Gestational Age Calculator
Enter your last menstrual period (LMP) to calculate your current gestational age, trimester, and estimated due date. The calculation uses ACOG trimester boundaries and term classification.
Calculate Your Gestational Age
Default is 28 days. If your cycle is longer or shorter, adjust this for a more accurate due date.
Leave blank to calculate from today.
How It Works
Gestational age is calculated from the first day of your last menstrual period (LMP), not from the date of conception. This is the standard method used by healthcare providers and is recommended by ACOG.
The Formula
Cycle-corrected LMP = LMP + (cycle length − 28)
Estimated due date (EDD) = Cycle-corrected LMP + 280 days
Gestational age = Reference date − Cycle-corrected LMP
The result is expressed in weeks and days. For example, 22 weeks and 3 days is written as 22+3.
Why Cycle Length Matters
The standard pregnancy calculation assumes a 28-day cycle. If your cycle is longer, ovulation likely occurred later, and your due date should be adjusted forward. If your cycle is shorter, ovulation likely occurred earlier.
Many calculators ignore this adjustment entirely. We include it because it is the most common source of error in pregnancy dating.
ACOG Trimester Boundaries
| Trimester | Weeks | Days |
|---|---|---|
| First | 0 0/7 – 13 6/7 | 0 – 97 |
| Second | 14 0/7 – 27 6/7 | 98 – 195 |
| Third | 28 0/7 – birth | 196+ |
Source: ACOG Committee Opinion, Definition of Term Pregnancy.
ACOG Term Classification
| Classification | Weeks |
|---|---|
| Preterm | Before 37 0/7 |
| Early term | 37 0/7 – 38 6/7 |
| Full term | 39 0/7 – 40 6/7 |
| Late term | 41 0/7 – 41 6/7 |
| Postterm | 42 0/7 and beyond |
Source: ACOG Committee Opinion, Definition of Term Pregnancy.
What This Calculator Does Not Tell You
- It does not confirm pregnancy. Only a pregnancy test or ultrasound can do that.
- It does not account for irregular cycles. If your cycle varies significantly, the calculation is less reliable.
- It does not replace ultrasound dating. A first-trimester ultrasound is more accurate than LMP-based dating and may change your due date by up to a week.
- It does not predict your delivery date. Only about 5% of babies are born on their exact due date.
When to Talk to Your Clinician
Contact your healthcare provider if:
- You are unsure of your LMP date
- Your cycles are irregular
- You have bleeding or pain
- You have questions about your due date or gestational age
Related Tools
- How Many Weeks Pregnant — same calculation, conversational framing
- Trimester Calculator — which trimester are you in?
- Due Date Calculator — four modes: LMP, conception, IVF, ultrasound
- Pregnancy Milestone Dates — dated list of key milestones