Pregnancy Calculator · Medically Reviewed

Due Date Calculator

Estimate your baby’s due date from your last period, conception date, or IVF transfer date — then see exactly where you are on the 40-week timeline.

Used in Naegele’s rule: LMP + 280 days.
Adjusts for cycles longer/shorter than 28 days.
Estimated Due Date
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You are currently
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Viability milestone
This estimate uses standard obstetric dating formulas and is accurate for roughly 70% of births within ±2 weeks. Only about 1 in 20 babies arrive on the exact estimated date — your clinician’s ultrasound dating (especially from the first trimester) takes priority over this calculator.

Your 40-Week Pregnancy Journey

Calculate your due date above to highlight where you are today. Each marker is a real clinical checkpoint, not a generic week count.
40-week pregnancy timeline showing three trimesters and key milestones A horizontal timeline from week 0 to week 40, divided into three trimesters (weeks 1–12, 13–27, 28–40), marking implantation, first prenatal visit, anatomy scan, viability at 24 weeks, and full term at 39 weeks. TRIMESTER 1 TRIMESTER 2 TRIMESTER 3 Weeks 1–12 Weeks 13–27 Weeks 28–40 Wk 4 Implantation Wk 8 First prenatal visit Wk 12 NT scan window closes Wk 20 Anatomy scan Wk 24 Viability milestone Wk 39 Full term
Conception & early development Key scans Viability & full term

How the Due Date Calculator Works

Pregnancy is dated from the first day of your last menstrual period (LMP), not from the day you conceived — even though conception happens roughly two weeks later, in the middle of an average cycle. This convention exists because most people know their LMP date with confidence, while the exact day of conception is usually unknown.

Naegele’s Rule (LMP method)

The standard formula adds 280 days (40 weeks) to the first day of your last period. If your cycle isn’t exactly 28 days, the calculator adjusts the estimate forward or backward to account for a later or earlier ovulation day.

Conception Dating

If you tracked ovulation with a fertility monitor, basal body temperature, or LH strips, this method adds 266 days to your known or estimated conception date — more precise than LMP dating when ovulation timing is uncertain.

IVF Transfer Dating

For IVF pregnancies, the embryo’s age at transfer is known precisely, making this the most accurate dating method available. A Day-5 blastocyst transfer adds 261 days; a Day-3 transfer adds 263 days.

Ultrasound Dating

A first-trimester crown-rump-length ultrasound (typically weeks 8–13) is considered the most accurate dating method of all and will override LMP-based estimates if the two differ by more than 5–7 days.

How Accurate Is This Due Date?

Due dates are an estimate, not a prediction. Research consistently shows that only about 1 in 20 babies are born on their exact estimated due date. Most births happen within a two-week window around it — roughly 80% of babies arrive between 37 and 42 weeks. Use this calculator as a planning tool for prenatal visits, leave, and preparation, and rely on your care provider’s ultrasound-confirmed dating as the clinical standard.

Trimester Breakdown

  • First trimester (weeks 1–12): Conception, implantation, organ formation, and the first prenatal visit.
  • Second trimester (weeks 13–27): Often called the “golden trimester” — fetal movement begins, and the anatomy scan happens around week 20.
  • Third trimester (weeks 28–40): Rapid weight gain, viability milestone at 24 weeks, and full term beginning at week 39.

More Calculators and Articles

Explore every tool in our pregnancy calculator hub, and read in-depth articles about dating, symptoms and each trimester on the PregCalc blog.

Related Reading

Also see our calculator FAQs, pregnancy FAQs and pregnancy terms glossary.

Frequently Asked Questions

Yes, and this is normal and expected. ACOG (Committee Opinion 700) sets specific thresholds for when a due date should be revised based on ultrasound: before 9 weeks, a difference of more than 5 days triggers redating; from 9 0/7–13 6/7 weeks, more than 7 days; from 14 0/7–15 6/7 weeks, more than 7 days; from 16 0/7–21 6/7 weeks, more than 10 days; from 22 0/7–27 6/7 weeks, more than 14 days; and at 28 weeks or beyond, more than 21 days. Once a due date is set (ideally from a first-trimester ultrasound, which is accurate to within about 5–7 days), ACOG recommends it generally should not be changed later in pregnancy.
A due date is 280 days (40 weeks) from your LMP, which is closer to 9 months and 1 week — a quirk of counting in 7-day weeks rather than calendar months.
Yes — IVF dating is considered the gold standard because the embryo’s exact age is known. The standard formula is: due date = transfer date + 266 days − embryo age in days. For a day-5 (blastocyst) transfer, that’s transfer date + 261 days; for a day-3 transfer, transfer date + 263 days. Because ovulation-timing guesswork is removed entirely, IVF dating typically isn’t revised later based on LMP.
If your periods are irregular or you’re unsure of the date, an early ultrasound (ideally before 14 weeks) is the most reliable way to establish or confirm your due date — ask your provider to schedule one as soon as possible.
Medically reviewed by Dr. Ruqaiya Khan
[Credential — e.g. MBBS, FCPS Obstetrics & Gynaecology] · [Verified profile link]

Dr. Khan reviews all clinical content on PregCalc for accuracy against current ACOG, NHS, and Mayo Clinic guidance. Last reviewed: .

Sources: American College of Obstetricians and Gynecologists (ACOG) · NHS Pregnancy Dating Guidelines · Mayo Clinic

Want to explore more of your pregnancy timeline? Try our Gestational Age Calculator to double-check your dating, or read official guidance from the American College of Obstetricians and Gynecologists on how a due date calculator estimate is used in prenatal care.

Plan, track and prepare with our other medically reviewed tools.

Ovulation Calculator

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Gestational Age Calculator

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Conception Calculator

Work back from your due date to estimate when you conceived.

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IVF Due Date Calculator

Calculate your due date from an embryo transfer date.

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