An estimated date of delivery cannot be derived for one third of pregnancies because:
In such circumstances, ultrasound provides a useful estimate of gestational age.
In the first trimester, the crown-rump length is used.
In the second and third trimesters, the biparietal diameter and abdominal circumference are measured.
An experienced operator will usually be able to predict the date of delivery to within 7 days.
Sources of error in dating a pregnancy include:
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