One of the first things I did when finding out I was pregnant was calculating my due date. In the US and UK, the due date is calculated by counting 40 weeks from the first day of the woman’s last menstrual cycle. This includes 2 weeks until conception and 38 weeks of gestation.
When I went to see my doctor in France for the first time, I was confident that my due date was October 3. To my surprise, he said that it was October 10. After disbelief that my French doctor was wrong, I knew there had to be a logical explanation why he was calculating 41 weeks instead of 40 weeks of pregnancy.
I found out that doctors in France calculate the due date by starting 2 weeks from the first day of the woman’s last menstrual cycle and then count 9 months. So 2 weeks until conception and 39 weeks of pregnancy – resulting in 41 weeks of pregnancy. This is why my doctor’s date was one week further than my calculation.
After my first ultrasound, the doctor changed my due date to October 6. He said that the ultrasound revealed that the date of conception was January 6 (less than the 2 weeks standard). In the USA, this would make my due date now September 29.
So when is my due date? Apparently, in the US it is September 29 and in France it is October 6. Since only 5% of babies are actually born on their due dates, I am not too worried about which date is the “right” one. I will count on October 6, but hope for September 29.
Only my little bun in the oven knows for sure when she will be done baking. Until then, I will be counting down using arbitrary numbers and daydreaming
Summary: Calculating Your Pregnancy Due Date in France
Count two weeks from the first day of your last period and then add 9 months.
For example, pretend that the first day of your last period is January 1. Two weeks from this date is January 15. Add 9 months, so the due date would be October 15.
Image Credit: flequi on Flickr.com
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I just moved to France, and my due date in the USA is October 3. Does that mean in France it is October 10? I went to see my OB/GYN doctor here, who doesn’t speak a word of English and he said that it was still the right due date, but now I’m worried.
No worries
I was also very confused with my OB/GYN about the due date for months! If your due date was originally October 3rd in the USA, then it will be October 10 in France. However, if you had an ultrasound then the doctor may change the due date (as did mine) based on the baby’s development. We found that I was one week further along than calculated, which means I conceived a week earlier. The 40 week USA calculation (and 41 week France calculation) both include 2 weeks from the point of the first day of the last period until conception – I only needed 1 week 
So look at your ultrasound results or ask your OB/GYN to find out what your actual date of conception is, then add 38 weeks for the USA date or 39 weeks for the French date. If he told you that October 3 is still your right due date, your ultrasound may have put you as having an earlier conception date like me. I would just deduct 7 days from October 3 to get your USA due date.
I talked to my doctor and I explained to him the differences in due dates (actually my father-in-law who is also a doctor called him). He told me that he would change the due date to October 10. Pheww I feel better now
I would much rather extend my time than shorten it. 
Thank you so much for your help. I would have no idea about this unless I had read this website. 