Because most women don’t know the exact date of conception, due date calculators typically use the first day of the last menstrual cycle and tack on 40 weeks. This estimate leaves a two-week grace period (be it early or late), making for less than accurate results. Even if you do know the date of conception, sperm can hang around up to five days waiting to fertilize an egg, so there’s still a little wiggle room.
The best bet
For the most accurate prediction of baby’s arrival, the American Pregnancy Association recommends scheduling a prenatal appointment between 8 and 18 weeks’ gestation. Fetuses develop at approximately the same rate during that time, so there’s a greater chance of a correct guess. After the first trimester, however, genetics take over, and it becomes harder to determine baby’s arrival.
The missed mark
If your due date comes and goes with nary a contraction felt, take comfort in knowing there’s likely no cause for concern. Nearly all women give birth by 42 weeks, and you’ll probably be induced if you haven’t. It could be that the date was miscalculated or that your bun needs to bake just a little longer. Rest assured he won’t stay in there forever—and will be worth the wait.