In the late 1980s and early 1990s, researchers hypothesized that aspirin could be used to prevent or delay the onset of preeclampsia. This hypothesis was tested in numerous trials which showed limited, but positive results. Subsequently, aspirin has been used in an attempt to improve pregnancy outcomes in women who have both antiphospholipid antibodies and a history of recurrent loss, and has also been used in an attempt to improve the success of in vitro fertilization. In theory, aspirin has both positive and negative effects on reproduction. Aspirin, which suppresses cyclooxygenase, has the potential to interfere with implantation, but also has the potential to support the maintenance of pregnancy. Aspirin is prescribed with increasing frequency to reduce the risk of maternal thrombosis and reduce the risk of miscarriage and poor pregnancy outcome. Aspirin alone, however, is not considered sufficient to prevent thrombosis and even in women with the antiphospholipid syndrome, the question as to whether low-dose aspirin improves pregnancy outcomes has not been answered affirmatively. Aspirin has potential risks. Aspirin inhibits platelet function and can contribute to maternal and fetal bleeding. Aspirin crosses the placenta. Although aspirin has not been associated with other congenital anomalies, it has been associated with an increased risk of vascular disruptions, particularly gastroschisis and possibly premature closure of the ductus arteriosus. Nonetheless, large trials demonstrate low-dose aspirin's relative safety and generally positive effects on reproductive outcomes.