Gestational Age Calculator
Enter the first day of your last menstrual period to see how many weeks and days pregnant you are, your current trimester, your estimated due date and how many days remain.
Last updated: June 20, 2026 · Medically reviewed by TG we-Calculate Editorial TeamLast menstrual period (LMP)
Reference date (today)
You are 12 weeks 6 days pregnant.
- 1
Days since LMP
90 - 2
Remaining days within week
90 mod 7 = 6 - 3
Completed weeks
floor(90 ÷ 7) = 12Gestational age in completed weeks; the extra days are shown in the helper text above.
How many weeks pregnant am I?
Gestational age is counted from the first day of your last menstrual period (LMP). Subtract the LMP from today to get the number of days, then divide by 7: the whole number is your weeks and the remainder is the extra days. A full-term due date is 280 days (40 weeks) after the LMP.
Formula
How this is calculated
The calculator works entirely from two calendar dates. The "last menstrual period" (LMP) is the first day of your last period, and the "reference date" is the day you want the age measured for (today by default). It counts the whole number of days between them, comparing the dates at local midnight in UTC so daylight-saving shifts never add or drop a day.
That day count is then split into weeks and days: dividing by 7 and taking the floor gives completed weeks, and the remainder gives the extra days — so 90 days reads as "12 weeks 6 days". Trimesters follow standard clinical cut-offs: first is under 14 weeks, second is 14 up to 28 weeks, and third is 28 weeks onward. The estimated due date is LMP + 280 days (40 weeks), Naegele's rule, and days remaining is 280 minus the days elapsed.
This assumes a regular 28-day cycle with ovulation on day 14, so dating is from the LMP rather than conception (which is roughly two weeks later). If the reference date is before the LMP, no result is shown. LMP-based dates are estimates; a first-trimester ultrasound is more accurate.
Examples
| Input | Result |
|---|---|
| LMP 2026-01-01, today 2026-04-01 | 12 weeks 6 days, first trimester, due 7 Oct 2026 |
| LMP 2026-01-01, today 2026-07-09 | 27 weeks 0 days, second trimester |
| LMP 2026-01-01, today 2026-08-06 | 31 weeks 0 days, third trimester |
About this calculator
Gestational age is the standard way pregnancy is dated in clinical practice. It is measured from the first day of the last menstrual period (LMP), not from conception, which typically occurs about two weeks later. A full-term pregnancy is counted as 40 weeks (280 days) from the LMP, an approach known as Naegele’s rule.
This calculator subtracts your LMP from a reference date to give your age in completed weeks plus extra days (for example, "12 weeks 6 days"). It also reports your trimester — first (up to 13 weeks 6 days), second (14 to 27 weeks 6 days) or third (28 weeks onward) — your estimated due date, and the number of days still to go. Because cycle lengths and ovulation timing vary, LMP-based dates are estimates; an early ultrasound is the most accurate way to confirm gestational age.
Frequently asked questions
Yes. Gestational age is the standard measure of how far along a pregnancy is, counted in completed weeks and days from the first day of your last menstrual period.
The last menstrual period is a date most people can identify, while the exact day of conception is rarely known. Conception usually happens about two weeks after the LMP, so gestational age runs roughly two weeks ahead of "fetal age".
It is a good estimate when periods are regular, but cycle length and ovulation timing vary. An ultrasound in the first trimester is the most reliable way to confirm gestational age and the due date.
Also known as
Sources & references
TG we-Calculate Editorial Team. (2026). Gestational Age Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/gestational-age-calculator
TG we-Calculate Editorial Team. "Gestational Age Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/gestational-age-calculator.
TG we-Calculate Editorial Team, "Gestational Age Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/gestational-age-calculator
@misc{wecalculate_gestational_age_calculator, title = {Gestational Age Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/gestational-age-calculator}}, year = {2026}, note = {TG we-Calculate} }
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