EDD Calculator — Your Estimated Due Date

Your 40-week timeline

Where you are today on the 40-week journey — the marker moves with your dates.

Keep exploring

Three companion tools for pregnancy and dates.

How it works

Three steps from one date to your estimated due date.

The math behind your due date

Go deeper

Why the rule works — and why no due date is exact.

How due dates are estimated

Three clinical methods — last period, ultrasound and IVF — and why the earliest good measurement is the one that sticks.

Every estimated delivery date comes from one of three anchors: the first day of the last period, an ultrasound measurement, or — for IVF pregnancies — the embryo transfer. This expected due date calculator offers all of them (plus a conception-date route), and clinics work the same way. The method guidance behind them is the American College of Obstetricians and Gynecologists’ Committee Opinion 700, Methods for Estimating the Due Date (2017, reaffirmed 2025), and this page follows it throughout.

Dating from the last period. The default route is Naegele’s rule: 280 days from the first day of the last menstrual period. An LMP due date calculator is really pricing in a standard cycle — it assumes ovulation, and with it conception, happened on day 14 of a 28-day cycle. That assumption is the method’s one soft spot. If your cycle is regularly 32 days, ovulation most likely fell around day 18, so an EDD by LMP runs about four days earlier than the conception-based truth; a regular 24-day cycle bends it the other way. Nothing is wrong with the arithmetic — the rule is exact for the cycle it assumes — which is why the estimate is always worth confirming against an early scan when one is offered. Used as a Naegele’s rule calculator or a simple day count, the LMP method is the starting estimate, not the final word. The same date also settles “how far along am I based on last period” — the week count runs from that same first day, worked through in the next section.

The size of the shift is simple arithmetic: ovulation sits about 14 days before the next period, so in a regular cycle of N days it falls on day N − 14, and the LMP estimate moves by N − 28. A regular 32-day cycle with a last period of January 1, 2026 puts ovulation around January 19 and the conception-based due date at October 12, 2026 — four days later than the October 8 the unadjusted rule prints. A regular 24-day cycle pulls it about four days earlier instead. This is an adjustment of days, not weeks, and it only applies when the cycle is genuinely regular at that length; a varying cycle is exactly the case where the scan, not the calendar, should set the date.

Dating by ultrasound. ACOG’s guidance is that ultrasound measurement in the first trimester — up to 13 weeks and 6 days — is the most accurate way to establish or confirm gestational age, because early embryos grow at a near-identical rate. A scan does not automatically move your date, though. The due date is revised only when the scan disagrees with the LMP date by more than a threshold set for that stage: more than 5 days up to 8 weeks and 6 days, more than 7 days from 9 weeks to 13 weeks and 6 days, and wider margins later — 10 days, then 14, then 21 — because growth varies more as pregnancy goes on. Once a date is set from an early scan it is normally left alone, and ACOG notes that a pregnancy with no ultrasound confirming or revising the date before 22 weeks counts as suboptimally dated. Later scans measure growth; they are not dating tools.

Dating an IVF pregnancy. IVF removes the guesswork: the fertilisation day is known, so the embryo transfer due date is built from the transfer date and the embryo’s age — transfer plus 266 days, minus 5 for a day-5 blastocyst (261 days) or minus 3 for a day-3 embryo (263 days). ACOG’s guidance is that this ART-derived date is used as assigned and is not revised by later ultrasound measurements; an IVF pregnancy due date calculator is therefore the one route on this page whose answer a scan will not overrule.

One fact belongs in this section rather than buried at the bottom: only about 5% of babies — roughly 1 in 20 — are born on their estimated due date. The EDD is the centre of a window, not an appointment. Most births land in the weeks either side of it, and a pregnancy running past the date is common and usually unremarkable. Treat every method above as producing a well-reasoned estimate, and let your care provider confirm the date that goes in your notes.

Weeks, trimesters and gestational age

The counting frame the whole family of tools shares — what the words mean, and how to count your own weeks.

What is gestational age? Gestational age is the age of the pregnancy counted from the first day of the last menstrual period (LMP), written as weeks plus days — 12 weeks, 3 days, say. The full gestation period is conventionally 40 weeks, or 280 days, on that count. It surprises many people that the clock starts about two weeks before conception: in the standard 28-day cycle, ovulation falls on day 14, so the baby’s fetal age — time since conception — runs roughly two weeks behind the gestational age. At a gestational age of 12 weeks 0 days (March 26, 2026, in this page’s worked example), the fetal age is about 10 weeks. Every gestational calculator, this one included, counts the LMP way, because the LMP is a date people know and conception usually is not.

How to calculate how many weeks pregnant you are. Count the days from the first day of your last period to today, then split them into whole weeks and leftover days. A last period of January 1, 2026 makes January 29 exactly 4 weeks (4 weeks, 0 days) and February 12 exactly 6 weeks. That is all “calculate pregnancy weeks” ever involves — the pregnancy weeks are counted from the period, not from conception and not from a positive test. Working the other way, from a due date: subtract the time remaining from 40 weeks. With a due date of October 8, 2026, on September 10 there are 28 days — 4 weeks — to go, so the pregnancy is at 36 weeks. And how to know how many weeks pregnant you are when the LMP itself is unknown or cycles are irregular? That is the case an early ultrasound exists for: the measurement sets the count directly, which is why scan-based weeks carry more weight than calendar weeks in that situation.

One reference point ties the count to everyday experience: in a 28-day cycle, the period that does not arrive is due at about 4 weeks 0 days, which is why a home test usually turns positive at roughly four weeks pregnant — the first two of those weeks were before ovulation, and the count had already reached two weeks on the day conception happened. “Two weeks pregnant”, in gestational counting, is the moment of conception itself.

