The bizarre and imprecise science of baby due dates

The science behind baby due dates is centuries old and frequently inaccurate.
The science behind baby due dates is centuries old and frequently inaccurate. Photo credit: Getty

Jacinda Ardern is currently in a position common to many first-time mothers. It's three days after her due date, and there's still no sign of baby.

The Prime Minister was expected to give birth on June 17, according to a confusing and arguably ineffective calculation from the 1800s.

Naegele's rule, named after German obstetrician Franz Karl Naegele, estimates that a woman will give birth 280 days from the first day of her last menstrual cycle.

Bizarrely, this means that the 40 weeks of a standard pregnancy begin before the baby is even conceived.

The Naegele's rule calculation is wrong far more frequently than it's right: just three percent of babies are born on their estimated due date (EDD).

So why is it still used when the level of accuracy is so low?

Jacqui Anderson, midwifery adviser from the College of Midwives, agrees Naegele's rule is "not a perfect science".

"But it's about as accurate as you'll get."

The calculation is all based on averages, so a period-disrupting illness around the time of conception can throw the whole estimate off. If women aren't sure of the exact dates of their period (and let's be real, who is?) that can also compromise the accuracy of their due date.

"Women ovulate at different stages in the first two weeks of the cycle," explains Ms Anderson.

She says the length of a menstrual cycle can also vary wildly - while the average is 28 days, some women have 30-day, 35-day, even 42-day cycles.

"In the old days they just went by the phase of the moon."

Scans, which are also used to calculate due dates, can be unreliable if they're done too early or too late in the pregnancy.

The accuracy of a scan can vary anywhere from four or five days up to two weeks either side of the EDD.

First-time mothers are more likely to have overdue babies, but no one's quite sure why. Ms Anderson says it could be because the body isn't used to experiencing pregnancy so it takes a while for the hormones and systems to kick into gear.

Ms Ardern told The AM Show she'll induce labour if the baby takes too long. It's generally not recommended to induce before the 41st week of pregnancy, so she's still got a few days to go into labour 'naturally'.

Inductions basically trick the body into thinking labour has started, by the use of artificial hormones, a balloon catheter in the cervix (yikes) or forcibly breaking the waters with a tiny hook (also yikes).

There are some risks that come with induction, Ms Anderson says. It might not work, requiring the mother to eventually undergo a caesarean. Or it might work too well, causing too many contractions which can create extra stress for both mother and baby.

If the water is artificially broken and the induced labour takes a long time, this can leave the baby unprotected and at greater risk of infection.

Women who are induced sometimes require an IV drip, which restricts their movement and often means they find it difficult to "get comfy" - as comfy as is possible during childbirth, at least.

Whenever the Prime Minister gives birth, it probably won't be a minute too soon for her.


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