Diethylstilbestrol syndrome (DES syndrome) refers to developmental or health problems caused by exposure to DES before birth (in utero), such as reproductive tract differences, infertility, and an increased risk for certain cancers. DES is a synthetic form of the female hormone estrogen that was prescribed to pregnant women between about 1940 and 1971 to prevent miscarriage and premature labor. DES also may have been prescribed to women to inhibit lactation, as hormone replacement therapy for menopause symptoms, and as a post-coital emergency contraceptive (“morning-after pill”). It was first thought to be safe and effective, but studies later found that DES was not effective in preventing pregnancy complications, and caused health problems in some children of mothers exposed during pregnancy, as well as some of the women exposed during pregnancy or for other reasons. It is important to note that not all people exposed to DES have health problems due to the exposure. Females exposed to DES in utero may have reproductive tract differences affecting the uterus, fallopian tubes, cervix, and/or vagina. Examples include incomplete development of the uterus or cervix, a differently-shaped uterus or cervix, and a transverse vaginal septum. These differences may increase the risk of infertility. Females exposed to DES in utero also are more likely to experience various pregnancy complications including ectopic pregnancy, miscarriage, and preterm birth. However, most have healthy babies. Additionally, females exposed in utero are at increased risk to develop cancers such as vaginal or cervical clear cell adenocarcinoma, squamous cervical cancer, and breast cancer. Males exposed to DES in utero have not been studied as extensively as females, but may be at increased risk for epididymal cysts, undescended testes, and inflammation or infection of the testicles. Males exposed to DES do not appear to have an increased risk of infertility. Of note, women exposed to DES after birth (those exposed while they were pregnant or those exposed for other reasons) have an increased risk of developing breast cancer. The children of women exposed to DES in utero (grandchildren of mothers exposed during pregnancy) may also have an increased risk for cancer, as well as higher infertility rates. Treatment for health problems associated with DES exposure depends on each person's signs and symptoms. Women who took DES during pregnancy or for other reasons are encouraged to inform their doctors and children of the exposure, and children exposed in utero should inform their doctors so they can be monitored for possible health problems that may arise.