Calculate This Body · Due Date
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Pregnancy
Due Date

Estimated delivery date from last menstrual period, conception date, or ultrasound, plus a full timeline of trimesters and pregnancy milestones.

See the math behind this calculator
Standard term
280 days · 40 weeks

Full-term window
37 to 42 weeks

On exact due date
~5% of births
Calculate from
Last menstrual period
Estimated due date -
Current gestational age-
Weeks remaining-
Conception (est.)-
Trimester-
▸ Milestone timeline
Week Milestone Date

The math

The standard pregnancy due date (what doctors call the "EDD" or estimated date of delivery) is calculated by Naegele's rule: add 280 days to the first day of the last menstrual period.

EDD = LMP + 280 days
or: EDD = LMP + 9 months + 7 days

The 280-day number assumes a 28-day cycle with ovulation at day 14. Conception then happens around day 14, and pregnancy is dated from the LMP, so "40 weeks of pregnancy" actually includes about 2 weeks before conception. For longer or shorter cycles, the calculator adjusts the conception date accordingly.

Frequently asked

Why does pregnancy start before conception?

For historical reasons, and because the LMP is a date most people can remember. The actual conception date usually can't be pinpointed. So the medical convention dates pregnancy from the last period, even though the embryo doesn't exist until about two weeks in.

How does ultrasound dating work?

Early ultrasounds (before 14 weeks) measure the embryo or fetus's size (particularly the crown-rump length) and compare it to growth charts. Early in pregnancy, growth is highly predictable across individuals, which makes it the most accurate dating method. Later in pregnancy, individual variation makes ultrasound less precise.

Should I be worried if I'm past my due date?

About 5% of babies are born on their estimated due date. About two-thirds are born within a week on either side. Most providers will discuss induction options between 41 and 42 weeks if you haven't delivered. Going to 41 weeks is normal; going past 42 is when most providers intervene.

Show the working

Four different starting points, all resolved back to a last menstrual period date, from which everything else is counted. If any of it looks wrong, it might be. Tell us what we got wrong.

From last menstrual period due = LMP + 280 days + (cycle length − 28)
From conception date LMP = conception − 14 days due = conception + 266 days
From an ultrasound gestational age in days = weeks × 7 + days LMP = scan date − gestational age due = LMP + 280 days
Working backwards from a due date LMP = due date − 280 days
Current gestational age days = today − LMP weeks = floor(days ÷ 7), days = remainder

Constants used

  • 280 days is Naegele's rule, 40 weeks from the last menstrual period
  • 266 days is 38 weeks from conception, which is the same thing with the two-week pre-ovulation period removed
  • cycle length − 28 is the adjustment for cycles that aren't 28 days, since ovulation shifts with cycle length rather than the period

Only about 5% of babies arrive on the due date. It's the midpoint of a distribution, not an appointment, and an early ultrasound is a more accurate basis than an LMP date for anyone with irregular cycles.

The sources

Due dates are a clinical standard rather than a calculation anyone gets to invent, so this page follows the published method exactly. The sources below are the guideline itself and the work on how accurate the method actually is.

Naegele's rule, as implemented here EDD = LMP + 280 days + (cycle length - 28)
  1. American College of Obstetricians and Gynecologists. Committee Opinion 700: Methods for Estimating the Due Date, 2017. The governing clinical guideline. Establishes the estimated due date as 280 days from the first day of the last menstrual period, and that an early ultrasound outranks LMP dating where the two disagree.
  2. Pregnancy Dating. StatPearls, NCBI Bookshelf, National Library of Medicine. Sets out Naegele's rule and, more usefully, its two standing fallacies: recall of the LMP date, and the assumption that every cycle is 28 days with ovulation on day 14.
  3. Committee Opinion No. 700: Methods for Estimating the Due Date. Obstet Gynecol 129(5), 2017. The indexed record of the same guideline, for anyone who wants the citation rather than the practice bulletin.
  4. Gestational length assignment based on last menstrual period, first trimester crown-rump length, ovulation, and implantation timing. PubMed Central, NIH. Compares the dating methods directly against each other, which is what justifies the cycle-length adjustment this page applies.
  5. Khambalia AZ, Roberts CL, Nguyen M, et al. Predicting date of birth and examining the best time to date a pregnancy. Int J Gynaecol Obstet 123(2), 2013. Where the birth-timing figures come from. Among 18,708 women with spontaneous labor, 5% of births fell on the estimated date and approximately 66% within 7 days of it.

The cycle-length adjustment is the part worth understanding. Naegele's rule assumes ovulation on day 14, so a longer cycle shifts ovulation and not the period, which is why this page adds the difference from 28 rather than scaling the whole 280 days. A due date is a midpoint, not an appointment, and most births fall in a two-week band around it.