However, if you want to know how what is behind a due date predictor calculator, its easy to understand.While you may not know the exact date of conception, you can work out your due date from the date of you last period. Your last period is the benchmark date that everyone uses as the “beginning” of pregnancy. Since pregnancy lasts around 280 days, give or take, or 40 weeks – then all you do is recall the first day of your last period – and add 280. This calculation gives you the gestational age.

By the way, if your doctor mentions “ovulatory age” then all they are doing is using your ‘conception’ date – about 2 weeks before your last period – as a due date predictor.