There are no studies that found a higher chance of pregnancy loss, preterm delivery(delivery before 37 weeks of pregnancy), or low birth weight when ondansetron was taken during pregnancy.
At the higher doses, there are several reports showing that ondansetron use might cause a heart rhythm problem(called QT interval prolongation) in the person taking an abnormal heart rhythm known as Torsades de Pointes. If you are taking this medicine you can ask your doctor about how to notice changes in your heart rhythm.