There are several "due date calculators" out there, and they all pretty much seem to give the same due date ... except in France!

See a striking example: http://www.babycenter.com/pregnancy-due-date-calculator It calculates the due date to May 26.

Then go to the French version of the SAME site: http://www.babycenter.fr/t400215/calculer-votre-date-daccouchement It calculates June 2nd.

Same goes if you go to a French doctor vs a US doctor (or Israeli, where I am).

This is science, biology, right? Why would there be a difference?

  • 5 or 6 days is not a striking difference. That due date has an accuracy measured in weeks, not days
    – Chrys
    May 23, 2014 at 11:15

2 Answers 2


The reason for these is around the way they estimate the date of conception. In general, the first missed period is a useful indicator, but at what point in the previous few weeks do you set the conception point? It isn't obvious.

So although pregnancy is about 40 weeks, the start point may be guesstimated anywhere in a 2 or 3 week window, and that is obviously what has been done here.

I don't know why it is done differently in France (or if it is possibly just down to the calculators you have tried) but the thing to remember is:

  • odds of baby turning up on the stated due date are not that good anyway

So don't worry about it - during pregnancy checks, the due date estimate can be revised based on the development of the fetus, and he/she will turn up when ready :-)

  • 1
    "odds ... not that good" : statistically, about 4% of the due date being correct (in the UK, anyway). The BBC covered this a few months ago: bbc.com/news/magazine-31046144 May 28, 2015 at 18:03
  • Good to have an actual number - thanks Graham
    – Rory Alsop
    May 28, 2015 at 18:37

When we took a birth preparation course, the midwife (being close to retirement) given the course told us that she loved the french system as most babys born (here in Germany) where expected too early, as they often are still covered with vernix. The French system expects them a little later. In fact, she told us, that especially for the first child, some extra days were sometimes added.

As the labor is usually induced if the due date has been passed by a certain amount of days, she told us that in her experience, the French system was more accurate as she had the impression the babys in Germany, today, are "fetched" a little too early.

However, she didn't know what the reason of the difference is, I guess it comes down to experience and history. I'm unsure what the medical/general cosensus is here.

  • That's interesting. I think that the reason for inducing early is that babies who are too overdue eventually start overwhelming the ability of the placenta and the other systems there to clean out waste; I would assume the "no later than" date would be determined scientifically based on that. But the due date itself might lead to more inductions or voluntary C-sections on that day (so, earlier in the non-French systems) since that might be chosen by those who prefer to do things that way.
    – Joe
    May 28, 2015 at 20:04

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