In general, sex in your third trimester is safe, according to Dr. Alberto Del Pilar Jr., a family medicine practitioner in Oswego, New York. He notes that there is a small possibility that prostaglandins present in semen may trigger contractions in your third trimester. Prostaglandins encourage cervical ripening and stimulate uterine contractions. "This is more prevalent in women who have had premature labor in previous pregnancies or have a strong family history of pre-term labor," Del Pilar explains. He advises that if you have a personal or family history of delivering prior to term, you may want to avoid having sex in your third trimester.