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Calculator Answers

Due Date Calculator

Source: ACOG and SMFM Committee Opinion No. 579, Definition of Term Pregnancy (reaffirmed 2025), Box 1 · Source verified August 22, 2026

Blake Boege
Written by Blake Boege · Founder, Calculator Answers

A due date calculator is a clinical tool used to project the expected date of birth for a child. It determines the date by adding two hundred eighty days to the mother's last menstrual period or two hundred sixty-six days to the conception date. This calculation assumes a regular cycle with ovulation occurring on day fourteen. The resulting date is an estimate, as natural delivery windows vary. Clinicians use it to date pregnancies and plan prenatal care schedules.

Estimate your pregnancy due date with our free due date calculator. Enter the first day of your last menstrual period (LMP) or conception date to instantly calculate your estimated due date (EDD), trimester milestones, and pregnancy timeline.

See also: Pregnancy Due Date Calculator, Date Calculator, and Days Until Date Countdown.

Quick Answer

Estimate your baby's due date. Select your calculation method, enter the reference date, and view the estimated delivery date and gestational age.

LMP is the most common reference for Naegele's rule.

Estimates assume a 28-day cycle with ovulation around day 14. Ultrasound and clinician assessment may revise dates.

Pregnancy due date

Estimated due date

Thu, Apr 1, 2027

Currently 8 wk 4 d along

LMPThu, Jun 25, 2026
Estimated due dateThu, Apr 1, 2027
Estimated conceptionThu, Jul 9, 2026
Gestational age8 wk 4 d
TrimesterFirst trimester

Estimates only. Naegele's rule uses LMP + 280 days (40 weeks); conception mode uses + 266 days (38 weeks). Ultrasound dating is more accurate, especially in the first trimester. For prenatal care decisions, work with a clinician.

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Examples

LMP 60 days ago

due ≈ 220 days from today · ~8 wk pregnant

Conception 46 days ago

due ≈ 220 days from today · ~9 wk pregnant

LMP 200 days ago

due ≈ 80 days from today · ~28 wk · third trimester

How it works

How the due date is calculated

Our due date calculator runs calculations using two standard methods based on your known starting reference date:

  • Last Menstrual Period (LMP) Method: Uses Naegele's Rule, adding 280 days (40 weeks) to the first day of your last period. This is the standard gestational age convention.
  • Conception Date Method: Adds 266 days (38 weeks) directly to the date of conception (ovulation date).

Pregnancy week-by-week milestones

Pregnancy is divided into three trimesters spanning roughly 40 weeks from the last menstrual period (LMP). Here are the major developmental milestones at each stage:

FIRST TRIMESTER (Weeks 1-13)
  • Week 4: Implantation complete; pregnancy test becomes positive
  • Week 6: Heartbeat detectable on ultrasound (~110-160 bpm)
  • Week 8: Embryo becomes a fetus; all major organs forming
  • Week 10: Fingers and toes formed; size of a strawberry
  • Week 12: End of first trimester; risk of miscarriage drops significantly
  • Week 13: Sex organs developed (but not yet visible on ultrasound)
SECOND TRIMESTER (Weeks 14-27)
  • Week 16: First fetal movements possible (quickening) — though most first-time moms feel it around 18-22 weeks
  • Week 18-22: Anatomy ultrasound; biological sex often visible
  • Week 20: Halfway point; baby is the size of a banana
  • Week 24: Viability threshold — baby has a chance of survival outside the womb with intensive medical support
  • Week 26: Eyes open; baby responds to sound
  • Week 27: End of second trimester; baby weighs about 2 lbs
THIRD TRIMESTER (Weeks 28-40)
  • Week 28: Brain rapidly developing; baby can dream (REM sleep observed)
  • Week 32: Baby practices breathing movements; about 4 lbs
  • Week 36: Baby usually settles head-down for birth
  • Weeks 37 and 38: ACOG's 'early term', defined as 37 0/7 weeks through 38 6/7 weeks
  • Weeks 39 and 40: ACOG's 'full term', 39 0/7 through 40 6/7. ACOG's own claim about this band is narrower than the 'optimal delivery window' this page used to call it: "the frequency of adverse neonatal outcomes is lowest among uncomplicated pregnancies delivered between 39 0/7 weeks of gestation and 40 6/7 weeks of gestation". That is an observation about risk, not advice about when to deliver
  • Week 41: ACOG's 'late term', 41 0/7 through 41 6/7. The bodies differ here: WHO recommends induction from 41 0/7. ACOG's own wording is in Practice Bulletin 146, which is behind its member wall and which this page has NOT read; it is quoted here through Amis, writing in the Journal of Perinatal Education in 2019, who reproduces both conclusions with a page cite. As a Level B conclusion, meaning limited or inconsistent evidence, "induction of labor between 41 0/7 weeks and 42 0/7 weeks can be considered"; as a Level A conclusion, meaning good and consistent evidence, "induction of labor after 42 0/7 weeks and by 42 6/7 weeks of gestation is recommended". Bulletin 146 was reaffirmed in 2024 and carries a January 2025 Clinical Practice Update alongside it
  • Week 42 and beyond: ACOG's 'postterm', 42 0/7 and beyond
  • ACOG also asks that the bare word 'term' be retired: "the use of the label 'term' to describe all deliveries between 37 0/7 weeks of gestation and 41 6/7 weeks of gestation should be discouraged"

How the date math works

Two standard rules cover the two common reference dates. The calculator runs whichever matches the date you know, then computes gestational age from today.

