How to Calculate Your Pregnancy Due Date

Calculation of the pregnancy due date is the single most important part of prenatal care.Everything in pregnancy depends on what your exact due date is and how many weeks along the pregnancy has come.

Decisions to treat, test, and deliver depend on knowing the exact due date.

Determing the due date is done best early in pregnancy. A sonogram done before 12 weeks can estimate the due date within 5-7 days, between 12 and 20 weeks the range of error is about 2 weeks, and after 20 weeks the range is 3-4 weeks.  That is why it is important to make a clear determination of the exact due date as early as possible in pregnancy.

In order of accuracy, here are the ways to determine the due date:

1. In-Vitro IVF transfer day (Correct to the due date day)
2. Basal body temperature chart (Correct to a few days)
3. Ultrasound before 6-8 weeks (Correct to about 5-7 days)
4. Ultrasound 8-12 weeks (Correct to about 7-10 days)
5. Ultrasound 12-24 weeks (Correct to about 2 weeks)
6. Ultrasound after 24 weeks (Correct to between 3-4 weeks) 
7. Last menstrual period (Correct to within a few weeks)
8. Size of uterus (not that correct)
9. First positive pregnancy test (only if there was a negativce test within a week before)
10. hCG levels (only works before 6,000 mIU/cc and then only within a few weeks of accuracy)

Using a due date calculator is just one of many ways to determine the exact due date. The due date calculator will display the estimated date of ovulation and the expected due date of your baby. This estimate is based on the last menstrual period and the estimated date of conception.

One important tool to calculate the due date is the BabyMed due date calculator, in which you can enter the first day of your last menstrual cycle and other information.

Your pregnancy due date is calculated by adding 40 weeks (280 days) to the approximate first day of your last menstrual period or 266 days to the day of ovulation.

Only 5% of women deliver their babies on their projected due date, so this date is just an estimate and delivery usually happens within 2 weeks before or after the due date.

Do you know your due date but want to know exactly when you did get pregnant? Try our Reverse Due Date Conception Calendar and Calculator. There, you can enter your due date and we will calculate the exact days of conception. More importantly, we will provide the range of dates that you may have made love and got pregnant.

Our pregnancy due date calculator will calculate your due date based on your own data. The due date is calculated first by calculating your date of ovulation and fertilization (14 days prior to your next estimated period), and then by adding 266 days to that date.

Just enter the first day of your last menstrual period plus the average length of the menstrual cycle and we will calcuate the due date for you.