Thanks for your query.
First of all understand the cycle, the ovulation that occurs around 14 days before the next expected day of period; the 3 + and - days is most fertile period. The pregnancy occurs only if you have sex during the ovulation period.
If you have a 28-day normal period cycle, it is not possible to have pregnancy if you have unprotected sex on the last day of the period.
I hope this answers your query.
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