Growing numbers of pregnant women across the world now routinely have ultrasound scans as part of antenatal care, including in low-income countries. This article presents the findings of anthropological research on the use of obstetric ultrasonography in routine antenatal care in Hanoi, Viet Nam. The findings come from observation, a survey and interviews with women seeking ultrasound scans at a main maternity hospital and interviews with doctors providing ultrasound there. We found a dramatic overuse of ultrasound scanning; the 400 women surveyed had had an average of 6.6 scans and 8.3 antenatal visits during pregnancy, while one-fifth had had ten scans or more. Doctors considered obstetric ultrasound an indispensable part of modern antenatal care. For two-thirds of the women, the main reason for frequent scans was reassurance of normal fetal development. However, the women often also said their doctor had recommended the scans. This overuse must be seen in the context of growing commercialisation in the Vietnamese health care system, where ultrasound provides an important source of revenue for both private and public providers. There is an urgent need in Viet Nam for policy and practice guidelines on the appropriate use of ultrasonography in pregnancy and how best to combine it with essential antenatal care, and information dissemination to women.