How the Pregnancy Weeks Calculator Works
This calculator uses Naegele's rule, the international medical standard for determining gestational age and the estimated due date (EDD). It is the same method used by obstetricians and midwives around the world.
The Calculation Method
The calculation starts from the first day of the last menstrual period (LMP -- Last Menstrual Period). From this date, elapsed days are counted and divided by 7 to obtain complete weeks. The notation "12+3" means 12 weeks and 3 days of gestation.
The estimated due date is obtained by adding 280 days (40 weeks) to the date of the last period. This method was developed by the German obstetrician Franz Karl Naegele in the 19th century and remains the clinical standard.
The Three Trimesters of Pregnancy
Pregnancy is divided into three trimesters, each with specific characteristics:
- First trimester (weeks 1-12): this is the embryonic development phase. All major organs form, the heart begins to beat, and the nervous system develops. This is when folic acid supplementation is most important.
- Second trimester (weeks 13-26): the baby grows rapidly. The mother begins to feel the first fetal movements (quickening), typically between weeks 16 and 20. The anatomy scan ultrasound is performed between weeks 19 and 21.
- Third trimester (weeks 27-40): the fetus completes lung maturation and accumulates subcutaneous fat. The mother prepares for delivery. A birth is considered full-term from week 37.
When to Use This Calculator
This tool is useful for:
- Checking your pregnancy week when you know your last period date
- Calculating your estimated due date to plan appointments and tests
- Tracking pregnancy progress week by week
- Sharing with your partner exactly where you are in pregnancy
When the Calculation May Not Be Accurate
If your menstrual cycle is irregular or if you do not remember the exact date of your last period, ultrasound dating in the first trimester (CRL -- Crown-Rump Length measurement) provides a more precise estimate. First-trimester ultrasound has a margin of error of plus or minus 5 days, while Naegele's rule assumes a regular 28-day cycle.