Objective To compare median levels of maternal serum inhibin A in the second trimester blood samples of women who subsequently develop pre-eclampsia and those who do not.

Design Retrospective analysis of 13–18 week samples from a bank of serum stored at −40°C, originally taken for Down's syndrome screening.

Setting Antenatal clinics in a teaching hospital.

Sample Twenty-eight pregnancies with pre-eclampsia and 701 controls. Samples were taken, on average, 22 weeks before the diagnosis.

Main outcome measure Median inhibin A level.

Results The median inhibin A level in the cases was 2.01 multiples of the gestation-specific median in the controls, a statistically significant elevation (P < 0.001). Twenty-three (82%) had levels above the normal median; 19 (68%), 15 (54%), and 11 (39%) exceeded the normal 75th, 90th and 95th centiles, respectively.

Conclusions In pre-eclampsia the maternal serum inhibin A level can be increased months before the onset of symptoms. This provides an opportunity to study the early natural history of the disease and possibly to conduct treatment trials.