Due Date in France: How It’s Calculated & What to Expect

How France Calculates Your Due Date

In France, pregnancy duration is counted in semaines d’aménorrhée (SA), weeks of aménorrhée, or weeks since the first day of your last menstrual period (LMP). A full-term pregnancy in France is 41 SA, not 40. 

This is one of the most common points of confusion for American women pregnant in France. You may have always thought of pregnancy as 40 weeks. In France the full term is 41 SA, and it is considered normal to give birth anywhere between 39 and 41 SA. Induction is typically not recommended before 41 SA for a low-risk pregnancy.

How to calculate your due date — France method

The French method is based on aménorrhée dating from the first day of your last menstrual period. The simple formula: 

First day of last menstrual period + 41 weeks = terme présumé (estimated due date in France) Or equivalently: first day of LMP + 287 days.

Tip: The online calculator at ameli.fr allows you to enter the first day of your last period and will calculate your terme présumé instantly, along with your expected prenatal appointment schedule.

What semaines d’aménorrhée (SA) means

SA counts the number of weeks since the start of your last period, not since conception. This is important to understand when your midwife or doctor refers to your pregnancy week, as it will be 2 weeks further along than the gestational age (weeks since conception).

For example: if you are 13 SA, you are 13 weeks from the start of your last period, but approximately 11 weeks from conception. All prenatal appointments, ultrasound schedules, and maternity leave dates in France are calculated in SA.

How the US Calculates the Due Date

In the United States, pregnancy is counted in gestational weeks from the first day of the last menstrual period, with a full term of 40 weeks. The standard calculation method is Naegele’s Rule, based on a 28-day menstrual cycle:

First day of LMP − 3 months + 1 year + 7 days = estimated due date. Or equivalently: first day of LMP + 280 days.

The due date in the US is considered more of a fixed target. It is increasingly common in the US to discuss induction after 40 weeks if there are no signs of labour beginning, whereas in France induction before 41 SA is generally not recommended for low-risk pregnancies.

If you know your exact date of conception, particularly relevant if you have been tracking ovulation or have undergone fertility treatment, you can also calculate from conception by adding 266 days. 

Key Differences at a Glance

The First Ultrasound and Confirming Your Due Date

Your estimated due date is initially calculated from the first day of your last period. However, the first official ultrasound, the échographie de datation et de dépistage, typically performed between 11 and 13+6 SA, will confirm and may adjust this date based on the size and development of the embryo.

After this ultrasound, the date confirmed by the imaging is the one used for the rest of your pregnancy. All subsequent appointments, your maternity leave start date, and your birth preparation schedule are calculated from this confirmed SA dating.

Important: The confirmed SA date from your first ultrasound is what your employer and the Assurance Maladie use to calculate your congé maternité dates. Make sure the compte rendu from your ultrasound is in your file.

Why Does France Use 41 Weeks?

This is one of the most common questions from American women going through their first pregnancy in France. The short answer is that the French medical system considers 41 SA to be the natural endpoint of a full-term pregnancy, based on obstetric research showing that the risk of complications does not increase significantly before 41 SA in a low-risk pregnancy.

The US figure of 40 weeks is also based on Naegele’s Rule, which itself is based on historical assumptions about a 28-day menstrual cycle. Neither figure is an exact biological constant, they are statistical averages. France simply uses a slightly different statistical baseline and is more conservative about intervention before 41 SA.

In practice, most babies arrive between 39 and 41 SA in both countries. The difference is in when medical intervention is discussed, not in when babies are actually born.

Prenatal Care Comparison: France vs. United States

To give you the full picture, here is how the prenatal visit schedule compares across both systems. This is based on a low-risk pregnancy and is an average, individual cases will vary. 

Note: The French system has fewer total visits but more structured mandatory appointments, more comprehensive bloodwork coverage, and a fully funded birth preparation program. The US system has more frequent monitoring visits, particularly in the third trimester, but coverage of additional services varies significantly by insurance plan.

What Comes Next

Understanding your due date and how SA dating works is one of the first practical steps to navigating pregnancy in France. The rest of this series covers the full journey:

•   Declaring your pregnancy in France: the administrative step that unlocks your coverage
•   Your first trimester appointment: what to expect at the premier examen prénatal
•   The entretien prénatal précoce: the early prenatal interview
•   The 7 childbirth preparation classes
•  Congé maternité explained:  how maternity leave works in France