Pregnancy Due Date Calculator

Background.

This pregnancy due date calculator estimates the day your baby is most likely to arrive — the estimated date of delivery, or EDD — from either the first day of your last menstrual period (LMP) or a known conception date, and reports how many weeks pregnant you are today, which trimester you are in, and how many days remain until delivery. The default mode uses Naegele's rule, the formula every obstetric textbook in the English-speaking world has taught since the German obstetrician Franz Karl Naegele published it in 1812: add 280 days (40 weeks, or 9 calendar months plus 7 days) to the first day of the last menstrual period. The conception alternative adds 266 days to the conception date instead — the same 280 minus the fortnight of cycle that precedes ovulation in a textbook 28-day cycle.

The calculator also accepts a cycle-length input from 21 to 45 days and adjusts the EDD by exactly (cycleLength − 28) days, because a person with a 32-day cycle ovulates roughly four days later than the Naegele assumption and a person with a 25-day cycle ovulates roughly three days earlier; uncorrected, that drift would push the EDD off by the same amount. Internally every date is anchored at UTC midnight and arithmetic is done in whole-day deltas, so daylight-saving transitions and timezone shifts cannot move the answer by even an hour.

The result is an estimate, not a prediction — only about 4 percent of babies are born exactly on the EDD calculated by Naegele's rule (ACOG Committee Opinion 700, reaffirmed 2024), with roughly 70 percent arriving within ten days of it and the remainder distributed across the 37-to-42-week term window. First-trimester ultrasound dating (crown-rump length between 8 and 13 6/7 weeks) is more accurate than LMP-based dating and supersedes the LMP estimate when the two disagree by more than seven days; that is the official ACOG redating threshold and is the reason your obstetrician may quietly shift your EDD at your dating scan.

This tool gives you the LMP-only number — the starting point your doctor uses before the first ultrasound — together with the gestational age and trimester math that drives every prenatal appointment, screening test, and clinical decision point from the nuchal translucency scan (11–13 6/7 weeks) to the anatomy scan (18–22 weeks) to the glucose tolerance test (24–28 weeks) to the GBS swab (35–37 weeks).

What is pregnancy due date calculator?

An estimated due date (EDD) is the calendar day, calculated from a known menstrual or conception reference, on which a pregnancy is expected to deliver if it goes the full 40 weeks from the last menstrual period. Gestational age is the elapsed time from the first day of the LMP (or LMP-equivalent for IVF and conception-dated pregnancies) measured in completed weeks plus extra days — written as 'X weeks + Y days' or simply 'XwYd' on clinical charts. Gestational age starts at 0w0d on the LMP, not at conception; this convention dates back to a time when LMP was the only datable event in pregnancy and persists because it produces a consistent number across both spontaneous and assisted-reproduction pregnancies. The three trimesters under the current ACOG convention are: First trimester — week 0 through the end of week 13 (days 0–97). This covers organogenesis, the highest miscarriage risk, the nuchal translucency scan, NIPT cell-free DNA screening, and the CVS window. Second trimester — week 14 through the end of week 26 (days 98–188). The 'easy' middle, anatomy scan around week 20, quickening (felt movement) typically 18–22 weeks for first pregnancies and earlier for subsequent ones, viability threshold (under modern NICU care) around 22–24 weeks. Third trimester — week 27 onward (day 189+). Glucose tolerance test, Rh-immune-globulin if needed, fetal-movement counts, GBS swab at 35–37 weeks, term window 37–42 weeks (early term 37 0/7 to 38 6/7, full term 39 0/7 to 40 6/7, late term 41 0/7 to 41 6/7, post-term 42 0/7 and beyond per ACOG Reaffirmed Committee Opinion 579).

How to use this calculator.

1. 01Pick a dating method. Choose 'Last menstrual period (LMP)' if you know — or can reasonably estimate — the first day of your most recent period before pregnancy. Choose 'Known conception date' if you tracked ovulation, had an IVF transfer (use the transfer date and the calculator will treat it as conception), or had a single act of intercourse you can date.
2. 02Enter the reference date in YYYY-MM-DD format (ISO 8601). For LMP, this is day one of bleeding, not the last day. For conception, this is the day of ovulation, fertilisation, or IVF transfer.
3. 03Enter today's date in YYYY-MM-DD format. It defaults to today and is used to compute your current gestational age and the days remaining until the EDD.
4. 04If you are using the LMP method and your cycles are not 28 days long, enter your average cycle length (21–45). The calculator shifts the EDD by (cycleLength − 28) days — a 32-day cycle pushes it four days later, a 25-day cycle pulls it three days earlier. Leave at 28 if you do not know or if you are using the conception method.
5. 05Read the primary output 'Days until due date' to find your EDD: add that many days to today. Read 'Gestational age' and 'Plus days' together as the clinical 'X weeks + Y days' notation. The trimester field tells you which prenatal appointment window you are in.
6. 06Cross-check against your dating ultrasound. ACOG redates the pregnancy if first-trimester ultrasound CRL disagrees with LMP-based EDD by more than seven days (or more than five days in the early first trimester); after the first trimester the redating threshold widens. If you have had a dating scan, use its EDD over this LMP calculation.

