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.