Among all women who had ever been pregnant, 21% reported ever having had a miscarriage. The results of the National Women's Health Study confirmed some well-established risk factors, including increased maternal age and previous history of miscarriage and infertility, and also the protective effect of nausea. The response rate for the study was 73% for the more targeted Stage 2 questionnaire. Six hundred and three cases and 6116 controls were included in the case-control analysis of risk factors for first-trimester miscarriage. The results obtained from this study, after adjusting for year of conception, maternal age, previous miscarriage and previous livebirth, are listed in this chapter. The reduced risks associated with taking vitamins, consumption of fresh fruit and vegetables and feeling happy and relaxed during pregnancy are perhaps not surprising, but further work is needed to establish causal pathways and whether these results can be explained by selection or reporting biases.