Due date calculator: how it's really calculated

“When's the baby due?” is the first question anyone will ask the moment they find out you're pregnant. The answer usually comes in the form of a single date — “March 14th” — but that date, in reality, is just a statistical estimate. Let's see how it's really calculated, why it's often off, and how to make the most of the BabyWish pregnancy calculator.

What the estimated due date (EDD) is

The estimated due date — abbreviated EDD, in Italian DPP (Data Presunta del Parto) — is the date on which, statistically, the baby should be born if the pregnancy lasted exactly 40 weeks. The key word is statistically: only 4-5% of babies are actually born on their EDD. All the others arrive earlier or later, in a window considered physiological between the 37th and 42nd week.

Naegele's rule: how it's calculated

The standard method used in obstetrics since 1812 is called Naegele's ruleand it relies on a single piece of information: the first day of the last menstrual period (LMP). The formula is simple:

EDD = first day of last menstrual period + 280 days
(equivalent to +1 year, -3 months, +7 days)

Example: if your last period started on January 1, 2026, your estimated due date is October 8, 2026. This is the calculation you'll find pretty much everywhere, including the BabyWish calculator.

Why Naegele's rule “gets it wrong”

The formula assumes three things that in real life are almost never all true at the same time:

1. A regular 28-day cycle, exactly. If your cycle is 32 days, ovulation (and therefore conception) happened later: the EDD calculated from the LMP is systematically a few days early.
2. Ovulation exactly on day 14. In reality, ovulation fluctuates between day 10 and day 20 even in regular cycles.
3. Average pregnancy length of 280 days. Recent studies show that the median duration is actually 280-282 days for first-time mothers and slightly longer for women who have given birth before.

For this reason, from the second check-up onward, the EDD is corrected based on the first-trimester ultrasound (usually between the 11th and 13th week). The measurement of the embryo's crown-rump length (CRL) gives a much more accurate estimate, with a margin of error of just ±3 days.

How the BabyWish calculator works

Our due date calculator is free, requires no registration, and in exchange for a single piece of information it gives you three useful things back:

• Estimated due date calculated with Naegele's rule starting from the first day of your last menstrual period.
• Current week of pregnancy — not just the number, but also what's happening to the baby in that week (heartbeat, facial features, first movements, etc.).
• Trimester and main milestones ahead: anatomy scan, glucose tolerance test, prenatal class, the right time to open the baby registry.

If you have a very irregular cycle or don't remember the date of your last period, you can also enter the conception date directly (if you know it) or theEDD given by your doctor: the calculator works in both directions.

EDD from ultrasound vs EDD from LMP: which one wins?

Practical rule used by almost all OB-GYNs:

• If the difference between the two dates is ≤ 7 days, the EDD calculated from the LMP is kept.
• If the difference is > 7 days, the EDD is updated based on the first-trimester ultrasound.

From that point on, the “official” EDD is the one from the ultrasound, and all pregnancy weeks are counted from there.

What knowing the EDD is actually for

Knowing your due date isn't just about “counting down the days.” It's the reference point that almost the entire pregnancy calendar depends on:

• Exams and visits — each check-up has a precise window in weeks.
• Anatomy scan — typically performed between the 19th and 21st week.
• Glucose tolerance test — between the 24th and 28th week.
• Opening the baby registry — the ideal moment is right after the anatomy scan.
• Maternity leave — calculated on a weekly basis starting from the EDD.
• Prenatal class — usually starts between the 28th and 32nd week.

Having a reliable (and up-to-date) estimate helps you not fall behind on any of these appointments.

Frequently asked questions

I have an irregular cycle. Does the calculator still work?
Yes, but the estimate will be less precise until the first ultrasound. Enter the last date you remember anyway and update the data after your visit.

Does the calculator account for assisted reproduction?
If you had IVF or an embryo transfer, the EDD is calculated differently (starting from the transfer date + embryo age). The BabyWish calculator lets you enter the conception date directly, which in this case is known precisely.

Does the EDD change during pregnancy?
Yes, at two moments: after the first-trimester ultrasound (physiological correction) and — more rarely — if later check-ups show growth that's well off the curve. Your doctor will tell you explicitly.

What happens if I pass my EDD?
Nothing dramatic. Between the 40th and 41st week the pregnancy is considered “at term.” Beyond the 41st week it's called a post-term pregnancy and closer monitoring begins. Labor induction is typically proposed between the 41st and 42nd week.

In summary

The estimated due date is a useful estimate, but not an appointment to mark rigidly on the calendar. Calculate it once from your last menstrual period, then compare it with the first-trimester ultrasound, and use the updated one as your reference.

To run the calculation right now, with no registration and localized to your week of pregnancy, try our due date calculator. All it takes is one date to have your entire pregnancy calendar laid out in front of you.