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The Great Due Date Debate

5 min read

You’ve discovered you are pregnant and you’ve been told when your little bundle of joy will arrive! But how accurate is that date? Is there really a point to having a due date during pregnancy? It may give you an idea as to how long you have to decorate the nursery, shop for tiny cute things and complete your prenatal classes but you’ll soon see why I think a fixed due date actually does more harm than good.

A quick poll of two of my Facebook “mom” groups showed a huge range of actual birth dates compared to due dates. With just a small sample size of 34 women, there were babies born anywhere from 30 days before their due date all the way up to 14 days after! That’s a little hit and miss, wouldn’t you say?

So, how did we get to this point and how is your due date calculated, anyway?

Almost everyone, including doctors and midwives, will calculate your due date by adding 40 weeks (280 days) onto the first day of your last menstrual period (LMP). This calculation, also known as Naegele’s Rule, was first publicized by a German obstetrician by the name of Franz Karl Naegele, and has become the generally accepted way of calculating a woman’s due date.

There are a few factors that make Naegele’s rule somewhat flawed. Firstly, it assumes that every woman remembers the exact date her LMP started and that she has a perfect cycle, lasting 28 days, with ovulation happening on the 14th day. Secondly, some embryos may take longer to implant in the uterus, a factor that varies from woman to woman. Thirdly, it has never been clear whether or not Naegele actually intended to start counting from the first or last day of the LMP. In the beginning, up until the 1900s, it was common practice to add to the last day of your LMP. Then for some unknown reason, doctors changed and began counting from the first day of the LMP. This change has essentially caused a woman’s due date to be set 4–5 days sooner than it was originally intended to be, resulting in more “overdue” babies.

Starting in the 1970s, the use of ultrasound measurements have allowed for a slightly more accurate estimate of the date of baby’s arrival, if performed in early pregnancy. This method of predicting due dates quickly replaced Naegele’s Rule as the most reliable way to determine gestational age. Despite this increase in accuracy, the due date still has a 2-week margin of error! Ultrasounds performed in the third trimester of pregnancy are sometimes used to change the estimated date of delivery. This method, however, is even less accurate than early pregnancy ultrasounds because they are basing the date on the size of the baby as compared to a “typical” sized baby. If the baby is slightly larger than average, this could incorrectly move up the due date.

In 2001 a study by Gordon C.S. Smith was published on 1514 women whose LMP due date and first-trimester ultrasound due date were perfect matches. This research found that 50% of first-time mothers gave birth within 40 weeks and five days and 71% of women had given birth at 41 weeks and two days. Overall, the study suggests that due dates should be closer to 40 weeks plus five days.

So why should all this matter to you?

A major problem with due dates is that many women are unnecessarily having their labour induced… all because they passed that magical date. Women themselves are thinking that the baby is “late” instead of simply arriving when it is ready. According to the 2013 Listening to Mothers III survey, 44% of women said that they were induced because their baby was full term and it was “close to the due date.”

Risks associated with inductions can include:

  • hyperstimulation of the uterus (the uterus contracts too frequently, decreasing blood flow to the baby).
  • the use of extra interventions such as pain relief or continuous fetal monitoring.
  • a failed induction which may lead to a Caesarean.

According to an article written on the website Evidence Based Birth, the lowest C-section rates happen in women who go into spontaneous labour on their own. The next-lowest C-section rates happen in women who are induced for being late term or post-term. The highest C-section rates happen in women who choose to wait for spontaneous labour but end up having an induction later on.

Given the above information, I propose that we change the term “due date” to “due month” and recognize that there is a normal range of time in which most women will give birth!

About Christie Weber

Christie Weber is a registered doula who wants to help new and expecting parents access resources and information. Feeling that she might have made different choices during her labour if she had known more, she is producing Birth Fair, a two-day fun, educational event for all stages of conception, pregnancy and childbirth, Feb 25 and 26 at the Cloverdale Agriplex benefiting the BC Women’s Hospital Foundation.

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