There is limited information concerning the effects of chemotherapy administered during pregnancy, which consists mostly of case reports and small series. The National Cancer Institute maintains a registry of neonates exposed to chemotherapy but there are currently only several hundred cases. When chemotherapy is used during embryogenesis, there is an increased rate of spontaneous abortions and major birth defects. The risk of fetal malformations when chemotherapy is administered during the second and third trimesters is probably not greater than the background rate. Use in the second and third trimesters may increase the risk of premature birth, fetal growth restriction, maternal and fetal myelosuppression, and still births.