Gestational Age Calculator
Estimate pregnancy age in weeks and days, plus due date and trimester.
What this gest age calculator does
This tool estimates gestational age (often called “GA” or “pregnancy age”) in weeks and days. It also returns an estimated due date, estimated conception date, trimester, and whether the pregnancy is preterm, term, or post-term based on the date you selected.
In obstetrics, gestational age is generally counted from the first day of the last menstrual period (LMP), not from conception. That means pregnancy dating usually starts about two weeks before fertilization.
How to use the calculator
Method 1: From LMP
- Enter the first day of your last menstrual period.
- Enter your average cycle length (default is 28 days).
- Choose the date you want to calculate on (today, appointment date, etc.).
Method 2: From due date
- Enter your estimated due date from your provider.
- Select the date to calculate gestational age on.
The output shows gestational age as weeks + days, which is the standard clinical format (for example, 24 weeks 3 days is written as 24w3d).
Why cycle length matters
A standard due date formula assumes a 28-day cycle. If your cycles are regularly longer or shorter, ovulation may happen later or earlier than average. Adjusting cycle length can improve rough dating from LMP.
- Longer cycle: ovulation often occurs later, so true embryonic age may be slightly younger.
- Shorter cycle: ovulation often occurs earlier, so true embryonic age may be slightly older.
This calculator applies a simple cycle-length adjustment. Your clinician may use ultrasound to refine dating, especially in early pregnancy.
Understanding the result fields
- Gestational age: Time elapsed since pregnancy dating start (LMP-based).
- Estimated due date (EDD): Usually 280 days (40 weeks) from adjusted LMP.
- Estimated conception date: About 14 days after adjusted LMP.
- Trimester: First, second, or third trimester based on gestational week.
- Pregnancy timing status: Preterm, early term, full term, late term, or post-term.
Quick medical context
Gestational age helps guide prenatal testing windows, growth scans, viability discussions, timing of screening, and delivery planning. Even so, all date estimates are approximate.
Frequently asked questions
Is gestational age the same as fetal age?
No. Fetal age (conceptional age) is usually about two weeks less than gestational age. Clinical care generally uses gestational age.
What if my ultrasound date differs from LMP date?
In many cases, early ultrasound can be more accurate than recalled LMP. Your provider will decide which date to use for official pregnancy dating.
Can I use this after IVF?
IVF pregnancies are usually dated from known embryo transfer timelines with specific clinical conventions. Use your fertility clinic’s official dating whenever available.
Final note
A gest age calculator is a helpful planning tool for appointments and understanding milestones, but your OB-GYN, midwife, or maternal-fetal medicine specialist should always be your source of definitive dating and clinical guidance.