Ultrasound screening for estimation of gestational age is one of the most important advances made in obstetrics. In this study on the effects of our crown-rump length (CRL) screening program, a most significant (25%) proportion of the pregnant women had a difference between menstrual age and gestational age estimated on the basis of crown-rump length, exceeding 7 days. Regular menstrual cycles and reliable menstrual history could only reduce this frequency to 19%. Postmature deliveries (greater than 294 days menstrual age) were reduced from one in fifteen to one in three hundred. A screening program with CRL is useless if the woman books in too late, since a CRL above 90 mm is unreliable because of differences in fetal bending of the spine. In our catchment area more than 78% of the women delivered booked early enough for a fetal CRL measurement. If this result can be expected in a population, CRL measurement should be used because of its better accuracy than biparietal diameter measurement for the estimation of gestational age.