Pregnancy Countdown Calculator

Estimate your due date and countdown using Naegele's Rule (last menstrual period + 280 days), or calculate from a known conception or due date instead.

Quick Facts

Formula
Naegele's Rule: due date = LMP + 280 days
From conception, it's 266 days instead; full term is 37-42 weeks.

Your Results

Calculated
Estimated due date
-
Naegele's Rule (280 days from LMP)
Days remaining
-
Countdown to due date
Gestational age
-
Weeks + days since LMP
Trimester
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Current stage of pregnancy

Ready

Choose a method, enter the date, then press Calculate.

About the Pregnancy Countdown Calculator

This calculator estimates a due date and shows how far along a pregnancy is today, using the same 280-day convention widely used for routine dating. Choose whichever date is known — the first day of the last menstrual period (LMP), an estimated conception date, or an already-confirmed due date — and the calculator works out the rest.

Naegele's Rule: the standard due-date formula

The standard method for estimating a due date is Naegele's Rule: add 280 days (40 weeks) to the first day of the last menstrual period, assuming a typical 28-day cycle with ovulation around day 14. If the average cycle is longer or shorter than 28 days, this calculator shifts the estimate by the difference — a 35-day cycle pushes the due date about a week later, a 21-day cycle pulls it about a week earlier.

Given a conception date instead (from ovulation tracking or fertility treatment), the calculator adds 266 days (38 weeks) — the same 280-day gestation measured from conception, since conception typically occurs about two weeks after the LMP under the 28-day assumption. Given an already-confirmed due date (often set or adjusted by a clinician using ultrasound), the calculator uses that date directly and works backward to an LMP-equivalent starting point for the gestational-age calculation.

Gestational age and trimesters

Gestational age is counted in completed weeks and days since the last menstrual period, not since conception — this is the convention used throughout prenatal care, so a pregnancy "due" at 40 weeks gestational age is only about 38 weeks past fertilization. A full-term pregnancy generally spans 37 to 42 completed weeks. The three trimesters are commonly divided as weeks 1-12 (first), weeks 13-27 (second), and weeks 28 onward (third).

What this calculator does not do

It applies a fixed, well-established formula to the date provided — it does not replace an ultrasound dating scan, which is generally considered more accurate than LMP-based dating, particularly for irregular cycles. Use the result for planning and general tracking, and defer to a prenatal care provider's dating for medical decisions.

Frequently Asked Questions

How is the due date calculated?
By default this calculator uses Naegele's Rule: it adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation around day 14. If the cycle is longer or shorter than 28 days, the estimate shifts by the difference. Choosing conception date instead adds 266 days (38 weeks). Choosing an already-confirmed due date uses that date directly.
Is gestational age the same as fetal age?
No. Gestational age is counted from the first day of the last menstrual period, which is the convention used in prenatal care. Fetal (conceptional) age is counted from conception, roughly two weeks later. A pregnancy described as 10 weeks gestational age is about 8 weeks past conception.
How accurate is a due-date estimate?
A due date from Naegele's Rule is an estimate built on an assumed 28-day cycle and day-14 ovulation, not a guarantee. A full-term pregnancy spans a range of about 37 to 42 completed weeks, so delivery on the exact estimated date is only one possible outcome within that window. An ultrasound dating scan, especially in the first trimester, is generally considered more accurate than LMP-based dating and is what most clinicians rely on.
What if I do not know my last menstrual period exactly?
Switch the calculation method to conception date if ovulation was tracked or fertility treatment was used, or enter the due date already confirmed by a clinician (often via ultrasound). Either alternative avoids relying on an uncertain LMP date.

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