Using ultrasound to assess the fetus at risk.

In early pregnancy, ultrasonic scanning is valuable if the patient's menstrual history is unreliable or if uterine size is less than expected. The crown-rump length of the embryo can be measured from the sixth week of gestation, and pulsation of the heart detected from the seventh. The crown-rump length of the early embryo gives precise information about maturity. Somewhat later in pregnancy, fetal maturity and growth rate can be assessed accurately by measurement of the biparietal diameter. The head circumference and the fetal upper abdominal circumference together give the head-abdomen ratio, which can be helpful in assessing fetal nutrition. The abdominal circumference also may be used to predict fetal weight.