Orgasm is known to cause contractions of the uterus. During orgasm, a woman’s internal and external reproductive organs become engorged with blood to varying degrees, contract and harden, and the muscles of her uterus also undergo rhythmic activity and contraction. That is why sex is forbidden during early pregnancy (first trimester) and late pregnancy (second trimester). In the first trimester, the fetus is still in an unstable stage of development, so if you have sex and have an orgasm at this time, it will cause uterine contractions and lead to miscarriage, while the late pregnancy may prompt uterine contractions that can easily lead to preterm delivery.