 21 days (7 somites). C. 22 days (17 somites). (After drawings and models in the Carnegie Institute.)
The schematic timeline is shown in Figure 4-3. During the third week, fetal blood vessels in the chorionic villi appear (Fig. 4-4). In the fourth week a cardiovascular system has formed, and thereby, a true circulation is established both within the embryo and between the embryo and the chorionic villi. By the end of the fourth week, the chorionic sac is 2 to 3 cm in diameter, and the embryo is 4 to 5 mm in length (Figs. 4-5, 4-6, and 4-7). Partitioning of the primitive heart begins in the middle of the fourth week. Arm and leg buds are present, and the amnion is beginning to unsheathe the body stalk, which thereafter becomes the umbilical cord.
Embryofetal development according to gestational age determined by the first day of the last menses. Times are approximate.
Early human embryos. Ovulation ages: A. 22 days. B. 23 days. (After drawings and models in the Carnegie Institute.)
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