At late stage of pregnancy, sex can help to get you into labor, but only if your body is ready to do so. Interestingly enough, sperm contains the same hormone prostaglandin as the synthetic counterpart used to induce labor. Also, when you are aroused, your body will release the hormone oxytocin. This helps to stimulate your uterus and start contractions. However, if you have a condition called placenta praevia, or a history of miscarriages, bleeding in pregnancy, or premature labour, you might be better avoid sex. If you are not sure, always consult your midwife or doctor.