Health

Due Date Calculator

Estimate your baby's arrival date, track gestational age, and see weekly development milestones using Naegele's rule.

Quick Answer

A pregnancy due date is estimated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). Full-term pregnancy ranges from 37 to 42 weeks, and only about 5% of babies are born on their exact due date. Each trimester lasts approximately 13 weeks, with the first trimester being weeks 1-12.

This is the standard method used by healthcare providers.

Important: This tool provides estimates only. Only about 5% of babies are born on their estimated due date. Your actual due date may differ based on ultrasound measurements and individual factors. Always consult your healthcare provider for medical advice regarding your pregnancy.

About This Tool

The Due Date Calculator estimates your baby's expected arrival date using Naegele's rule, the same method used by obstetricians and midwives worldwide. Named after German obstetrician Franz Naegele (1778-1851), this formula calculates the estimated due date (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP).

How Naegele's Rule Works

The formula assumes a 28-day menstrual cycle with ovulation occurring on day 14. From the first day of your last period, the calculator adds 280 days to arrive at the estimated due date. If you know your conception date instead, the calculator subtracts 14 days to estimate the LMP, then applies the same 280-day rule. While simple, this method has been the clinical standard for over two centuries and aligns closely with first-trimester ultrasound dating for most pregnancies.

Understanding Gestational Age vs. Fetal Age

Gestational age is measured from the first day of the last menstrual period, not from conception. This means the "clock starts" approximately two weeks before conception actually occurs. A baby born at 40 weeks gestational age is actually about 38 weeks from conception. Healthcare providers universally use gestational age because the LMP date is typically more reliably known than the exact date of conception.

Accuracy of Due Dates

Only about 5% of babies are born on their estimated due date, and just 50% are born within one week of it. The due date is best thought of as the middle of a window rather than a precise prediction. First-trimester ultrasounds (performed between 8 and 13 weeks) can refine the due date with an accuracy of plus or minus 5 to 7 days. Later ultrasounds are less precise because of increasing variability in fetal size. Your provider may adjust your due date based on early ultrasound measurements, especially if there is a discrepancy of more than 7 days from the LMP-based calculation.

When to Contact Your Provider

Schedule your first prenatal visit as soon as you suspect you are pregnant, ideally between 6 and 8 weeks. Contact your provider immediately if you experience severe abdominal pain, heavy bleeding, persistent headaches with vision changes, sudden severe swelling, or if your baby's movements decrease significantly after 28 weeks. Remember that every pregnancy is unique, and the due date is an estimate. Term pregnancy ranges from 37 to 42 weeks, and healthy babies can arrive at any point within this window.

Frequently Asked Questions

How accurate is the due date calculation?
The due date based on Naegele's rule is an estimate. Only about 5% of babies are born on their exact due date. Most healthy pregnancies deliver between 37 and 42 weeks. A first-trimester ultrasound can refine the estimate to within 5-7 days of accuracy. If your cycle is longer or shorter than 28 days, the actual due date may shift by a few days.
What is the difference between LMP and conception date?
The LMP (last menstrual period) is the first day of your last period before becoming pregnant. Conception typically occurs about 14 days later, during ovulation. Healthcare providers use LMP because it is a more reliably known date for most women. If you used ovulation tracking, fertility treatments, or know your conception date precisely, you can use that option for a potentially more accurate estimate.
What are the three trimesters of pregnancy?
The first trimester spans weeks 1-12, during which major organs begin forming. The second trimester (weeks 13-26) is when the baby grows rapidly and most women start feeling movement. The third trimester (weeks 27-40) is the final stretch when the baby gains weight and prepares for birth. Each trimester brings different developmental milestones and physical changes for the mother.
Can my due date change during pregnancy?
Yes. If an early ultrasound (8-13 weeks) shows a significant discrepancy from the LMP-based due date (usually 7+ days), your provider may adjust it. Due dates are less likely to change based on later ultrasounds since size variability increases as pregnancy progresses. Conditions like gestational diabetes can also affect fetal growth without changing the actual gestational age.
What does 'full term' mean?
The American College of Obstetricians and Gynecologists (ACOG) defines full term as 39 weeks 0 days through 40 weeks 6 days. Early term is 37-38 weeks, late term is 41 weeks, and post-term is 42 weeks or later. Babies born at full term generally have the best health outcomes. Induction or cesarean delivery before 39 weeks without a medical indication is discouraged because the final weeks are critical for brain and lung development.

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