This paper reports indirect evidence that prenatal sex selection is a contributor to the recent increase in sex ratio at birth in Vietnam. The paper uses birth data from the Population Change Survey 2006 to assess the associations between sex ratio at birth and variables that predict increased opportunities to practise prenatal sex selection, including maternal knowledge of foetal sex before birth, the use of ultrasound for foetal sex determination, the gestation week when foetal sex was disclosed, and access to abortion services. The high sex ratio of most recent births was significantly associated with the use of ultrasound to determine the foetal sex in gestation weeks 12–22 and with access to family planning services that provide abortion. Prenatal sex selection in health facilities are likely to contribute to the recent increase in sex ratio at birth in Vietnam.