When fertilization occurs, a sequence of events is triggered to prepare the body for pregnancy. The body starts to produce higher amounts of progesterone and the levels in the blood are kept at certain high level during the whole pregnancy.
In non-pregnant women the drop of progesterone in the second half of the menstrual cycle is actually a sign for the menstrual period to start. Because such a hormonal drop is missing in pregnancy, there is no menstrual period during pregnancy.
This might be helpful when there is a suspicion of pregnancy. if a woman starts her period after intercourse, then pregnancy might be ruled out.
However, a non-medical person might mistake something else for a menstrual period. Implantation bleeding, which happens when the egg implants in the uterine wall, is usually lighter then the normal period, but can be easily mistaken by an inexperienced person. You might want to refer to a gynecologist for more information on this issue.
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