Due Date Calculator

Calculate your estimated due date based on your last menstrual period, gestational weeks, or known due date.

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How is the Due Date Calculated?

The due date calculator uses Naegele's rule, the standard method used by healthcare providers worldwide. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This method assumes a regular 28-day menstrual cycle with ovulation occurring on day 14.

While the estimated due date gives you a target date, only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after. Your healthcare provider may adjust the date based on ultrasound measurements, especially if done in the first trimester.

Understanding Gestational Age

Gestational age is measured from the first day of your last menstrual period, not from conception. This means that in the first two weeks of "pregnancy," you haven't actually conceived yet. Conception typically occurs around week 2-3 of gestational age.

Pregnancy is divided into three trimesters: the first trimester (weeks 1-13), the second trimester (weeks 14-26), and the third trimester (weeks 27-40). Each trimester brings different developmental milestones for your baby and physical changes for you.

When to See Your Doctor

Once you have a positive pregnancy test, schedule your first prenatal appointment. Your doctor will confirm the pregnancy, estimate the due date (often with an early ultrasound), and begin your prenatal care plan. Regular check-ups throughout pregnancy are essential for monitoring both your health and your baby's development.

How this tool works

How we calculate

The calculator uses Naegele's rule: it adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). If you enter how many weeks along you are, it counts the time left to 40 weeks; if you already know your due date, it works out today's gestational age from it.

Trimesters follow the split used across BabyLingo: up to 13 weeks, 14 to 26, and 27 onward.

Limitations

  • The rule assumes a 28-day cycle with ovulation on day 14; longer, shorter, or irregular cycles shift the date.
  • When there is a first-trimester ultrasound, the date it gives is the most accurate and is the one your care team uses.
  • For IVF pregnancies, the due date is set from the embryo age and the transfer date.
  • It is an estimate: few babies are born exactly on their due date.

Sources

  1. ACOG — Committee Opinion 700: Methods for Estimating the Due Date
  2. MedlinePlus (NIH) — Gestational age

Sources checked on November 6, 2026. How we produce our content

Frequently Asked Questions

How accurate is the due date calculator?
The due date calculator provides an estimate based on Naegele's rule (LMP + 280 days). While it's the standard method used by doctors, only about 5% of babies are born on their exact due date. Most arrive within a 2-week window around the estimated date. An early ultrasound can refine the estimate.
Can I calculate my due date without knowing my LMP?
Yes! You can use the "Gestational Weeks" method if your doctor has told you how far along you are, or the "Known Due Date" method if you already have a date from an ultrasound. Each method provides the same detailed results.
What is the difference between gestational age and fetal age?
Gestational age is counted from the first day of your last menstrual period (LMP), while fetal age (or embryonic age) is counted from conception, which happens about 2 weeks later. Doctors use gestational age as the standard measure, so your baby's fetal age is typically 2 weeks less than the gestational age.
What is the 5-1-1 rule for contractions?
The 5-1-1 rule is a guideline for when to go to the hospital: contractions every 5 minutes, lasting 1 minute each, for at least 1 hour. Use our Contraction Timer tool to track this pattern automatically.

Further reading

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