Use this pregnancy week calculator to estimate gestational age, due date, and trimester status.
What is gestational age?
Gestational age is the standard way clinicians describe how far along a pregnancy is. It is measured in weeks and days, starting from the first day of the last menstrual period (LMP), not from conception. That is why gestational age is usually about two weeks more than embryonic or fetal age.
How this age gestational calculator works
This tool supports two common methods used in pregnancy dating:
- LMP method: Calculates gestational age from the first day of your last period.
- Due date method: If you already have an estimated due date, it back-calculates your current gestational age.
Formulas used
- Gestational age in days: As-of date minus LMP date
- Estimated due date (EDD): LMP + 280 days (adjusted by cycle length when provided)
- Estimated conception date: LMP + (cycle length - 14 days)
Understanding your results
After you click calculate, you will see:
- Current gestational age in weeks and days
- Estimated due date
- Estimated conception date
- Current trimester
- Days remaining until 40 weeks (or days overdue)
Trimester ranges
- First trimester: 0w0d to 13w6d
- Second trimester: 14w0d to 27w6d
- Third trimester: 28w0d to delivery
Why gestational dating can vary
Pregnancy dating is an estimate, not a guaranteed schedule. Ovulation timing, cycle irregularity, implantation timing, and ultrasound findings can all shift expected dates. A first-trimester ultrasound is often considered the most accurate method for refining due date estimates.
Practical tips for best accuracy
- Enter the exact first day of your last menstrual period if known.
- Use your typical cycle length (for example 26, 28, 30 days).
- Update the as-of date if you are checking progress on a future appointment date.
- Compare calculator results with your provider’s chart and ultrasound dating.
Important medical note
This gestational age calculator is for informational purposes only and does not replace clinical care. Always rely on your OB-GYN, midwife, or maternal-fetal medicine specialist for diagnosis, dating adjustments, and treatment decisions.