Calculatrice de grossesse
The detailed guide below is currently available in English.
How pregnancy dating works
Clinical pregnancy dating counts from the first day of the last menstrual period (LMP), not from conception, because the LMP is usually easier to pinpoint. Gestational age is therefore about two weeks ahead of embryonic age. A typical pregnancy lasts 280 days (40 weeks) from the LMP, which is how the estimated due date is set. Only about one in twenty babies arrives exactly on its due date — anywhere from 37 to 42 weeks is considered normal term.
Choosing the right method
Pick the method matching what you know. The due-date method works backwards from a date you were already given. Use the LMP method if your cycles were regular and you remember the date. If you know when conception or ovulation occurred, the conception method adds the standard two-week offset for you. An early ultrasound is the most accurate option: entering the scan date and the gestational age reported by the sonographer reconstructs your dating from the best available evidence. For IVF, the transfer date is known exactly, so day-3, day-5, and day-6 embryos each use their own equivalent LMP. All results are estimates, not medical advice.
Questions fréquentes
Why is gestational age counted from my last period?
Because ovulation and conception usually happen about two weeks after the LMP, the LMP provides a consistent, observable starting point. When the true ovulation date is known, it can be used directly with the same 280-day framework shifted accordingly.
How accurate is an ultrasound due date?
First-trimester ultrasound predicts the due date within about 3–5 days, which is more accurate than LMP dating when cycles are irregular. Accuracy gradually declines in the second and third trimesters as babies vary more in size.
What are the trimester boundaries?
Commonly, the first trimester runs to the end of week 13, the second from week 14 to week 27, and the third from week 28 to birth. This calculator follows those cut-offs.