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Gestational Age Calculator

Find your gestational age — how far along your pregnancy is, in weeks and days — measured from the first day of your last menstrual period. This is the figure clinicians use to track a pregnancy and time prenatal tests. Because it is dated from the last period rather than conception, gestational age runs about two weeks ahead of the actual time since conception.

Calculate

Default result: 13

Gestational age is measured from this date, not from conception.

Today by default — the date you want the gestational age for.

Gestational Age Calculator · Result

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Weeks

13

2026-01-01 × 2026-04-02

Days
0
Trimester
1
Total days
91
13

This calculator provides an estimate for general information only and is not medical advice. Due dates, gestational age, and fertility windows are estimates — babies rarely arrive exactly on the estimated due date, and individual cycles and pregnancies vary. Always confirm dates and any health decisions with your healthcare provider or OB-GYN.

Reviewed by the calculators.dev team · Last updated 2026-06-24

Formula reviewed against Gestational age and Naegele's rule (review), PubMed 33079400; ACOG trimester definitions

How to calculate

Enter the first day of your last menstrual period and the date you want the age for (today by default). The calculator counts the whole days between them and converts to completed weeks plus the remaining days, then maps that to a trimester.

total days = as-of date − first day of last period. weeks = total days ÷ 7 (rounded down), and the remainder is the days. Trimester boundaries (ACOG): first trimester is weeks 0–13, second is weeks 14–27, and third is week 28 onward.
Example calculation

From a last period on January 1, 2026, April 2, 2026 is 91 days later. Dividing by 7 gives 13 weeks and 0 days — the first trimester. Gestational age is always measured from the last menstrual period, so it runs about two weeks ahead of the time since conception.

weeks
13
days
0
trimester
1
totalDays
91

Assumptions

  • Gestational age is measured from the first day of the last menstrual period (LMP), not from conception — that is the obstetric convention.
  • The result is expressed as completed weeks plus the leftover days, e.g. '13 weeks 0 days' means 13 full weeks have passed.
  • Trimesters use the ACOG boundaries: weeks 0–13 (first), 14–27 (second), and 28 onward (third).
  • The LMP method assumes a regular 28-day cycle. If your cycle is irregular, an early ultrasound usually gives a more accurate gestational age.
  • An as-of date before the last period is not a valid pregnancy age and is reported as an error rather than a negative number.

Common mistakes

  • Confusing gestational age with time since conception. Gestational age is dated from the last period, so it is roughly two weeks more than the embryo's actual age.
  • Assuming the LMP date is exact when cycles are irregular. An early ultrasound is the more reliable dating method in that case.
  • Reading '13 weeks 0 days' as being in week 13. Thirteen completed weeks means you are in your 14th week.

Frequently asked questions

Is gestational age measured from conception?

No. Gestational age is measured from the first day of your last menstrual period, which is about two weeks before conception. That is why it reads roughly two weeks higher than the time since conception.

How many weeks pregnant am I?

Enter your last period date and today's date. The calculator returns completed weeks and days — for example, 13 weeks 0 days — and the matching trimester.

When does each trimester start?

Using ACOG boundaries, the first trimester is weeks 0–13, the second is weeks 14–27, and the third begins at week 28.

What if my cycle is not 28 days?

The last-period method assumes a 28-day cycle. If your cycle is longer, shorter, or irregular, an early ultrasound gives a more accurate gestational age, which your provider will use for your care.

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