Gestational Age Calculator
Calculate gestational age in weeks and days based on last menstrual period or ultrasound dating. Track pregnancy progress and trimester milestones throughout pregnancy.
🤰 Pregnancy📐 Gestational Age = Today - LMP date🏥 Health
Last menstrual period date
Please enter valid values.
Formula & Reference
| Variable | Symbol | Formula | Units |
|---|---|---|---|
| Gestational Age Calculator | — | Gestational Age = Today - LMP date | weeks + days |
Step-by-Step Examples
Example 1
Early Pregnancy
LMP was 8 weeks 3 days ago.
- Total days = 8×7+3 = 59 days since LMP
- Gestational age: 8 weeks, 3 days
- First trimester (weeks 1-13)
✓ 8 weeks 3 days — first trimester
Example 2
Mid-Pregnancy
LMP was 20 weeks 0 days ago.
- Total days = 140 days since LMP
- Gestational age: 20 weeks, 0 days
- Second trimester (weeks 14-27)
✓ 20 weeks 0 days — second trimester
Example 3
Late Pregnancy
LMP was 35 weeks 2 days ago.
- Total days = 247 days since LMP
- Gestational age: 35 weeks, 2 days
- Third trimester (weeks 28-40)
✓ 35 weeks 2 days — third trimester
Real-World Applications
Prenatal Care
Gestational age determines appropriate timing for screening tests, ultrasounds, and prenatal visit schedules.
Obstetric Practice
Accurate gestational dating guides clinical decisions throughout pregnancy management.
Milestone Tracking
Expecting parents track gestational age to follow expected fetal development milestones.
Childbirth Education
Understanding gestational age terminology helps parents engage with prenatal education materials.
Common Mistakes to Avoid
⚠️
Confusing gestational age with fetal age
Gestational age counts from LMP, which is about 2 weeks before actual conception — fetal/embryonic age is always about 2 weeks less than gestational age.
⚠️
Assuming LMP-based dating is always most accurate
For those with irregular cycles or uncertain LMP dates, first-trimester ultrasound measurements are often considered more reliable for precise dating.
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Not updating dating if ultrasound dating differs significantly from LMP
If clinical ultrasound dating differs by more than about a week from LMP-based dating, providers often adjust the official dating to the ultrasound estimate.
Frequently Asked Questions
How is gestational age calculated? ▾
Counted in weeks and days from the first day of the last menstrual period (LMP), even though actual conception typically occurs about 2 weeks after LMP.
What are the trimester boundaries? ▾
First trimester: weeks 1-13. Second trimester: weeks 14-27. Third trimester: weeks 28-40 (or until delivery). These boundaries can vary slightly by source.
Why is pregnancy considered 40 weeks if gestation is really about 38 weeks from conception? ▾
The standard 40-week pregnancy timeline counts from LMP, which is about 2 weeks before conception — true embryonic/fetal development time is closer to 38 weeks.
What if I don't remember my exact LMP date? ▾
First-trimester ultrasound dating (ideally between 8-13 weeks) provides accurate alternative dating based on fetal measurements when LMP date is uncertain or unknown.
What is considered full term? ▾
Full term is generally 39-40 weeks 6 days. Early term: 37-38 weeks 6 days. Late term: 41-41 weeks 6 days. Post-term: 42+ weeks.