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Method · Naegele’s rule

Pregnancy Due Date Calculator

Estimate your due date, gestational age and trimester from your last menstrual period.

In short

A pregnancy due date is estimated with Naegele's rule: the first day of your last menstrual period plus 280 days (40 weeks). Enter your LMP and cycle length and this calculator returns your estimated due date and current gestational age. It is an estimate — a dating scan is more precise.

Due Date

Estimate from your last period

Estimating your due date

This calculator estimates your due date using Naegele’s rule, the standard method used in maternity care. It takes the first day of your last menstrual period (LMP), adds about 280 days (40 weeks), and adjusts for your average cycle length. It also estimates your current gestational age in weeks and days, your trimester, and the days remaining.

Gestational age is counted from the first day of your last period, not from conception, which is why a “40-week” pregnancy is really about 38 weeks from conception. The calculator follows the same convention your provider uses, so the numbers line up with clinical milestones.

Important: This is an estimate for general guidance only. Only about 1 in 20 babies arrive on the exact estimated date, and most are born within two weeks either side. Always rely on your midwife or doctor and any dating scan for medical decisions.

If your cycle isn’t 28 days

The classic formula assumes a 28-day cycle. If yours is longer or shorter, enter your average length and the estimate shifts accordingly — a longer cycle generally means a slightly later due date.

FAQ

Common questions

How is my due date calculated?
It uses Naegele’s rule: the first day of your last period plus roughly 280 days, adjusted for your cycle length. This is the same starting method used in most maternity care.
Is the due date accurate?
It’s a reliable estimate, but only an estimate. Most births happen within two weeks before or after the date. A dating ultrasound gives a more precise figure.
What is gestational age?
It’s how far along the pregnancy is, counted from the first day of the last period — reported in weeks and days, e.g. “12w 3d”.
What if I don’t know my last period date?
If you’re unsure, your provider can date the pregnancy with an ultrasound, which is more accurate than LMP for irregular cycles.
From our guides

Related reading: How Old Is My Baby in Weeks & Months · Born on February 29

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