Diagnosis

A. The accurate diagnosis of postdates pregnancy can be made only by proper dating. The estimated date of confinement (EDC) is most accurately determined early in pregnancy. An EDC can be calculated by subtracting 3 months from the first day of the last menses and adding 7 days (Naegele's rule). Other clinical parameters that should be consistent with the EDC include maternal perception of fetal movements (quickening) at about 16 to 20 weeks; first auscultation of fetal heart tones with Doppler ultrasound by 12 weeks; uterine size at early examination (first trimester) consistent with dates; and, at 20 weeks, a fundal height 20 cm above the symphysis pubis or at the umbilicus.

Clinical Estimates of Gestational Age

Parameter

Gestational age (weeks)

Positive urine hCG

5

Fetal heart tones by Doppler

11 to 12

Quickening Primigravida Multigravida

20 16

Fundal height at umbilicus

20

B. In patients without reliable clinical data, ultrasound is beneficial. Ultrasonography is most accurate in early gestation. The crown-rump length becomes less accurate after 12 weeks in determining gestational age because the fetus begins to curve.

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