The calculator works on according to medicine's most accepted calculation, according to which a pregnancy lasts for 266 days from conception to birth. Conception is likely to take place during ovulation, which should take place on the 14th day of the 28 day period cycle.
But since, we often only have exact information about our last period, and not about he moment of conception, our calculation must begin with information about the last period. In this way, the length of a pregnancy is is calculated to be 280 days or 40 weeks after the last menstruation cycle.
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