Calculating Baby’s due date

Ah, the due date. That day, month, and year is going to etch itself deep into your mind as you traverse the nine months until delivery, but how do you figure out when it is? Babies on average need about 38 weeks in the womb from the time of conception, but you won’t really know the exact date when that magic happened. So healthcare providers have several different methods that they can use to determine when Baby is due.

Last menstrual period (LMP)

Most healthcare providers will use this method because the vast majority of moms-to-be don’t know exactly when they conceived. When calculating using this method, healthcare providers count 40 weeks forward from the first day of your last menstrual period, a figure that includes the 38 weeks of gestation plus an assumed 2 week span between your LMP, and ovulation. While this method works well for women who have 28-day menstrual cycles with ovulation squarely in the middle (day 14 or 15), most do not, so your healthcare provider may have to re-adjust your due date further down along line.

Date of conception

If you know the exact date of conception, you can simply add 38 weeks, and you’ve got your due date. This should make calculating a due date easy, right? Well, it’s not that simple. But why? It’s because the exact date of conception is really hard to know.

Conception is when sperm meets egg. And, remember, intercourse or introduction of sperm can lead to pregnancy any time in the five days leading up to ovulation, which is when an egg is released from an ovary. So if, for example, you had intercourse once during your fertile window and conceived, the actual fertilization may have taken place as many as five days after that intercourse.

To sum up, after you have sex or introduce sperm, sperm might hang around for a few days before an egg arrives and conception actually takes place. Since you can’t get a handy dandy text notification or receive a status update to know exactly when – hooray! – sperm and egg have finally met, it’s hard to have more than an estimate of when conception takes place. But a due date is just an estimate anyway.


Your healthcare provider will monitor Baby’s size and development throughout your pregnancy, and will adjust your due date if he or she has any reason to (e.g. if Baby is smaller than a typical fetus at an 8-week ultrasound). Because the normal variance in size between babies increases as pregnancy progresses, earlier ultrasounds are generally regarded as more accurate in determining due date.

Put it in perspective

Just remember: your due date is an estimate, not an exact calculation. In fact, 95% of babies aren’t born on their due dates. Due dates are simply there to help you prepare for Baby on a timetable, and for your healthcare provider to monitor Baby’s size and health against the averages.

Reviewed by Dr. Jamie Lo

Read more


  • “What to Expect After Your Due Date: FAQ069.” ACOG. American College of Obstetricians and Gynecologists, 8/11/2015. Web.
  • “Due date statistics: A study on the length of pregnancy.” SpaceFM. SpaceFM, n.d. Web.
  • “Due Date Calculator.” March of Dimes. March of Dimes, n.d. Web.
  • “Ultrasound during pregnancy.” March of Dimes. March of Dimes, 10/14/2015. Web.

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