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Contemporary obstetrics incorporates physiology and pathophysiology of the fetus, and its development and environment. As a result, the fetus is considered a patient and is given the same meticulous care provided for the mother. Section 6 is dedicated to the fetal patient, however, 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. 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, and with this, term pregnancy averages 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 stems from 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 (2019) together recommend the following:

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

  2. In conceptions achieved with in vitro fertilization (IVF), the embryo age and egg transfer date are 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) is 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 gestational age differs by more than 5 days prior to 9 weeks’ gestation, or by more than 7 days later in the first trimester, the EDC is changed. These and discrepant values in the second and third trimester are discussed further in Chapter 14 (p. 248).

Naegele Rule

An EDC based on the LMP can be quickly estimated as follows: add 7 days to the first day of the LMP and subtract 3 months. For example, if the first day of the LMP was October 5, the due date is 10–05 minus 3 (months) plus 7 (days) = 7–12, or July 12 of ...

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