Gestational Age Calculator
Convert between gestational age in weeks/days, days, and months. Calculate from LMP or ultrasound measurements.
Results
Visualization
How It Works
Gestational age counts from the first day of the last menstrual period (LMP). Fetal age is about 2 weeks less since conception occurs around day 14 of the cycle.
The Formula
Gestational Age = Days since LMP
Fetal Age = Gestational Age - 14 days
CRL formula: GA (days) = 42 + 3.73 × CRL - 0.001 × CRL2
Variables
- GA — Gestational Age — time since last menstrual period
- CRL — Crown-Rump Length — most accurate dating measurement in first trimester
Worked Example
Day 87 since LMP = 12 weeks, 3 days gestational age = approximately 2.9 months = 10 weeks, 3 days fetal age.
Practical Tips
- Gestational age is always ~2 weeks more than fetal age.
- First trimester CRL is the most accurate method for dating a pregnancy.
- Months of pregnancy do not correspond neatly to calendar months (4.3 weeks per month).
- Pregnancy is 10 lunar months (28 days each) or about 9 calendar months.
- If ultrasound dating differs from LMP by more than 7 days, ultrasound is used.