First-trimester spontaneous abortions have been predicted from alterations in the gestational sac size and embryonic heart rate. To evaluate which parameter or combination of parameters has the greatest predictive value, a prospective study was carried out during the first trimester of pregnancy. Pregnancy sac size, crown-rump length and heart rate were measured in 603 embryos, of which 580 continued beyond 13 weeks' gestation and 23 ended in first-trimester spontaneous abortions. Based on the continuing pregnancies, we constructed nomograms relating the ratio of mean sac diameter to crown-rump length (S/CR) to gestational age (last menstrual period [LMP]), to crown-rump length (CRL) and to embryonic heart rate (EHR). The S/CR ratios of the 23 embryos that spontaneously aborted in the first trimester were plotted on these nomograms to determine the significance of values falling outside the 95% confidence limits. Plotting the S/CR ratios of the aborted fetuses on the S/CR versus LMP nomogram gave a sensitivity of 78.3%, a specificity of 97.8% and a false-positive rate of 2.2%. The same population was evaluated for abortion risk using EHR nomograms, giving a sensitivity of 65.2%, specificity of 98.2% and false-positive rate of 1.8%. Our findings suggest that S/CR measurement in early pregnancy may be a simple and reliable method of predicting first-trimester abortions.