Free online gestational age calculator

Gestational age is measured in weeks and days from the first day of your last menstrual period, not from conception. This calculator determines where you are in your pregnancy and.

Date method

How to use

Choose Last period (LMP) or Ultrasound using the toggle.
If using LMP, enter the first day of your last menstrual period.
If using ultrasound, enter the scan date and the gestational age (weeks + days) assigned at that visit.
Click Calculate to see gestational age, trimester, due date, and upcoming milestones.

The formula

LMP method (Naegele’s rule)

gestationalDays = today − lmpDate
dueDate         = lmpDate + 280 days

Source: Naegele FC. Lehrbuch der Geburtshilfe. 1830.

Ultrasound method

gestationalDays = (ultrasoundWeeks × 7 + ultrasoundDays) + (today − ultrasoundDate)
dueDate         = equivalentLMP + 280 days

Where equivalentLMP = today − gestationalDays.

Pregnancy milestones

Week	Milestone
6	Heartbeat detectable
10	Embryo period ends — now a fetus
12	End of first trimester
20	Anatomy scan (mid-pregnancy ultrasound)
24	Viability threshold
28	Third trimester begins
37	Full term
40	Estimated due date

Worked example

LMP: October 1, 2025. Checked on April 19, 2026.

Days since LMP: 199 days
Gestational age: 28 weeks 3 days (Third trimester)
Due date: October 1, 2025 + 280 days = July 8, 2026
Days until due: 280 − 199 = 81 days

Notes

Source: ACOG Committee Opinion 700, 2017: Methods for estimating due date.
This calculator is for informational planning. Consult your healthcare provider for clinical dating and prenatal care.
Valid gestational age range: 0–45 weeks.

Frequently asked

What is gestational age?

Gestational age counts from the first day of your last menstrual period (LMP), not from conception. Because ovulation typically occurs about 14 days after the start of the period, the first two weeks of "gestational age" predate conception. Full-term pregnancy is 40 gestational weeks.

How is the due date calculated?

The calculator uses Naegele's rule: due date = LMP + 280 days (40 weeks). This assumes a 28-day cycle with ovulation on day 14. If you use the ultrasound method, the calculator works backwards from your gestational age at the ultrasound date to determine an equivalent LMP, then applies the same rule.

What are the three trimesters?

First trimester: weeks 1-12. Second trimester: weeks 13-27. Third trimester: weeks 28-40. Some sources place the second-trimester start at week 14 — this calculator uses week 13.

How accurate is this estimate?

The LMP method has a margin of several days because cycle length varies. Ultrasound dating in the first trimester (before week 14) is typically more accurate. ACOG recommends first-trimester ultrasound dating when available. Only about 5% of babies are born on their exact due date — most arrive within two weeks before or after.

When should I use the ultrasound method?

Use the ultrasound method when your OB or midwife has already established a gestational age from a scan. Enter the date of the ultrasound and the gestational age assigned at that visit (weeks and days). The calculator then projects forward to today.

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