Dating a pregnancy

An estimated date of delivery cannot be derived for one third of pregnancies because:

• the woman cannot remember the date of her LMP
• the menstrual cycles were irregular
• pregnancy occurred after coming off the contraceptive pill

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:

• third trimester measurements are the least accurate because of the greater variablility of foetal size in late pregnancy
• dolichocephaly (long narrow head)
• failure to measure the correct diameter