Pregnancy Due Date Calculator

This page answers one question: what estimated due date follows from the dating fact you already have? It counts forward. It leaves “how many weeks am I today?” to the Gestational Age Calculator, an around-date fertilization estimate to the Conception Date Calculator, and embryo-transfer arithmetic to the IVF Due Date Calculator.

ACOG Committee Opinion 700 states the usual convention: the estimated due date is 280 days after the first day of the last menstrual period. That 280-day span is the calendar form of Naegele’s rule. The older month shortcut (add 7 days, subtract 3 months, add 1 year) can land a day away from a true 280-day count because months are not all 30 days long. This calculator adds 280 days. It does not use the month shortcut.

Committee Opinion 700 also says that convention assumes a regular 28-day cycle with ovulation on day 14, and that it does not absorb an uncertain period date, a different cycle length, or ovulation that falls on another day. When you enter a usual cycle length, this calculator shifts the 280-day result by (cycle length − 28) days. A 32-day cycle moves the estimate 4 days later; a 24-day cycle moves it 4 days earlier. The accepted range here is 21 through 35 whole days. That box is this calculator’s input limit. ACOG’s redating thresholds are the ultrasound differences listed later on this page, and a cycle length in this field does not prove that ovulation occurred on a particular day.

A well-supported ovulation date or a documented fertilization date uses a different offset. The Merck Manual describes estimated delivery as 266 days after a known conception date, which is 280 minus the 14 days between a conventional period start and ovulation. An intercourse date belongs in the ovulation field only when that ovulation date is already known from a separate record. The full fertile-window explanation lives on the Ovulation Calculator and in the trying and testing guide.

The dated-record path is for a pregnancy age that a record already states, together with the date that age was written. The calculator steps back by those completed weeks and extra days to a pregnancy-calendar start, then adds 280 days. It does not measure a crown–rump length, choose a biometric formula, or decide that an ultrasound should replace a period date.

First day of the last period: January 1, 2024. Usual cycle length: 28 days. Estimated due date = January 1, 2024 + 280 days = October 7, 2024. The same period with a 32-day cycle uses an adjustment of 32 − 28 = 4 days, so the estimate is October 11, 2024.

If the supported ovulation or documented fertilization date is January 15, 2024, the estimate is January 15 + 266 days, which is also October 7, 2024. That agreement is the 28-day model working as designed: day 14 after January 1 is January 15, and 266 + 14 = 280.

A record that says 8 weeks and 4 days on March 1, 2024 is 60 days of pregnancy age. March 1 minus 60 days is January 1, 2024, and adding 280 days again gives October 7, 2024. Week counts on other days belong on the gestational age calculator.

The date in the result is an estimated due date. Mayo Clinic describes due-date estimation as part of prenatal care and an inexact science: many births fall in the weeks before or after the estimate. ACOG’s patient FAQ on going past the due date treats the conventional pregnancy calendar as 40 weeks and the due date as an estimate. The result on this page is that estimate for the inputs you entered. Labor can start on a different day.

Read the line under the date before you compare it with another calculator. A 280-day period result, a cycle-adjusted result, a 266-day ovulation result, and a dated-record result can disagree when they start from different facts. Averaging them does not create a more official date. If a prenatal record already shows a current EDD, that documented date is the one to use for care. The pregnancy dating guide walks through why two honest methods can still differ, and the methodology records the exact day counts.

Committee Opinion 700, written with the American Institute of Ultrasound in Medicine and the Society for Maternal-Fetal Medicine, says ultrasound measurement of the embryo or fetus in the first trimester, through 13 weeks 6 days, is the most accurate ultrasound method to establish or confirm gestational age, using crown–rump length rather than mean sac diameter. The World Health Organization recommends one ultrasound scan before 24 weeks of gestation to estimate gestational age, among other purposes, in its antenatal care ultrasound recommendation.

The same opinion gives thresholds for when ultrasound dating and last-period dating differ enough that the due date should be changed to the ultrasound date. The cutoffs widen as pregnancy advances:

• Through 8 weeks 6 days, using crown–rump length: a difference of more than 5 days.
• From 9 weeks 0 days through 13 weeks 6 days, using crown–rump length: more than 7 days.
• From 14 weeks 0 days through 15 weeks 6 days, using biparietal diameter, head circumference, abdominal circumference, and femur length: more than 7 days.
• From 16 weeks 0 days through 21 weeks 6 days, using those biometric measurements: more than 10 days.
• From 22 weeks 0 days through 27 weeks 6 days: more than 14 days.
• From 28 weeks 0 days onward: more than 21 days.

ACOG notes that a smaller difference can still justify a change early in the first trimester when the period date is uncertain, and that a third-trimester ultrasound alone is a weak basis for redating because a smaller measurement can reflect growth. A pregnancy with no ultrasound confirming or revising the due date before 22 weeks 0 days is called suboptimally dated in that opinion. This page lists the thresholds. It does not apply them to a measurement. Embryo-transfer dating uses a different rule, on the IVF Due Date Calculator and in How IVF due dates are calculated.

These sources describe due-date methods and limits. They do not review or endorse Pregnancy Metrics.

• ACOG Committee Opinion 700 — Methods for Estimating the Due Date. The 280-day last-period convention, the 28-day assumption, first-trimester ultrasound, the redating thresholds, and embryo-transfer dating.
• ACOG — When Pregnancy Goes Past Your Due Date. The due date as an estimate on a 40-week calendar.
• Mayo Clinic — Pregnancy due date calculator. Counting 40 weeks from the last period, and births before or after the estimate.
• Merck Manual — Evaluation of the Obstetric Patient. The 280-day and 266-day estimated-delivery conventions.
• WHO — Ultrasound before 24 weeks of pregnancy. One scan before 24 weeks to estimate gestational age, among other purposes.
• NICE guideline NG201 — Antenatal care. An ultrasound offered between 11 weeks 2 days and 14 weeks 1 day to determine gestational age.