Le Lévitique 15:18 Halakha : Mishneh Torah, Heave Offerings & Sefer HaChinukh

When an Israelite has relations with a daughter of a priest, we do not suspect that she became pregnant. Instead, she may immerse herself [in the mikveh] and partake [of terumah] in the evening. If she was married to an Israelite and her husband died, she may immerse herself and partake of terumah in the evening [and continue doing so] for 40 days. If her fetus is recognized [afterwards], retroactively, her [actions] are objectionable from the fortieth day onward. For, throughout the forty days, the embryo is not considered as a fetus, merely as water.

To not have intercourse with a menstruant woman: To not have intercourse with a woman when she is menstruant, as it is stated (Leviticus 18:19), “And to a woman in menstrual impurity, you shall not approach.” And the time of her menstruation continues for seven days, as it is written (Leviticus 15:19), “seven days shall she be in her menstruation.” And whether she [experiences blood] once during them or gushes all seven, she is a menstruant (Niddah 73a). And the entire time that she does not immerse (in a mikveh) — even after the seven — she is [considered] a menstruant; since the verse made it dependent upon days and immersion, as it is stated with impure people (Leviticus 15:18), “and they shall wash in water.” And they, may their memory be blessed, said [that it is] a constructive paradigm (binyan av) to any impure one, that he is with his impurity until he immerses. (See Rabbenu Bachya on Leviticus 15:19 in the name of the Geonim.) And likewise, they, may their memory be blessed, also expounded (Shabbat 64b), “‘Shall she be in her menstruation’ — she shall be in her menstruation until she goes into water.”