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Gestational Age Calculator

Health

Calculate gestational age in weeks and days, plus trimester, from the number of days since your last menstrual period (LMP) instantly and free.

Reviewed by the thecalcu.com team · Last updated August 1, 2026

1300

Gestational Age (Weeks)

20
Additional Days
0
Trimester
2

This calculator computes your Gestational Age (Weeks), Additional Days, Trimester from the values you enter.

Inputs
Days Since Last Menstrual Period (LMP)
Outputs
Gestational Age (Weeks)Additional DaysTrimester

What is a Gestational Age?

The Gestational Age Calculator computes gestational age in weeks and days, along with trimester, from the number of days since your last menstrual period (LMP), the standard medical method for dating pregnancy.

For a related pregnancy dating tool, see the Pregnancy Due Date Calculator and Fundal Height Calculator.

Why Use a Gestational Age Calculator?

Gestational age is referenced constantly throughout prenatal care, and converting a raw day count into the standard weeks-and-days format (and knowing which trimester you're in) is a common but repetitive calculation. This calculator does it instantly.

Who Should Use This Calculator?

  • Expecting parents wanting to quickly convert days since LMP into weeks and days.
  • Students of midwifery or nursing learning how gestational age is calculated and reported.
  • Healthcare educators demonstrating LMP-based dating conventions.
  • Anyone researching how pregnancy dating terminology works.

What Insights Does the Gestational Age Calculator Give You?

The Gestational Age (Weeks) is the primary result. The Additional Days shows the remainder days into the current week, giving the standard "weeks + days" format used in prenatal care. The Trimester shows which of the three pregnancy stages you're currently in.

How to use this Gestational Age calculator

  1. Enter the number of days since your last menstrual period (LMP).
  2. Read the Gestational Age, Additional Days, and Trimester instantly.
Show formula & methodology ↓Show less ↑

Formula & Methodology

Gestational Weeks = ⌊Days Since LMP ÷ 7⌋

Additional Days = Days Since LMP mod 7

Trimester: 1st (weeks 0-12), 2nd (weeks 13-27), 3rd (weeks 28+)

Worked example, 140 days since LMP:

Gestational Weeks = ⌊140 ÷ 7⌋ = 20 weeks

Additional Days = 140 mod 7 = 0 days

Trimester = 2nd

Frequently Asked Questions

What is gestational age?
Gestational age is the standard way pregnancy duration is measured, counted in weeks and days starting from the first day of the last menstrual period (LMP), rather than from the actual date of conception.
Why is gestational age counted from LMP instead of conception?
The exact date of conception is often unknown or difficult to pinpoint, while the first day of the last menstrual period is generally a known, consistent reference point, which is why LMP became the medical standard for dating pregnancy.
How is gestational age calculated from days since LMP?
The calculator divides the total days since LMP by 7 to get whole weeks, with the remainder showing the additional days into the current week.
How are trimesters determined?
This calculator uses commonly cited trimester boundaries: the first trimester runs through week 12, the second trimester from week 13 through 27, and the third trimester from week 28 onward.
Is gestational age the same as fetal age?
No, gestational age (counted from LMP) is typically about two weeks ahead of fetal age (counted from actual conception), since ovulation and conception usually occur roughly two weeks after the start of the last period.
How does this relate to the Fundal Height Calculator?
The [Fundal Height Calculator](/fundal-height-calculator/) estimates gestational age from a physical measurement taken during a prenatal visit, offering an alternative (though less precise) reference point compared to LMP-based dating.
Why might ultrasound dating differ slightly from LMP-based dating?
Ultrasound dating, especially in early pregnancy, measures fetal size directly and can sometimes differ slightly from LMP-based calculations if ovulation didn't occur exactly on the typical cycle day assumed by LMP dating.
How does this relate to the Pregnancy Due Date Calculator?
The [Pregnancy Due Date Calculator](/pregnancy-due-date-calculator/) projects forward to estimate a due date from LMP, while this calculator shows the current gestational age at any point during the pregnancy.
What if I don't know my exact LMP date?
If your LMP date is uncertain, an ultrasound dating scan (particularly one done in the first trimester) generally provides a more reliable estimate of gestational age than LMP-based calculation alone.
Is a full-term pregnancy a fixed number of weeks?
Full term is generally considered to span 39 to 40 weeks of gestational age, though babies born anywhere from 37 to 42 weeks are typically categorized within the broader term range.
Also known as
gestational age calculator from LMPweeks pregnant calculatorpregnancy weeks and daystrimester calculator