It is not possible to conceive owing to an intercourse that takes place a few hours before the periods. That is because roughly around two weeks before the period is the time when ovulation takes place in a woman's body and the ovulation (release of an egg) is a vital step for pregnancy to occur. Basically the period after ovulation is what is termed as Safe Period by people use natural methods such as the calendar method to time intercourse as a means of contraception.
The only alternative possibility is if a woman has breakthrough-bleeding during ovulation and mistakes it for a period or if she has a post-coital bleeding (seen in women with pelvic infections, cervical ersosions etc. ) and mistakes it for a period. These bleeding episodes are typically scantier than the regular period.
Hope this information helps to clear your doubts.
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