Naegele (LMP) · due date = LMP + 280 days

Conception · due date = conception + 266 days

Assumes a 28-day cycle with ovulation around day 14. Ultrasound may revise the date.

How due dates are calculated

The estimated due date (EDD) is calculated using Naegele's Rule:

LMP method: Add 280 days (40 weeks) to the first day of your last menstrual period. This is the standard method used by doctors for the first ultrasound — it assumes a 28-day cycle with ovulation on day 14.

Conception method: Add 266 days (38 weeks) to the date of conception. This requires knowing exactly when conception occurred, which is usually only possible with IVF or precise ovulation tracking.

Ultrasound dating: Between 8-13 weeks, an ultrasound provides the most accurate due date estimate (within 3-5 days). If the ultrasound dating differs from your LMP-based date by more than 5-7 days, doctors typically use the ultrasound date.

Only about 4-5% of babies are actually born on their estimated due date. About 80% are born within 10 days on either side of the EDD. The due date is a target, not a deadline — pregnancies between 37 and 42 weeks are considered normal.

Pregnancy trimester guide

FIRST TRIMESTER (Weeks 1-13): The most critical period for fetal development. All major organs form. Common symptoms include nausea (morning sickness), fatigue, breast tenderness, and food aversions. Miscarriage risk is highest in this trimester (~10-20% of confirmed pregnancies), dropping sharply after week 12.

SECOND TRIMESTER (Weeks 14-27): Often called the 'easiest' trimester. Nausea typically subsides, energy returns, and the baby bump becomes noticeable. The anatomy ultrasound at 18-22 weeks is a major milestone. Baby becomes viable (could potentially survive outside the womb) around week 24.

THIRD TRIMESTER (Weeks 28-40): The home stretch. Baby gains weight rapidly — about half a pound per week. Common symptoms include heartburn, back pain, swelling, and difficulty sleeping. Preparations for labor and delivery intensify. Most prenatal appointments increase from monthly to weekly during this trimester.

Related pregnancy tools

If you need more general pregnancy calculations or fertility tracking, check out these related utilities:

Pregnancy CalculatorOvulation CalculatorPregnancy Due Date Calculator

Health note. Estimated due dates are not exact. Ultrasound dating in the first trimester is the most accurate single measure. For pregnancy care, including any concerns about the timeline, work with a qualified clinician.

Related Calculators

More tools from Health

Pregnancy Due Date CalculatorEstimate the pregnancy due date from last menstrual period or conception date. Returns due date, gestational age, and trimester.Date CalculatorCalculate days between dates, add or subtract days from a date, count business days, and find days from today. Works as a date calculator, date duration calculator, days between dates calculator, add days calculator, subtract days calculator, business days calculator, calendar calculator, countdown calculator, and weekday calculator.Days Until Date CountdownCalculate exactly how many days, weeks, hours, and seconds are left until any custom date, birthday, or anniversary.Ovulation CalculatorEstimate ovulation date and fertile window from last menstrual period, cycle length, and luteal phase length.

Frequently asked questions

From LMP, add 280 days (40 weeks). This is Naegele's rule. From conception date, add 266 days (38 weeks). Both assume a 28-day cycle with ovulation around day 14. Cycles vary, so the date is an estimate, not a guarantee.

Most people know LMP more reliably than the exact day of conception. Pregnancy is dated from LMP by convention, so 'gestational age' is two weeks longer than the actual time since conception. Ultrasound can refine the date, especially in the first trimester.

Gestational age is how far along the pregnancy is, expressed as weeks and days from the first day of the last menstrual period. A typical pregnancy reaches term at 37 to 42 weeks. The calculator computes gestational age from today's date.

First trimester is roughly 0 to 13 weeks, second trimester is 14 to 27 weeks, and third trimester is 28 weeks through delivery. Exact week boundaries vary slightly by source, but those are the common cutoffs.

Longer or shorter cycles shift ovulation away from day 14, which shifts the conception date. For irregular cycles, ultrasound dating is more reliable than calendar dating. A clinician may revise the due date based on the first trimester ultrasound.

Yes. Ultrasound dating is the most accurate single estimate in the first trimester (typically within 5 to 7 days). If the calendar-based estimate differs significantly from the ultrasound, clinicians usually adopt the ultrasound date.

The LMP-based calculation is accurate within about ±1 week for women with regular 28-day cycles. For irregular cycles or unknown LMP, accuracy decreases. Early ultrasound dating (between 8-13 weeks) provides the most accurate estimate, typically within 3-5 days. Only about 4-5% of babies are born exactly on their due date — 80% are born within 10 days on either side.

Enter the first day of your last menstrual period in the calculator above. The "Gestational age" output shows your current pregnancy week and day (e.g., 14 weeks 3 days). Pregnancy is typically counted from the first day of your LMP, not from conception — which means by the time most women confirm pregnancy, they're already 4-6 weeks along by this count.

The second trimester begins at week 14 and ends at week 27. The first trimester is weeks 1-13. The third trimester begins at week 28 and runs to delivery (typically week 40). The calculator above shows your current trimester based on today's date and your due date.

It's unlikely but possible, especially for women with short cycles (21-25 days). Sperm can survive in the female reproductive tract for up to 5 days. If you ovulate early — say, day 10 or 11 of a short cycle — having sex during a longer period (days 6-8) could potentially result in pregnancy. For more on fertility windows, see our ovulation calculator.