How your pregnancy due date is calculated

What a due date is, and why it starts before you were pregnant

A pregnancy due date is an estimated date of delivery, usually written EDD, set at 40 weeks and 0 days of gestational age. Gestational age is counted from the first day of your last menstrual period (LMP), not from conception. MedlinePlus defines it the same way: how far along the pregnancy is, measured in weeks from the first day of the last menstrual cycle to the current date.

That convention surprises many people, because it means you are counted as roughly two weeks pregnant on the day ovulation happens, and a baby that is 'full-term' has really been developing for about 38 weeks. The reason for the convention is practical. The first day of a period is a date most people can remember. Conception is almost never observed. The American College of Obstetricians and Gynecologists (ACOG) states the convention plainly: by convention, the EDD is 280 days after the first day of the LMP.

Naegele's rule: the arithmetic

Naegele's rule has two equivalent forms. The calendar version says: take the first day of the LMP, add 7 days, subtract 3 months, and add a year. The day-count version says: add 280 days. The EveryFix calculator uses the day-count version, which is just 40 weeks written as 280 days.

Worked example. Suppose the first day of the last period was January 1, 2026. Calendar version: January 1 plus 7 days is January 8; minus three months is October 8; the due date is October 8, 2026. Day-count version: January 1 plus 280 days is also October 8, 2026. In this case the two versions land on the same day. They do not always match, because months have different lengths, and across the year the calendar version can land up to about three days away from the exact 280-day count. If your clinic's date differs from a calculator's by a day or two, that rounding is one common reason.

The calculator also shows your current gestational age in weeks and days while the entered date is within 300 days of today, counted from the date you entered. A last period that began on January 8, 2026 reaches 40 weeks on October 15, 2026, because 280 days after January 8 is October 15.

Calculating from a conception date instead

If you know the conception date, for example from a documented single act or from fertility treatment, the calculator adds 266 days instead. The logic is that a standard 28-day cycle puts ovulation about 14 days after the LMP, and 280 minus 14 is 266. So a conception date of January 15, 2026 gives October 8, 2026, the same date as an LMP of January 1.

One quirk to know about: when you choose the conception option, the calculator's current-age display counts weeks from the date you entered, which is conception, not from the LMP. Clinicians and pregnancy apps usually quote gestational age counted from the LMP, which is about two weeks higher. If the tool says 20 weeks from a conception date, the LMP-style gestational age your provider would use is about 22 weeks.

Assisted reproduction works similarly but more precisely. ACOG says that for an IVF pregnancy the due date is assigned from the age of the embryo and the date of transfer: for a day-5 embryo, 261 days from the transfer date, and for a day-3 embryo, 263 days. That is another reason to treat a calculator as a rough planning tool when treatment dates are involved.

Cycle length: the assumption behind the number

Naegele's rule assumes a regular 28-day cycle with ovulation on day 14. ACOG notes that this convention does not account for inaccurate recall of the LMP, irregular cycles, or variation in when ovulation occurs. ACOG also notes that it has been reported that approximately half of women accurately recall their LMP.

To see why cycle length matters, take the January 1 example. If a person ovulates later, say on cycle day 21 (a 35-day cycle with a 14-day luteal phase, which is an illustrative assumption and not a universal fact), conception is about January 22, and adding 266 days gives October 15, a week later than the standard calculation. A shorter cycle with earlier ovulation would shift the date earlier by the same logic. The EveryFix calculator does not ask for cycle length, so it will not make this adjustment; if your cycles are long, short or irregular, your provider will usually rely on an ultrasound measurement rather than on the LMP.

Why so few babies arrive on the due date

The calculator says that only about 5 percent of babies are born on the exact due date. A frequently quoted figure is 4 percent, and the NIH-affiliated study below cites 4 percent of women delivering at 280 days. Either way, a due date is not a prediction of the birth day.

That study, by researchers at the National Institute of Environmental Health Sciences, followed 125 naturally conceived singleton pregnancies in which the ovulation date was identified from urinary hormones. The median time from ovulation to birth was 268 days (38 weeks and 2 days), which is only two days longer than the 266 days Naegele's rule assumes. But the variation was large: even after excluding preterm births, the gestational length spanned 37 days. Half of the women delivered between 262 and 275 days after ovulation, a window about two weeks wide, and the middle 80 percent spread across 253 to 280 days.

The researchers also found that pregnancies counted from the LMP showed more variability than those counted from ovulation, because the LMP does not capture when ovulation really happened. Pregnancies with a quick progesterone rise after conception tended to run longer, and women whose other pregnancies were long tended to have long ones again. So some of the spread is just biology that no calendar formula can see.

How ultrasound dating changes the date

An early ultrasound can measure the embryo directly, which sidesteps the uncertainty in cycle timing. According to ACOG's Committee Opinion 700, measuring the embryo or fetus in the first trimester, up to and including 13 weeks and 6 days, is the most accurate way to establish or confirm gestational age. A crown-rump length measurement in that window has an accuracy of plus or minus 5 to 7 days.

ACOG's redating rules, in short, are these. If ultrasound dating before 14 weeks differs from the LMP date by more than 7 days, the due date should be changed to match the ultrasound; before 9 weeks, a gap of more than 5 days is enough. Between 14 weeks and 15 weeks 6 days the cutoff is more than 7 days, and between 16 weeks and 21 weeks 6 days it is more than 10 days. In one study cited by ACOG, 40 percent of women randomized to first-trimester ultrasound had their due date adjusted because of a discrepancy of more than 5 days.

ACOG also says a pregnancy without an ultrasound that confirms or revises the due date before 22 weeks should be considered suboptimally dated. None of this is something to do yourself with a calculator. It is the reason your provider may give you a different date than the one you computed, and a date revised by ultrasound is not a mistake in the first calculation.

Common mistakes and limits of any calculator

A calculator can only do the arithmetic you give it, so most errors come from the inputs and the interpretation.

• Using the wrong start date. The LMP means the first day of your last period's full flow, not the day it ended or the day you suspected pregnancy.
• Confusing LMP and conception. Entering a conception date with the LMP option adds two weeks of error, and the reverse takes two weeks off.
• Assuming a 28-day cycle when yours is not. The shift can be a week or more.
• Treating the due date as a deadline. Delivery from 38 to 42 weeks is described by MedlinePlus as a normal range, and being early or late by days or even a couple of weeks is common.
• Ignoring a revised date. If an ultrasound changes your due date, the provider's date replaces the calculator's.
• Using the calculator for multiples, IVF or high-risk pregnancies. The standard rule is not designed for those, and your care team will set the schedule.