Pregnancy & Period Calculator

How due dates and cycle estimates are calculated

The due-date estimator uses Naegele's rule, the standard obstetric method: add 280 days (40 weeks) to the first day of your last menstrual period, assuming a textbook 28-day cycle where ovulation happens around day 14. If you'd rather calculate from a known conception date instead, the tool adds 266 days (38 weeks), which is the average length of pregnancy counted from ovulation/fertilization rather than the last period.

Your current week and trimester are derived from how many days have passed since the (assumed) start of your cycle. Weeks 1-13 make up the first trimester, 14-26 the second, and 27 onward the third — though many clinicians define the exact cutoffs slightly differently. Because this method assumes a standard 28-day cycle, it will be less accurate if your cycles run consistently longer or shorter; an early ultrasound dating scan remains the most reliable way to confirm gestational age and due date.

The period tracker works on a simpler principle: your next period is estimated by adding your typical cycle length to the first day of your last one, ovulation is placed roughly 14 days before that, and the fertile window spans about five days before ovulation through one day after, reflecting sperm survival time and the egg's short viability window. Cycles naturally vary month to month due to stress, illness, travel, and hormonal changes, so treat these as planning estimates rather than guarantees — and if you're trying to conceive or avoid pregnancy, combine this with other tracking methods or medical guidance rather than relying on dates alone.

Frequently asked questions

How accurate is a due date from Naegele's rule? Only about 4–5% of babies actually arrive on the exact calculated date — it's a useful average, and most births happen within about two weeks either side of it, which is why doctors treat it as an estimate.

What if my cycle isn't exactly 28 days? The calculation adjusts for irregular cycles when you enter your average length, since ovulation timing shifts along with cycle length, and that shifts the estimated due date too.

Should this replace an ultrasound estimate? No. Once a first-trimester ultrasound is available, doctors typically consider it more accurate than a calculation based on the last period alone, since it measures the fetus directly.

Worked example

If the first day of your last period was March 1st and your average cycle is 28 days, standard due-date estimation (Naegele’s rule: last period date + 280 days) puts the due date around December 5th, with ovulation typically falling around March 15th.

Why does the calculator ask for cycle length instead of assuming 28 days? Naegele's rule was originally built around a 28-day cycle; if yours runs longer or shorter, ovulation shifts accordingly, so adjusting for your actual cycle length gives a more accurate estimate.

How accurate is a due date calculated this way? Only about 5% of babies arrive on the exact calculated date — it's a statistical midpoint, and a full-term pregnancy can reasonably arrive anywhere from 3 weeks before to 2 weeks after that estimate.