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Our knowledge concerning the physiology of the foetus has been markedly enriched during recent years; nevertheless, when compared with the adult, it offers many points concerning which we are but slightly informed or profoundly 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 25th 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 affect 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 actually 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 ovulation age, or the time in days or weeks from ovulation. Another term is postconceptional age, which is nearly identical to ovulation age.


Terminology used to describe pregnancy duration.

Until recently, clinicians customarily calculated menstrual age with term pregnancy averaging approximately 280 days, or 40 weeks between the first day of the LMP and birth. This corresponds to 9 and 1/3 calendar months. However, menstrual cycle length variability among women renders many of these calculations inaccurate. This realization, combined with the frequent use of first-trimester sonography, has led to more accurate gestational age determination (Duryea, 2015). Much of this change hinges on the accuracy of early sonographic measurement. As a result, the American College of Obstetricians and Gynecologists, the American Institute of Ultrasound in Medicine, and the Society for Maternal-Fetal Medicine (Reddy, 2014) together recommend the following:

  1. First-trimester sonography is the most accurate method to establish or reaffirm gestational age.

  2. In conceptions achieved with assisted-reproductive technology, this gestational age is used.

  3. If available, the gestational ages calculated from the LMP and from first-trimester sonography are compared, and the estimated date of confinement (EDC) recorded and discussed with the patient.

  4. The best obstetrical estimate of gestational age at delivery is recorded on the birth certificate.

The embryofetal crown-rump length in the first trimester is accurate ±5 to 7 days. Thus, if sonographic assessment of gestational age differs by more than 5 days prior to 9 weeks’ gestation, ...

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