What to Expect When You're Expecting
A widely used pregnancy book that some readers may find helpful after estimating a due date.
View on AmazonDue Date Calculator
Estimate your due date from your last period, conception date, or a known due date.
Short answer: Your due date is the first day of your last menstrual period plus 280 days (Naegele's rule), adjusted for cycle length. Few babies arrive exactly on the predicted day. Pregnancy normally lasts 37 to 42 weeks from the first day of the last period. See the methodology for formulas and sources.
How this works
Pregnancies are dated from the first day of your last menstrual period (LMP), not from conception. The full term is 280 days (40 weeks) from LMP, or about 266 days from conception. This rule (Naegele's rule) is the calculator's default estimate before any cycle-length or ultrasound adjustment.[1]
If your cycle is longer or shorter than 28 days, we adjust: a 32-day cycle pushes conception 4 days later, which moves the due date 4 days later too. Few babies arrive exactly on the predicted date. Pregnancy normally lasts 37 to 42 weeks from the first day of the last period.[2]
When to see a clinician
This tool is for planning, not diagnosis. Confirm your due date with your OB or midwife. An early ultrasound is the most accurate way to date a pregnancy.[1] After the baby arrives, our Baby Percentile Calculator can help put weight and length measurements in context.
Read more: Due Date vs Conception Date and First Trimester Week by Week.
Frequently asked questions
How accurate is a due date calculator?
A due date calculated from your last menstrual period is an estimate. Few babies arrive exactly on the predicted day. Pregnancy normally lasts 37 to 42 weeks from the first day of the last period.[2] An early ultrasound, especially in the first trimester, is the most precise way to date a pregnancy and is what your clinician will usually rely on.[1]
When should I calculate my due date?
You can calculate as soon as you know the first day of your last period or your estimated conception date. A clinician will usually confirm or adjust the date at your first prenatal visit using an ultrasound, which is why your pregnancy app and your chart may show slightly different dates early on.
What if my cycle isn't 28 days?
Our calculator lets you enter your cycle length. A longer cycle shifts ovulation and conception later, which moves the due date later by the same number of days. A shorter cycle moves it earlier. The adjustment matters most when your cycle is more than a few days off from 28.
Is LMP or ultrasound more accurate for dating?
A first-trimester ultrasound is the most accurate way to establish or confirm gestational age. LMP-based dating is close when cycles are regular, but it can be off by a week or more when cycles vary or ovulation was unusually early or late. The threshold for switching to ultrasound dating depends on gestational age and follows banded thresholds that vary from roughly 5 days in the first trimester to 21 days near term.[1] See our methodology page for the full table.
Can my due date change later in pregnancy?
Clinical guidance generally keeps the first-trimester due date once it is set, because early ultrasounds are more precise than later ones. After about 14 weeks, fetal size varies more between healthy pregnancies, so a later ultrasound is rarely used to reassign the due date unless no earlier dating exists.
What if I don't know my last period date?
Use the conception date option if you know when you conceived, or the due date option if a clinician has already given you one. If none of these are known, an early dating ultrasound is the most reliable path and will produce an estimate within a few days either way.