When’s the child due? How long have you been pregnant? Which week are you in?

These are probably the most asked questions following people’s congratulations on your pregnancy.


“October! I’m currently 6 weeks pregnant and there are 34 more weeks to go”, a typical answer taken from the numbers your doctor gave you at your last visit. But have you wondered about how your due date is calculated and if the dates make sense? Here we will be explaining the 3 most common due date myths about 40-week pregnancy and how due dates are calculated.

Myth 1: How is your Due Date estimated?

Time for some history: the commonly known Naegele’s rule was made known to the public in 1812 by a German doctor named Franz Naegele. According to Naegele’s rule, a woman’s due date is calculated by adding 9 months and 7 days to the first day of her last menstrual period. The assumption is based on women having a 28-day menstrual cycle with ovulation happening on Day 14. But this poses some uncertainties:

  • Not all women follow an exact 28 days of the menstrual cycle
  • Not all ovulations happen on Day 14
  • What if you don’t know when your last period happened?
  • What if you have irregular periods?
  • Every baby is different, not all children are born after 9 months and 7 days

The due date is only an estimate. Based on statistics, there is only about a 4-5% chance that your baby will be born exactly on their due date. Most women, around 80%, give birth between 38 to 42 weeks to a full-term baby, and the rest of 15% are more likely to give birth to a preterm or overdue baby. It is still very important to know your baby’s progress and when your baby will be born, but don’t get too clung on the idea of an exact due date.

Myth 2: Why does my doctor change my Due Date?

Doctors usually estimate your due date based on the dates of your last menstrual cycle before your fetus can be spotted in the ultrasound. At 6-9 weeks in the first trimester, your doctor will order an early ultrasound to take a glance at the tiny fetus that is now visible. The length of the fetus, also known as the Crown-Rump Length (CRL), will be measured to estimate the gestational age of your baby. This is when your initial due date may be overthrown if your fetus is significantly smaller or larger than the average fetus at your current stage of pregnancy.

The due date estimated from your first-trimester ultrasound will be the most accurate. Why? Your baby starts growing at different rates starting from 20 weeks of gestation and it can be hard to calculate a correct estimate based on the CRL alone. It becomes even harder in the third trimester, therefore having your first ultrasound taken in your first trimester is highly recommended.

Myth 3: Does it mean I’m overdue if I’m more than 40 weeks pregnant?

The traditional belief for decades is that pregnancies will last 40 weeks and should not last longer than 42 weeks or you’d be overdue. But a study in 2013 conducted by the US National Institute of Environmental Health Sciences has challenged the idea with their findings. From the 125 cases of pregnancies included in the analysis, the team found that due dates can vary up to 5 weeks, rather than just 2 weeks from our common knowledge.


They have also identified that the length of gestation can vary depending on the age and physical condition of the mother at the time of conception. Mums pregnant at an older age or with a heavier weight were more likely to have longer pregnancies.


Embryos that took longer to implant were delivered after a long period of pregnancy. They also found a correlation between the length of your previous pregnancies or subsequent pregnancies to the current one being studied, advising a consistency in the length of pregnancy in individual mums.


Key Takeaway: Even though the sample group was not big enough to make any clinical recommendations, mums should not be as worried about being overdue after 40-42 weeks of pregnancy for the natural variability can be greater than we think. That being said, you should still always monitor your baby’s condition and consult your physician about any risks of being overdue. It is okay to bring out a second opinion when your doctor suggests inducing labor with no other particular concerns than your pregnancy passing the 40-week mark.