Pregnancy weeks calculator
Gestational age in weeks and days, trimester and estimated due date.
Calculator inputs
Results
Enter your values and press “Calculate” to see the result.
In short
- What it calculates
- Gestational age in weeks and days, trimester and estimated due date.
- Formula used
Days of gestation = today − last period- Example
- Last period on 2026-02-01, appointment on 2026-08-21:
Pregnancy weeks calculator
Gestational age is counted from the first day of your last period, not from conception. That is why a pregnancy is usually already four or five weeks along when it is confirmed.
The “weeks + days” format is the one used in obstetric care.
How it works
Days since the last period are counted and divided by seven. The estimated due date falls at 280 days, that is 40 weeks.
Formula
Days of gestation = today − last period
Weeks + days = days ÷ 7 and its remainder
EDD = last period + 280 days
Trimesters: 1st through week 13, 2nd to week 27, 3rd from week 28
Worked example
Last period on 2026-02-01, appointment on 2026-08-21:
Days: 201
Gestation: 28 + 5 (28 weeks and 5 days)
Trimester: 3rd
EDD: 2026-11-08
Explanation
The two weeks you were not pregnant
Obstetric convention counts from the first day of the last menstrual period, not from conception. Since ovulation arrives about two weeks later, when you are told you are six weeks along the embryo is about four. It is not an error: it is the system all of obstetrics uses because the date of a period is remembered and the date of conception almost never is.
The trimesters and what happens in each
The first trimester runs to week 13, and organs form during it. The second, weeks 14 to 27, is when the anomaly scan is done and first movements are usually felt. The third runs from week 28 to birth. A pregnancy is considered at term from week 37.
The scan overrides the calculation
If the first-trimester scan gives a gestational age differing from the period-based calculation by more than a few days, the estimated due date is corrected to the scan. Measuring the embryo in those weeks is highly reliable and does not depend on cycle regularity.
This does not replace medical care
An approximate calculation helps you orient yourself between appointments, not make clinical decisions.
Frequently asked questions
Why count from the last period?
Because it is the date known with certainty, whereas the exact day of conception is rarely known.
What if my cycles are irregular?
The estimate loses accuracy. The first trimester ultrasound corrects gestational age and is the definitive clinical reference.
Why does the scan give a different date?
Because it measures the actual size of the embryo, while the period-based calculation assumes a standard cycle. Where they differ, the first-trimester scan prevails.
From which week is a pregnancy at term?
From completed week 37. Before that it is preterm, and from week 42 it is considered prolonged.
Need to calculate something else?
These tools are often used alongside this calculator.
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Ideal weight
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