Gestational Age Calculator
Find how many weeks and days pregnant you are today, your due date and your trimester from the first day of your last period, your due date or your conception date. Includes key pregnancy milestones with their dates. Example: a last period starting June 10, 2026 gives a due date of March 17, 2027.
Show steps
How to use the Gestational Age Calculator
- Choose which date you know: last period, due date or conception.
- Enter the date, and your cycle length if it is not 28 days.
- Press Calculate to see how far along you are and the key dates.
Formula and method
Naegele’s rule: due date = first day of last period + 280 days (40 weeks), adjusted by (cycle length – 28) days. Conception is about 14 days after the last period in a 28-day cycle, so due date = conception + 266 days. Gestational age = today minus the first day of the last period.
Trimesters at a glance
| Trimester | Weeks |
|---|---|
| First | 1 to 13 weeks 6 days |
| Second | 14 to 27 weeks 6 days |
| Third | 28 weeks to birth |
Pregnancy length terms
| Term | Gestational age |
|---|---|
| Early preterm | Before 34 weeks |
| Late preterm | 34 to 36 weeks 6 days |
| Early term | 37 to 38 weeks 6 days |
| Full term | 39 to 40 weeks 6 days |
| Late term | 41 weeks to 41 weeks 6 days |
| Post-term | 42 weeks and later |
Gestational age versus fetal age
Gestational age counts from your last period. Fetal (conceptional) age counts from conception and is about two weeks shorter, which is why a baby at 12 weeks of gestation is really 10 weeks old.
Why dating matters
Accurate dating guides screening tests, growth checks and decisions about induction, so confirm your due date with your healthcare provider.
Frequently asked questions
How is gestational age calculated?
From the first day of your last menstrual period.
How is the due date calculated?
Add 280 days (40 weeks) to the first day of your last period.
Is pregnancy 9 months or 10?
About 9 months from conception, and 40 weeks (nearly 10 lunar months) from the last period.
What if my cycle is not 28 days?
Enter your cycle length and the due date adjusts.
Which is more accurate, ultrasound or last period?
An early ultrasound usually is.
What is full term?
39 weeks through 40 weeks 6 days.
Sources
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