The formula.

Naegele's rule (LMP method):

EDD = LMP + 280 days + (cycleLengthDays − 28)

The constant 280 days is exactly 40 weeks. The cycleLength correction exists because Naegele assumed a textbook 28-day cycle with ovulation on day 14; in a person with a 32-day cycle, ovulation is closer to day 18, so conception (and therefore delivery) is shifted four days later. The correction is linear and symmetrical: −7 days for a 21-day cycle, +17 days for a 45-day cycle.

Conception method:

EDD = conceptionDate + 266 days

266 = 280 − 14, where 14 is the fortnight of follicular phase that precedes ovulation in the Naegele baseline. For IVF the rule is the same: transfer date + 266 days (some clinics adjust for embryo age — day-3 vs day-5 transfer — but the simple +266 from transfer is within 24 hours of the more refined calculation and is what most online tools use).

Gestational age (always measured from LMP or LMP-equivalent):

if method = lmp: lmpEquiv = LMP else: lmpEquiv = conceptionDate − 14 days

totalDays = today − lmpEquiv (signed integer day delta, UTC midnight anchored) gestationalAgeWeeks = floor(totalDays / 7) gestationalAgeDaysRemainder = totalDays mod 7 (always 0–6)

Trimester classification follows the ACOG convention:

weeks 0–13 → trimester 1 weeks 14–26 → trimester 2 weeks 27+ → trimester 3

Due-date offset and weeks remaining:

dueDateOffsetDays = EDD − today (positive = not yet due, negative = past EDD) weeksRemaining = floor(dueDateOffsetDays / 7)

Floor (rounding toward negative infinity) is used for weeksRemaining so that days 1–7 past due read as −1 week, days 8–14 as −2 weeks, etc. — matching clinical 'post-term' language.

All arithmetic uses UTC-midnight Date constructions and 86,400-second day units, which means the answer is timezone-independent and immune to daylight-saving transitions during the pregnancy.

A worked example.

Frequently asked questions.

References& sources.

1. [1]American College of Obstetricians and Gynecologists. Committee Opinion 700: Methods for Estimating the Due Date. Obstet Gynecol. 2017;129(5):e150–e154 (reaffirmed 2024). The definitive U.S. clinical guideline for LMP-based and ultrasound-based EDD assignment, redating thresholds, and the 280-day Naegele baseline.
2. [2]American College of Obstetricians and Gynecologists. Committee Opinion 579: Definition of Term Pregnancy. Obstet Gynecol. 2013;122(5):1139–1140 (reaffirmed 2022). Established the early-term / full-term / late-term / post-term sub-classifications used inside the third trimester.
3. [3]Mongelli M, Wilcox M, Gardosi J. Estimating the date of confinement: ultrasonographic biometry versus certain menstrual dates. Am J Obstet Gynecol. 1996;174(1):278–281. The reliability study that quantified the bias of LMP-based dating against ultrasound biometry across thousands of pregnancies.
4. [4]Cunningham FG, Leveno KJ, Bloom SL, Dashe JS, Hoffman BL, Casey BM, Spong CY. Williams Obstetrics, 26th edition. McGraw-Hill, 2022. Chapter 6 (Prenatal Care) covers the 266-day conception-based EDD calculation and the LMP-equivalent gestational-age anchor for ART pregnancies.
5. [5]Geirsson RT. Ultrasound instead of last menstrual period as the basis of gestational age assignment. Ultrasound Obstet Gynecol. 1991;1(3):212–219. The paper that quantified the cycle-length correction to Naegele's rule and argued for ultrasound primacy in dating.
6. [6]Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848–2855. The cohort study quantifying the actual delivery-day distribution around the Naegele EDD — only ~4% on the predicted day, with substantial individual variation.
7. [7]World Health Organization. ICD-11 for Mortality and Morbidity Statistics — Chapter 18: Pregnancy, childbirth or the puerperium. The international classification of gestational-age categories and term/preterm definitions used by national perinatal statistics.
8. [8]Grobman WA, Rice MM, Reddy UM, et al. Labor Induction versus Expectant Management in Low-Risk Nulliparous Women (ARRIVE trial). N Engl J Med. 2018;379(6):513–523. Defines current evidence for post-term management decisions and the 39-week elective induction threshold that has reshaped late-pregnancy obstetric care.