Lev 20:18 If a man lies with a woman during her menstrual period and has intercourse with her, he has laid bare the source of her flow and she has uncovered it. The two of them shall be cut off from the people.
Lev 15:24“’If a man has sexual relations with her and her monthly flow touches him, he will be unclean for seven days; any bed he lies on will be unclean.
Commentators agree that in the first case (20:18), both husband and wife were aware menses had started before engaging in intercourse, whereas in the second, (15:24), they were not i.e. her flow had not yet started and was probably precipitated by the sexual act itself.
“There are no research studies that demonstrate a definite relation to having sex and altering the start date of menses. That said, it is well known that orgasms (associated with intercourse, or without) cause contractions within the uterus as part of the autonomic response associated with that event....In theory, those contractions may precipitate the start of menses. For women that engage in penile-vaginal intercourse, semen causes the cervix to soften slightly, due to a type of chemical called a prostaglandin that is present within the ejaculate. This, coupled with orgasm, could potentially lead to the start of a period.”