Find Your Due Date

Getting the Dates Right

The calculation rests on one date, so the accuracy of everything that follows depends on how confident you are about it.

1. Enter the first day of your last period, not the day it ended.
2. Enter your usual cycle length if you know it, since the default assumes 28 days.
3. Read the estimated date alongside your gestational age in weeks and days.
4. Use your clinic's date instead if you have already had a dating ultrasound.
5. Take the result to your first prenatal appointment and let them confirm it.

A Rule Written Two Centuries Ago

Your due date comes from a rule set out by Franz Karl Naegele, a German obstetrician working in the early 1800s. Take the first day of the last period, add a year, subtract three months, then add seven days. That comes to 280 days, or forty weeks, and every clinic in Canada still starts there.

Look at what the rule assumes, though. It assumes every cycle runs 28 days, that ovulation lands on day 14, and that every pregnancy lasts the same length. None of those hold for everyone, and the first two do not hold for a great many people.

There is a stranger feature buried inside it as well. Counting begins at your last period, which falls roughly two weeks before conception, so on the day you find out you are pregnant you are already counted as several weeks along. The rule survives because it needs only one date, one you can usually remember, and no equipment at all.

What the Date Means

The word estimated does more work here than most people notice. Births happen near the date rather than on it, a full-term birth spans several weeks, and arriving early or late is ordinary rather than a problem.

The date still matters a great deal, which is why clinics take dating seriously. Accurate gestational age decides when clinics offer screening tests, how they track growth, and how they handle decisions near the end of pregnancy. The rest of your prenatal care depends on it.

Words You Will Hear

LMP. The first day of your last menstrual period, which is where the calculation starts.

Gestational age. How far along you are, counted from the LMP and written as weeks and days, so twelve weeks and three days appears as 12w3d.

EDD. Estimated date of delivery, which is the due date itself.

Crown-rump length. The measurement from head to bottom taken on an early scan, and the most accurate way to date a pregnancy.

Dating ultrasound. The scan that confirms or corrects your due date, usually carried out in the first trimester.

Why an Ultrasound Can Move the Date

An early ultrasound measures the pregnancy directly rather than working backwards from a calendar, and measuring beats assuming, so clinics usually go with the scan when the two disagree.

Crown-rump length is the measurement they use, and it works up to about thirteen weeks and six days. Beyond that the pregnancy has grown enough that size varies between individuals, and the dating becomes less precise.

Clinics do not rewrite your dates over a small difference. They keep the period-based date when the scan agrees closely, and switch to the scan once the gap passes a set number of days, with that allowed difference widening as pregnancy progresses. If your cycles are irregular or you are unsure of your last period, say so at your first appointment rather than guessing at a date.

When the Date Is Less Reliable

• Cycles longer or shorter than 28 days move ovulation away from the day 14 the rule assumes.
• An uncertain date for your last period makes the whole calculation uncertain.
• A pregnancy conceived soon after stopping hormonal birth control often has unreliable dates.
• Pregnancy following fertility treatment is dated from the procedure instead.
• Twins and more follow different growth patterns and are dated and monitored differently.
• A first scan after the first trimester carries a wider margin of error.