OBJECTIVE--To test the effect of extra non-directive information about prenatal testing, given individually or in a class. SETTING--Antenatal clinics in a district general hospital and a university hospital. DESIGN--Randomised controlled trial; participants allocated to control group or offer of extra information individually or in class. SUBJECTS--1691 women booking antenatal care before 15 weeks' gestation. INTERVENTIONS--All participants received the usual information about prenatal tests from hospital staff. Individual participants were offered a separate session with a research midwife in which prenatal screening was described in detail. Class participants were offered the same extra information in an early prenatal class. MAIN OUTCOME MEASURES--Attendance at extra information sessions; uptake rates of prenatal tests; levels of anxiety, understanding, and satisfaction with decisions. RESULTS--Attendance at classes was lower than at individual sessions (adjusted odds ratio 0.45; 95% confidence interval 0.35 to 0.58). Ultrasonography was almost universally accepted (99%) and was not affected by either intervention. Uptake of cystic fibrosis testing, high in controls (79%), was lowered in the individual group (0.44; 0.20 to 0.97) and classes (0.39; 0.18 to 0.86). Uptake of screening for Down's syndrome, already low (34%) in controls, was not further depressed by extra information in classes (0.99; 0.70 to 1.39) and was slightly higher in the individual group (1.45; 1.04 to 2.02). Women offered extra information had improved understanding and were more satisfied with information received; satisfaction with decisions about prenatal testing was unchanged. The offer of individual information reduced anxiety later in pregnancy. CONCLUSIONS--Ultrasonography is valued for non-medical reasons and chosen even by fully informed people who eschew prenatal diagnosis. The offer of extra information has no overall adverse effects on anxiety and reduces uptake of blood tests when background uptake rate is high (but not when it is already low). High uptake of prenatal blood tests suggests compliant behaviour and need for more information.