Pregnancy Week Calculator
The pregnancy week calculator works out your gestational age by counting weeks from the first day of your last menstrual period (LMP) or backwards from your due date, then shows your trimester.
verified_userReviewed by the Calculopedia editorial teamLast updated 2026-08-15
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quizExample
How this calculator works, with real numbers (no JavaScript needed):
Inputs
- Calculate from
- lmp
- First day of last menstrual period
- 2026-01-15
- Estimated due date
- 2026-10-22
- As of date
Results
- Pregnancy week
- 32
- Extra days
- 3
- Trimester
- Third trimester (weeks 28+)
- Due date
- 22 October 2026
- Weeks to go
- 7.6
- Pregnancy complete
- 81.1%
functionsThe formula
Pregnancy is counted in gestational weeks from the first day of your last menstrual period (LMP) — even though conception happens about two weeks later. This is the standard dating used everywhere, and it is the reason "how many weeks am I?" and "when is my due date?" are really the same question asked two different ways.
Why weeks, not months
Everyone asks "what month are you in?", but medically pregnancy is tracked in weeks. A week is a precise unit that maps cleanly onto developmental milestones — organs form in particular windows, scans are timed by week, and risks and expectations are described by gestational age. It also avoids the confusion of months having different lengths. The week count is measured as complete weeks plus days (for example "30 weeks and 1 day"), which is far more useful than a vague month number for planning your care.
From your LMP
Gestational age = days since LMP ÷ 7
If your LMP was 15 January 2026 and today is 14 August 2026:
211 days ÷ 7 = 30 weeks and 1 day
Your due date is LMP + 280 days (40 weeks) = 22 October 2026.
From your due date
Don't know your LMP? Work backwards: LMP = due date − 280 days. This is handy when your only date comes from a scan or your doctor. Because due dates from an early scan are often more accurate than LMP dates anyway, working backwards this way can actually give you a tighter estimate of your current week than counting forward from a remembered period.
Week-by-week
| Trimester | Weeks | What's happening |
|---|---|---|
| First | 1–13 | Organs form; morning sickness common |
| Second | 14–27 | Baby moves; scan can reveal the sex |
| Third | 28–40 | Rapid growth; getting ready to meet the baby |
A full pregnancy is 40 weeks. About 4% of babies arrive exactly on the due date — most come within two weeks either side. Remember that "the first week of pregnancy" (week 1) falls before conception, which surprises many people; your pregnancy week is counted from the LMP, not from the moment of conception.
This tool is for general information and is not medical advice. An early ultrasound is the most accurate way to date a pregnancy.
helpFrequently asked questions
question_markHow do I calculate how many weeks pregnant I am?
Count from the first day of your last menstrual period: divide the number of days since your LMP by 7. The calculator does this instantly — e.g. 211 days = 30 weeks 1 day.
question_markAre pregnancy weeks counted from conception?
No. Gestational age is counted from the first day of your last menstrual period, about two weeks before conception. Your pregnancy is dated from LMP, not from ovulation.
question_markHow many weeks is a full pregnancy?
A full-term pregnancy is about 40 weeks (280 days) from the first day of the last menstrual period, spanning roughly 9 calendar months.
question_markCan I calculate pregnancy weeks from my due date?
Yes. Subtract 280 days from your due date to get the LMP, then count weeks from there. This calculator supports both directions.