To find out the due date for a baby, it is possible to use a due-date calculator online to estimate the day, reports BabyCenter. Another alternative is to do a manual estimation.
To use a due-date calculator, a person enters either the date of conception or the date of the first day of the woman's last period, says BabyCenter. To do a manual estimation, health care providers say to count 40 weeks from the first day of the woman's last period. If the exact date of conception is known, which is rare, then the person reduces the number of weeks to 38.