Calculation for Pregnancy Due Date
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Calculation for Pregnancy Due Date

When you first learn you are pregnant, one of the first things you want to know is when you’re due. You can find due date calculators online, but your doctor will be able to give you the most accurate date. You must take a few details into consideration when calculating your due date.

How It’s Done

The gestational period for a human baby is 280 days. One of the easiest ways to calculate your due date is to add seven days to the date of your last menstrual period, then count back three months. For example, if your period started on July 12, you would add seven days to get July 19, the count back three months to have a due date of April 19. In the doctor’s office, your doctor may use a “pregnancy wheel,” which essentially does the same thing in an easy-to-view format.

Ultrasound Date

During the first trimester, your doctor will probably order an ultrasound to determine the ultrasound date. The technician will measure your baby from crown to rump and then use that data to determine your due date. Usually, it is around the same due date that you determined using the first method. If you are not sure when you last had your period, however, this can give you a more accurate due date.


Your due date is only an estimate of when your baby will be born. Your baby may be born sooner or later. In fact, the Healthline website reports that only about 4 percent of women deliver their babies on the due date.


In America, a pregnancy lasts nine months. A baby that was conceived in July will be born in April. However, lunar months are only 28 days, rather than the 30 or 31 days in the Gregorian calendar. Some countries, such as Japan, count the lunar month when talking about pregnancy, stating that a woman is pregnant for 10 months.


The typical calculation for pregnancy due date is based on the average cycle of 28 days. If your cycle is much longer and shorter than that, you may need to adjust the date, subtracting days if you have a short cycle and adding days if you have a long cycle. Discuss your concerns with your doctor if your cycle is far from average.

Photo Credit

  • pregnant woman image by Valentin Mosichev from
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