Knowing your accurate due date is important, not only because you are likely curious when to expect your little bundle of joy; it is also helpful and important to understand if your baby is early to arrive or past its due date and what that may mean for the two of you.

So, how is my due date calculated?

The date that a baby is due, also known as the EDD (Estimated Due Date), used to be calculated with “Naegel’s Rule”, coined by a German OB all the way back in 1812.  You take the first day of your last period, add 7 days, then count backwards 3 calendar months. For example, if your last menstrual period began on July 20th, you would add 7 days to get July 27th, then count back 3 months to get an EDD of April 27th (that’s around 280 days or 40 weeks). And yes, this 200+ year old rule is still used today, on occasion! That’s why many OBGyns and midwives will ask for your last menstrual period during your first prenatal visit.

Though fairly accurate, the rule lacks precision: it does not account for different cycle length (and subsequently different times of ovulation which is when your baby is conceived)¹. That means that your due date may be off by a couple of days or even a few weeks, which can be a big deal.

Today, most pregnancies are dated when the first ultrasound is done, usually somewhere between week 6 to week 12 of pregnancy, but possibly later with a little less precision. Embryos develop at a fairly consistent rate during the first trimester which makes measuring during that time period a quite accurate predictor of your due date, give or take a day or two.

If you used IVF or IUI  to conceive, your doctor will know the exact date of conception. Adding 266 days to this date will give you and your care team the precise due date, based on the average length of pregnancy. You may have also been tracking your hormones at home and know your ovulation date. Though many doctors won’t accept your own reported ovulation date to calculate your due date, it’ll give you a good gauge of how accurate your ultrasound estimate or Naegle’s Rule is.

Swell, so I know when baby is coming…not so fast:

A study conducted by epidemiologist Anne Marie Jukic et al at the National Institute of Environmental Health Sciences followed mothers to determine what contributes to natural variation in pregnancy length. The study found only 4% of mothers deliver on their due date, and around 70% of mothers deliver within 10 days of their due date². About half of all mothers have delivered by their due date, but the other half is still waiting.

There are a variety of factors that can affect the length of your pregnancy, making it slightly shorter or slightly longer than the average. First-time moms often have a slightly longer gestation period than mothers who have already given birth before. Above mentioned study also concluded that longer pregnancies were associated with higher age and body weight². Another factor that might affect pregnancy length is ethnicity: on average, black and Asian women naturally deliver slightly earlier, around 39 weeks, while white European women deliver slightly later, around 40 weeks³. And then of course, there is your and your baby’s individual make-up that will affect your pregnancy length, as well as your individual circumstances, such as your support structure. As an example, women that have support during pregnancy, like a birth doula,  are 10% less likely to deliver prematurely than women with less or no support.

The bottom line: your due date is just an estimate based on averages and it will never be 100% accurate for you as an individual; however, it gives you at least an idea when to expect your bundle of joy.

So what does that mean for me?

If you can, take your due date with a grain of salt and don’t focus quite so much on the exact date (and yes, that’s really hard when everybody is asking for your due date and the days feel like weeks). Your baby and your body will know when it is time to go.

According to the American College of Obstetricians and Gynecologists, a pregnancy is considered to be “term” between 39 to 42 weeks  (so up to 2 weeks past the calculated due date)⁴. If you know your due date reliably, it’ll help you understand during what time period you are likely to go into labor, and what to expect if you fall outside of that timeframe.

If your baby is born prematurely, which is true for about 10% of babies in the US today, your baby may need to spend some time in the NICU (neonatal intensive care unit) to be able to thrive on its own. Knowing your due date will help you and your provider evaluate up-front the potential need for additional support and can help you prepare mentally, emotionally and maybe even logistically.

Most women will go into labor on their own by or before 42 weeks of their pregnancy.  If your pregnancy, however, goes past your due date, discuss your next steps and timeline with your midwife or doctor.

In any of the above scenarios, your doula is a great resource to help you discuss your personal circumstances, to assess early signs of labor, and to get the support you deserve.


*Disclaimer: Any content provided by is intended for informational and educational purposes only and should not be used as a substitute for personalized medical advice by your doctor, midwife, or other healthcare professional. Click return to homepage.

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