ΣMyCalc
ΣMyCalc

Due Date Calculator

Find your pregnancy due date using LMP, conception date, or IVF transfer date methods.

Disclaimer: This tool is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making any health-related decisions or starting any diet or exercise programme.

How to Use the Due Date Calculator

This calculator supports three methods — choose the one that applies to you:

  • 🔵 LMP— Enter the first day of your last menstrual period. Best for those with regular 28-day cycles.
  • 🟢 Conception— If you know when you conceived (e.g. from ovulation tracking), enter that date directly.
  • 🟣 IVF— Enter your embryo transfer date. Choose Day 5 blastocyst or Day 3 cleavage stage.

🔬 Most Accurate Method

First-trimester ultrasound (8–11 weeks) measured by Crown-Rump Length (CRL) is the most accurate dating method, typically accurate to within 5–7 days. All calculator methods are estimates — confirm with your healthcare provider.

Formula

LMP method: Due Date = LMP + 280 days (40 weeks)

Conception method: Due Date = Conception + 266 days (38 weeks)

IVF Day-5 transfer: Due Date = Transfer + 261 days

IVF Day-3 transfer: Due Date = Transfer + 263 days

Why the IVF offsets differ:

A Day-5 blastocyst is already 5 days old at transfer, so subtract 5 from 266 = 261 remaining days. A Day-3 embryo is 3 days old, so 266 − 3 = 263 days.

History & Interesting Facts

💡 Did You Know?

The first IVF baby, Louise Brown, was born on 25 July 1978 in Oldham, UK. Today, more than 8 million people have been born through IVF worldwide. Yet the core formula for calculating an IVF due date is simply a modification of Naegele’s 1812 rule!

Origin & History

Due date calculation has evolved dramatically across three eras. Era 1: Naegele’s Rule (1812) — LMP + 280 days, still widely used despite being based on a 100-woman sample. Era 2: Ultrasound Dating (1970s–present) — Ian Donald in Glasgow pioneered obstetric ultrasound; Crown-Rump Length (CRL) measurement is now the gold standard for first-trimester dating, accurate within ±5 days. Era 3: IVF-Specific Dating (1978–present) — following Louise Brown’s birth, clinicians developed transfer-date formulas that are actually the most precise of all three methods, as the embryo age at transfer is known exactly. A landmark 2013 study in BJOG found Naegele’s Rule overestimates gestational length by 2–3 days for first-time mothers.

Fascinating Facts

  • 1

    The 280-day (40-week) gestational period corresponds to 10 lunar months — it’s not a coincidence. Ancient physicians noticed that pregnancy spans about 10 menstrual cycles.

  • 2

    Twins are delivered on average 3.5 weeks earlier than singletons; triplets arrive about 6.5 weeks early. The more fetuses, the shorter the typical gestation.

  • 3

    The longest confirmed human pregnancy lasted 375 days (approximately 53.5 weeks) in Los Angeles, 1945 — though this predates ultrasound and may reflect inaccurate dating.

  • 4

    Babies born at 37–38 weeks (early term) are technically full-term but have higher rates of learning difficulties than those born at 39–41 weeks — which has shifted recommendations to avoid elective early delivery.

  • 5

    Ultrasound dating is more accurate early in pregnancy: at 8–11 weeks, margin of error is ±5 days; at 20 weeks, it grows to ±10 days; after 28 weeks ultrasound is unreliable for dating.

  • 6

    Only about 4% of babies are born on their exact EDD, 80% are born within 2 weeks before or after, and 10% go post-term beyond 42 weeks.

Frequently Asked Questions

What if my cycles are irregular?

For irregular cycles, the LMP method is unreliable. Use the conception date (if known from ovulation testing/tracking) or IVF transfer date. A first-trimester ultrasound at 8–11 weeks is the most accurate option — it measures embryo size directly rather than relying on cycle assumptions.

How accurate are these calculations?

The LMP method has a ±2-week margin for women with regular cycles. The conception method is typically accurate to within ±3–5 days. IVF transfer date is the most precise. First-trimester ultrasound (Crown-Rump Length) is accurate to ±5–7 days. All are estimates — always confirm with your healthcare provider.

What is a blastocyst vs cleavage-stage embryo for IVF?

Cleavage stage (Day 3) embryos are 3 days post-fertilisation with 4–8 cells. Blastocyst (Day 5) embryos are 5 days post-fertilisation, more developed, and have higher implantation rates. The transfer date offset differs because the embryo is already older at transfer, so fewer days remain until birth.

What does "term pregnancy" mean exactly?

The WHO now defines term pregnancy as 37–42 weeks. Before 37 weeks = preterm. 37–38+6 = early term. 39–40+6 = full term (optimal). 41–41+6 = late term. 42+ weeks = post-term. Babies born in the "full term" window (39–41 weeks) generally have the best outcomes.

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