Our pregnancy due date calculator form calculates the conception date and the expected delivery date based on the first day of your last menstrual period. This is how most health care providers calculate your due date.
Note that the dates below are approximate calculations, the real date may be slightly different. Only about 5% of births actually take place exactly on the expected due date, most babies are born a week or two before or after.
How Is Your Due Date Calculated
Pregnancy starts from conception, when the man’s sperm and the woman’s egg meet together at the right time for fertilization. In general, a normal pregnancy can last about 40 weeks from the first day of the last period, or 38 weeks from conception.
Your due date is calculated by adding 280 days (40 weeks) to the first day of your last period, considering a cycle length of 28 days. If your cycle is different and you know its exact length, then we will calculate your due date by counting 266 days (38 weeks) from your conception date.
Conception occurs only at the time of ovulation and fertilization of the egg (when a sperm meets the egg). And this will only happen on one day, which can be so difficult to pin down since we don’t know exactly when a woman ovulates.
When are you most likely to conceive?
For most women, ovulation happens around 14 days before menstruation starts. Because sperm live 3 to 5 days, conception is more likely to occur when sex takes place a few days before ovulation.
If your average menstrual cycle is 28 days, you will ovulate on about day 14, and your most fertile days will be 10, 11, 12, 13 and 14. Conception is also possible after ovulation, but is limited to 12-24 hours after the egg has been released from the ovary.
Your Due Date Is Only An Estimation
Estimating the expected due date and the conception date is rarely exact, because it is difficult to know exactly when ovulation occurs. This also depends on the length of the menstrual cycle, which may vary from woman to woman.
Obviously, your due date is only an estimation. Most health practitioners use the first day of your last period as the starting point of your 40-week pregnancy. But it is also very probable that you will start giving birth any day within a week or two before or after.
On the other hand, if you can’t remember the first day of your last period or you have irregular periods, an ultrasound is one of the most accurate ways to estimate your gestational age.