Due date calculator

Last updated: September 6, 2026

Knowing when your baby will arrive is the first question of every pregnancy. This calculator gives you the estimated due date — and the context to understand it.

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How the estimated due date is calculated

The formula every obstetrician uses to calculate the EDD is called Naegele's rule, and it's surprisingly simple: add 280 days (40 weeks) to the first day of your last menstrual period. That's it. The date you get is the estimated due date, also called the EDD or FPP in Spanish.

The reason it starts from the last period rather than conception is practical: most women remember when their last period started, but very few know exactly when they ovulated. Ovulation typically occurs around day 14 of a 28-day cycle, so adding 266 days to conception — or 280 to the last period — gives approximately the same result.

If your cycle isn't 28 days, the EDD adjusts: with a 35-day cycle, ovulation occurs around day 21 instead of day 14, so 7 extra days are added to the calculated EDD. Our calculator makes this adjustment automatically.

How accurate is the due date?

The EDD calculated from the LMP is a statistical estimate, not a guaranteed date. Only 4–5% of babies are born exactly on the EDD; the vast majority arrive within the two weeks before or after. Birth is considered full term between weeks 39 and 40, and late term from week 42 onward.

The most accurate way to confirm the EDD is a first-trimester ultrasound before week 14. At that point, the crown-rump length (CRL) is measured — it has very little variation between babies of the same gestational age. If the ultrasound gives an EDD different from the LMP-based calculation, obstetricians typically use the ultrasound date, especially if the difference exceeds 5–7 days.

How pregnancy weeks are counted

Pregnancy weeks are counted from the first day of the last menstrual period, not from conception or fertilization. This means that in week 1 you technically haven't conceived yet. Conception occurs around weeks 2–3, implantation around weeks 3–4, and the first missed period coincides with weeks 4–5.

This system may seem counterintuitive but makes great practical sense: using the LMP as the starting point makes all calculations consistent and reproducible, regardless of exactly when ovulation or fertilization occurred. Week 40 is the calculated EDD, and a normal birth can occur between weeks 37 and 42.

The three trimesters of pregnancy

Pregnancy is divided into three trimesters of approximately equal duration. The first trimester spans weeks 1 to 13: it's the period of organogenesis, when all major organs form. Miscarriage risk is highest in this period, which is why many couples wait until it's over before announcing the pregnancy.

The second trimester runs from week 14 to 27: many women describe this as the most comfortable part of pregnancy. Early-pregnancy nausea usually subsides, the belly starts showing, and fetal movements are typically felt between weeks 16 and 22. The third trimester spans from week 28 to birth: the baby rapidly matures their lungs and brain, and the mother may notice more fatigue and discomfort as the baby takes up more space.

When an ultrasound changes your due date

The date this page calculates starts from your last period, and that reference assumes you ovulated mid-cycle. Because that is not always what happens, first-trimester ultrasound dating is more reliable: it measures the embryo's crown-rump length, and in those weeks every embryo grows at such a similar rate that the margin of error is only a few days.

The rule obstetric services follow is to adopt the scan date when it differs from the last-period date by roughly 5 to 7 days in the first trimester. If the gap is smaller, the last-period date stands. So walking out of your first scan with a different date from the one the calculator gave you is normal, and it does not mean anything is wrong.

From the second half of pregnancy onwards, ultrasound loses precision for dating: size differences between babies are genuine by then and the margin widens to two weeks or more. That is why the due date is not recalculated at late scans, even if the baby measures larger or smaller than expected.

What happens if you reach week 41

Any birth between weeks 37 and 41+6 counts as term, and post-term begins at week 42. Reaching week 41 with no sign of labour is common, especially in a first pregnancy, and on its own it is not a complication.

What does change is the monitoring. From around that point, checks are usually stepped up with cardiotocography and an assessment of amniotic fluid, because the safety margin narrows gradually as the days pass. Most protocols offer to schedule an induction between weeks 41 and 42, and the decision is made with the team looking after you, weighing your particular situation. None of that is decided by a calculator: what you get here is the reference date those conversations are built on.

Frequently asked questions

This content is informational and educational. The dates and probabilities it calculates are statistical estimates: they do not replace evaluation by your doctor or the professional managing your pregnancy, and no health decision should be based on them.

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