We investigate the problem of late diagnosis of twin pregnancy in Soweto, South Africa, where routine antenatal ultrasound is not available. One hundred consecutive pairs of twins were studied, using the notes of mothers who delivered twins at Chris Hani Baragwanath Hospital and the referring Soweto clinics. A positive history was found in 31 mothers (22 family history, nine previous history of twins). Six mothers did not attend for antenatal care. Twenty-five twin pregnancies were discovered at delivery, 15 of them in the second stage of labour, and 27 were diagnosed accidentally in the third trimester. Only 15 pregnancies were referred specifically for suspicion of twin pregnancy. Most twin pregnancies are detected only in the third trimester or at delivery. Until routine ultrasound is available to all pregnant women, the teaching of antenatal care in South Africa must give emphasis to clinical suspicion of twin pregnancy.