Your body starts producing pregnancy hormones immediately after a sexual encounter that results in pregnancy, says Dr. Manijeh Kamyar, maternal-fetal medicine specialist in obstetrics and gynecology at the University of Utah. An egg can be fertilized as soon as one hour after sex, and up to 24 hours after sex. Once the sperm has fertilized the egg, the pregnancy is initially supported by the corpus luteum, which supports the pregnancy by releasing progesterone. "Your baby releases human chorionic gonadotropin, which helps to ensure that the corpus luteum will continue to secrete progesterone," says Kamyar. "Around the eighth week of pregnancy, the placenta is able to take over the production of these pregnancy hormones."