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GESTATIONAL AGE ............................. 124
EMBRYONIC DEVELOPMENT . . . . . . . . . . . . . . . . . . . .. 125
FETAL DEVELOPMENT AND PHYSIOLOGY. . . . . . . . . .. 128
ENERGY AND . . . . . . . . . . . . . . . . . . . . . .. 137
PLACENTAL ROLE IN EMBRYOFETAL DEVELOPMENT. .. 139
Our knowledge concening the physioloy of the oetus has been markedy enriched during recent years; nevertheless, when com pared with the adult, it oers many points concening which we are but slighty inormed or prooundy ignorant.
-J. Whitridge Williams (1903)
Since these words were written by Williams in 1903, great strides in the understanding of fetal organogenesis and physiology have been gained. Contemporary obstetrics incorporates 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 gravidas. In our 2yh edition, the entirety of Section 5 is dedicated to the fetal patient, as are individual chapters in other sections. Indeed, virtually every aspect of obstetrics can afect the developing fetus.
Several terms 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 (LMP), a time that actully precedes conception. This starting time, which is usually approximately 2 weeks before ovulation and fertilization and nearly 3 weeks before blastocyst implantation, has traditionally been used because most women know their approximate last period. Embryologists describe embryofetal development in ovuation age, or the time in days or weeks from ovulation. Another term is postconceptional age, which is nearly identical to ovulation age.