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Updated: Feb 1

You always hear that pregnancy is 9 months long, but no one tells you that it’s a FULL 9 months. That means if you go overdue, like many first-time parents, then you’re actually into the 10th month. So, how do we settle on a due date?

Last Menstrual Period, plus 9 Months and 7 days, equals your Estimated Due Date

Calculating your due date is based on the first day of your last menstrual period (LMP). Halfway through your cycle (approximately 2 weeks in), you ovulate and the egg can be fertilized. This means that conception has not even happened during the first two weeks that count as Month 1.

Add 9 (full) months onto the date of the first day of your LMP.

Let’s just really rub it in here… add an extra week onto that date to make up for one of the *bonus* weeks you had at the start.

Your Estimated Due Date. Estimated… not what anyone really wants to hear but it’s a date to base things off. (Only about 4% of people give birth on their EDD!)

So, as an example: if your LMP was August 1, 2019, add 9 months (May 1, 2020), then add 7 days which brings you to May 8th, 2020.

What if you don’t know your LMP?

Likely you will be sent for a dating ultrasound. Dating ultrasounds are not routinely done in all parts of Canada. These ultrasounds, performed early in pregnancy, measure the crown to rump (head to bum) length of babies and are considered the most accurate before 12 weeks gestation. Does anything about this surprise you? Did you go before or after your due date? Let me know below!


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