“If I’m 4 weeks pregnant, when am I due?” Four weeks pregnant means 4 weeks since the LMP, so 36 of the 40 weeks — 252 days — remain. In the running example, 4 weeks falls on January 29, 2026, and the due date is still October 8, 2026. The general move: due date = today + (280 − days since your last period). This is also the shape of the month-specific questions (“pregnant in July, due when?”) — the answer always comes from one known date plus the standard intervals, never from the calendar month itself.

Trimesters. The 40 weeks divide into three blocks. The result panel above works as a trimester calculator on exactly these boundaries:

The three blocks as conventionally drawn — the same boundaries this calculator’s timeline and trimester result use.

Clinicians also label the last stretch of the third trimester more finely, using ACOG’s term definitions: early term runs from 37 weeks 0 days to 38 weeks 6 days, full term from 39 weeks 0 days to 40 weeks 6 days, late term from 41 weeks 0 days to 41 weeks 6 days, and postterm is 42 weeks 0 days and beyond. A birth anywhere from 37 to 42 weeks is inside the broad term window, which is one more reason the single due date should be read as the middle of a range.

Weeks into months — honestly. Months are the unit people ask about (“how many months am I?”) and the unit with no exact answer: calendar months run 28 to 31 days, so any week-to-month conversion is a convention, not a calculation. The mapping below is the one in common clinical use:

The conventional month mapping. Calendar months vary from 28 to 31 days, so treat the month as a rough label and the week count as the precise figure.

Notice what the table implies: 40 weeks is nine months plus about a week, not a clean nine months. Quote the week count when precision matters — it is the number every appointment, scan and result on this page is actually built on.

If you only know one date

Four single-date routes to the same estimate — last period, conception, transfer day or scan day.

You only ever need one date, because the four anchors of a pregnancy sit at fixed distances from each other: conception about 14 days after the last period, the due date 280 days after the period and 266 days after conception. Each route below is a complete way to estimate due date from a single date — pick whichever one you actually know and let this due date pregnancy calculator do the one addition or subtraction involved:

• From a last period — January 1, 2026. Add 280 days (Naegele’s rule): due October 8, 2026.
• From a conception date — January 15, 2026. Add 266 days: due October 8, 2026. Same answer, because the conception date sits 14 days after the period it followed.
• From an embryo transfer — January 20, 2026, day-5 embryo. Add 266 − 5 = 261 days: due October 8, 2026.
• From a scan — March 12, 2026, measuring 10 weeks 0 days. 280 − 70 = 210 days remain from the scan day: due October 8, 2026.
• From a last period with a longer cycle — January 1, 2026, regular 32-day cycle. Ovulation falls around day 18 (January 19), so count 266 days from there instead: due October 12, 2026 — the four-day cycle shift, applied.
• Backwards, from a due date — October 8, 2026. Subtract 280 days for the last period (January 1, 2026) and 266 for the likely conception (January 15, 2026) — the direction the Conception Calculator works in.

The month-shaped versions of the question resolve the same way. Conceive in mid-July 2026 — say July 15 — and the due date is April 7, 2027; conceive on August 15, 2026 and it is May 8, 2027. Nothing about July or August matters except the day you count 266 from.

Whichever route you took, read the result as the anchor of a pregnancy calendar by due date: the 40-week line runs from the last period to the estimated date, every week count is a position on that line, and the trimester boundaries fall at weeks 14 and 28 along it. The timeline at the top of this page draws exactly that — a baby due date calendar in miniature, with your own dates on it. That is all a “baby calculator due date” search is really asking for: one known date, one standard interval, one honestly-labelled estimate.

Methods compared — and the pregnancy wheel

Three anchors, one piece of card, and what to do when you only know one date.

The pregnancy wheel. Before due-date calculators, clinics used a paper disc called a pregnancy wheel or OB wheel: two rotating discs, one marked with calendar dates and one with weeks of pregnancy. Line the arrow up with the first day of the last period and the wheel reads off the due date and the current week at a glance. It is the same arithmetic as Naegele’s rule, done mechanically — last period plus 280 days — with no new information in it. The wheel also carries the method’s one assumption in plain sight: it is printed for a 28-day cycle, so for a regularly longer or shorter cycle the reading has to be shifted by the difference, exactly as the calculator’s cycle-length adjustment does. A wheel is a handy waiting-room prop, and this calculator is the same wheel with the adjustment built in.

The three methods, side by side. Dating from the last period is the default because the date is usually known, but it assumes ovulation on day 14 of a 28-day cycle, which many cycles do not follow. An early ultrasound dates the pregnancy by measuring the embryo directly, and it is the method clinicians trust most in the first trimester — when it disagrees with the period date by more than the margin allowed for that stage, the ultrasound date is the one that sticks, which is why the earliest good measurement is the anchor this page follows. IVF dating is the most exact of the three, because the transfer date and the embryo’s age are both known: the due date is the transfer date plus 266 days minus the embryo age. Each method is an estimate with its own honest margin; none of them moves the baby, and none of them should be treated as an appointment.

If you only know one date. Any single anchor is enough to start from. From a conception date, add 266 days. From an ultrasound’s estimated due date, the date itself is the answer. From “I am X weeks pregnant” with no remembered last period, count back X weeks to the estimated first day of the last period and add 280 days — the same interval, run from the other end. And from a due date already given, subtract 280 days to recover the last period, or 266 to recover the conception window. That is all a due-date search is really asking for: one known date, one standard interval, one honestly-labelled estimate.

Related guides

Two guides that take this page’s methods further — the dating methods compared side by side, and the conception-to-due-date line worked in both directions.

Related calculators

More tools for pregnancy, dates, and body metrics.