Pregnancy “Due Dates”

So you’ve peed on a stick and it’s positive. How is your “due date” calculated?

One of the main tools used to calculate your “due date” (we prefer to use the term “guess date” as we know that only about 4% of women go into labour on their so called due date, is something called “Naegele’s rule”. This rule, invented by a German obstetrician who was alive from 1778-1851, assumes that pregnancy lasts for 40 weeks, and uses the first day of your last menstrual period (LMP) to determine your so called “due date”. This date will be 7 days + 9 months (or subtracting 3 months) from that LMP date. It assumes that you have a 28 day cycle and that you ovulated on day 14, which for many women, is incorrect.

Per Naegele’s rule, for example:

  • First day of LMP = 1 March

  • “Due date” = 8 December

This is equal to counting forward 280 days from the date of your last period. Not only is there a dispute about the interpretation of Naegele’s rule in the first place, but we do not have any current evidence to support its use. But the approach persists.

The “due date” used by your care provider may be based on Nagele’s rule, on information from conception if it was an assisted (IVF) conception, or alternatively, from a dating ultrasound (which is considered to be more accurate when completed in the first trimester of pregnancy).

Accurate as to what however?

All of these methods use a 40 week pregnancy as the basis of this date. But we know that “full term” is anywhere between 37 and 42 weeks, and that the evidence suggests that somewhere closer to 41 weeks is a more accurate average gestation (accounting for the obvious difficulties with data when we have such high rates of induction).

So why the obsession with the date, and what else do you need to bear in mind when thinking about when your baby may or may not come? More on this, and hot topics like induction of labour, to come.

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