The value of routine antenatal booking ultrasound as an adjunct to, or as an alternative to the 18 to 20 week ultrasound, is not known. A study into the possible benefits of routine antenatal booking ultrasound was undertaken. One year's experience, involving 1,372 scans is described. The main medical benefits include the establishment or correction of gestational age (41.7% of all pregnancies) and the identification of multiple pregnancies (0.7%), nonviable pregnancies (1.4%) and other pregnancies requiring specialist antenatal clinic referral (0.7%), resulting in a total of 44.5% of patients in whom some benefit was obtained.