Oestriol, oestrone, and oestradiol‐17β concentrations in plasma were studied in a series of “high risk” pregnancies. A gas chromatographic method of measurement was used. Plasma determinations have an advantage over urine determinations, because the collection of 24 h urine specimens is ledious and particularly liable to error. Low and subnormal plasma oestriol values were found to be correlated with the subsequent occurrence of fetal asphyxia or intrauterine death. Plasma oestriol values well within normal limits, or increasing, indicated that fetuses were not at risk of the more severe grades of asphyxia, or of intrauterine death. An exception was observed when the mothers had impaired renal function. Oestrogen clearance is reduced and, therefore, apparently normal values are not significant. Low oestrogen concentrations may not be significant when patients are on (halogenated) steroid therapy. Plasma oestrone and oestradiol‐17β values in the asphyxia group showed a similar pattern to those of oestriol, but gave no additional information. In the groups without asphyxia, especially oestradiol‐17β values lay low in the normal range with many subnormal values. In many of these cases, there was also the discrepancy that plasma oestriol values were normal, while urine oestriol values were subnormal. The plasma analysis will probably become a valuable guide to care in the obstetrical clinic.