Estimation of pregnancy duration by means of ultrasonic measurements of the fetal crown-rump length.

In order to establish the reliability and accuracy of crown-rump length measurements of the fetus between the 7th and 14th wk of pregnancy, a series of 118 ultrasound tests has been performed on 60 patients, whose ovulation dates were known from basal body temperature records. During the tests, the investigator was not informed of the duration of the pregnancy. These data were statistically analyzed, using the equation t = 0.037 L2,24 (t is the duration of the pregnancy from the moment of ovulation onwards, L the crown-rump length in cm). The correlation coefficient was 0.942. This investigation proved that, if ultrasound measurements of the crown-rump length are taken in the first trimester, the maturity of the pregnancy can be estimated to within 1 wk.