The main reason is that the embryo is still unstable in the first trimester, so having sex may cause the smooth muscle of the uterus to contract and cause the gestational sac to peel away from the uterine wall, which may result in vaginal bleeding, preterm miscarriage, lower abdominal pain and other symptoms. If you have intercourse in the second trimester of pregnancy, it may cause premature birth, intrauterine infection or premature rupture of membranes, so it is recommended not to have intercourse in the first and second trimesters of pregnancy, and during the rest of the time if you have sex, you must pay attention to the frequency of sex, pay attention to the hygiene of sex, and be gentle during sex.