Calculating Estimated Due Dates for Pregnancy

Calculate your baby's due date.

Pregnancy is a time of anticipation and preparation and every eager mother-to-be wants to know when she can plan on the arrival of her bundle of joy. Calculating the estimated due date is based upon certain criteria, such as the date of the mother’s last menstrual period (LMP) but if the mother is unsure of that date or if she has irregular periods, estimating the due date is harder. Using some mathematical techniques, we can come up with a likely date but only 5 percent of babies are born on their due date.

Instructions

1. Figure out the first day of your last menstrual period, if possible. After pinpointing the date, count forward exactly 40 weeks. This will give you the estimated date for your baby’s birth. This calculation works well if you have regular periods.

2. Count forward from your ovulation date if you know it. For couples who follow the Basal Body Temperature method when trying to conceive, they have a pretty accurate idea when the female ovulates. The actual gestation period for humans is 266 days so if you known when you last ovulated, count forward 266 days and you will have a fairly accurate due date.

3. Schedule an appointment with your obstetrician for a fetal ultrasound if your periods are irregular and you have no idea when your last period occurred. During an ultrasound, the doctor will use sound waves to measure your baby’s head and he will make a determination of how far along your pregnancy is by comparing your baby's head size to the national gestational standard.

4. Use an online calculator if you know your LMP and you want your estimated due date figured for you. These calculators are slightly more accurate for women who have regular periods since they include the length of time between their periods as a factor.

5. Figure out the exact date of your baby’s birth if your obstetrician recommends birth by cesarean section. He will schedule a date on which he will induce your labor, usually one to two weeks prior to the time he feels you would go into labor naturally.

Tips & Warnings

• Expect to go into labor up to two weeks before or after your estimated due date. More births occur a few days early than they do late and if you’re a week past your due date; your doctor will monitor your progress closely.