The question on the minds of 8 out of 10 women who have just gotten a positive test is how to calculate the due date. It seems like alien math—a bunch of numbers and formulas that seem almost impossible to deal with, just like doctors do. Even for them, calculating the estimated due date (EDD) can be a bit tricky. But the truth is, calculating your due date can be easy if we use a few methods.

Pregnancy lasts approximately 280 days. For some women it lasts longer (due to late ovulation and implantation) and for others, less time if they have shorter cycles. However, the difference is not very big1.

 

Information for calculating your due date

To calculate your due date, you need to know the date of your last period, commonly called the LMP (last menstrual period). The LMP is essential for women with regular cycles. With it, it’s possible to calculate exactly when the baby might be born, and also from when a woman should start paying attention to signs of labor2.

This is because of the previously mentioned period of 280 days. For the baby, being born between 38 and 42 weeks of pregnancy is normal and safe. Beyond or below this window may cause problems for the baby, such as being born prematurely and not having fully developed lungs, or suffering fetal distress because the labor is late.

How to calculate the due date using your LMP

To calculate the due date using the last period, subtract the length of your menstrual cycle from 280 days. For example, if your cycle is 28 days, the calculation is as follows:

  • Set your last period date = September 1, 2015
  • Add 07 days = 01 + 07 = September 8
  • Subtract 90 days or 3 months = July 8, 2015
  • Add one year = July 8, 2016

So, by your period date, you would expect your baby on July 8, 2016. This is if your period date is accurate and there’s no difference between your actual and expected ovulation dates.

Generally, women with irregular cycles can only know their baby’s estimated due date through an ultrasound scan. The best ultrasound scan to consider is the very first one, ideally before the 9th week of pregnancy. It will tell you for sure when the baby was conceived and the exact fetal age. Unless you are having your first ultrasound much later in pregnancy (after 20 weeks), it’s possible to precisely determine when the baby was conceived, or at least the most likely days.

Chart to determine your estimated due date

The chart below is from the book Grávida e Bela3. When online calculators weren’t available, this was how I double-checked the calculations my doctor made. It always worked, even if there was a difference at the end of the pregnancy. Just look for your last period date above (month and day) and below, your estimated due date.

And finally, to make things easier, the due date calculator. These are reliable as long as you have regular cycles. Many websites offer a due date calculator, but here on this site you’ll find a very detailed one that even shows when each trimester ends and when the baby was likely conceived.

We should always remember that the calculation can vary by 15 days more or less, so keep an eye out if the date goes beyond that. Also, talk to your doctor about your due date and any discrepancies between your ultrasound and LMP. Differences of a few days are normal and not a problem, but if the difference is more than a week, it should be considered in calculating your due date. You can use the Famivita calculator by clicking here.

Also see: Cervix Effaced – It’s Time for Labor!

Photo: Grávida e Bela, phalinn