Contemporary obstetrics includes physiology and pathophysiology of the fetus, its development, and its environment. An important result is that fetal status 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. Normal fetal development is considered in this chapter. Anomalies, injuries, and diseases that affect the fetus and newborn are addressed in Chapter 33 and others.
Gestational Age Variously Defined
Several terms are used to define pregnancy duration, and thus fetal age (Fig. 7-1). 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 blastocyst implantation, has traditionally been used because most women know their last period. Embryologists describe embryo-fetal 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 pregnancy duration.
Clinicians customarily calculate gestational age as menstrual age. Approximately 280 days, or 40 weeks, elapse on average between the first day of the last menstrual period and the birth. This corresponds to 9 and 1/3 calendar months. A quick estimate of a pregnancy due date 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 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, 1998). The period of gestation can also be divided into three units, each 13 to 14 weeks long. These three trimesters are important obstetrical milestones.
Embryo-Fetal Growth and Development
The complexity of embryo-fetal development is almost beyond comprehension. Shown in Figure 7-2 is a schematic sequence of various organ systems as they develop. New information regarding organ development continues to accrue using modern technologies. For example, imaging techniques help evaluate the role of gene regulation and tissue interaction on eventual 3-dimensional organ morphology (Mohun, 2011). And, Williams and colleagues (2009) described the sequence of ...