Miscarriage is the most common complication of early pregnancy. This chapter summarizes the risk factors for first-trimester miscarriage. There is a strong relationship between infertility and miscarriage. Studies have shown that risk of miscarriage varies by socio-economic position, but the trends are unclear and most probably relate to exposure to environmental, occupational or behavioral risk factors. A very clear finding from the National Women's Health Study (NWHS) was the impact of stressful life events, a stressful job situation, and feelings of anxiety and depression on the risk of miscarriage. Most people seek an explanation of the cause of their miscarriage and treatment or guidance to prevent a recurrence. Efforts to gather research evidence have been hampered in the past by methodological difficulties and the lack of understanding by health professionals that at least a proportion of miscarriages are preventable.

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.