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🍼 Due Date Calculator

Calculate your estimated due date (EDD) using last menstrual period (LMP), conception date, or IVF transfer date. Shows current pregnancy week, trimester, and upcoming milestones.

What is a Pregnancy Due Date?

A pregnancy due date (EDD — Estimated Due Date) is the date 40 weeks from the first day of the last menstrual period (LMP), representing the most likely date for childbirth based on average gestational length. While only about 5% of babies are born exactly on their due date, the estimate provides a critical reference point for scheduling prenatal care, monitoring foetal development, and planning for birth.

The standard calculation method is Naegele's Rule: add 280 days (40 weeks) to the first day of the LMP. This assumes a regular 28-day cycle with ovulation on day 14. Women with irregular cycles or known conception dates may use conception-based calculations instead. For IVF pregnancies, transfer date and embryo age (day 3 or day 5) provide the most accurate baseline.

In clinical practice, the first-trimester ultrasound (weeks 8–13) is the most accurate method for confirming gestational age and adjusting the EDD. Crown-rump length (CRL) measurements can shift the due date by several days from the LMP estimate. The calculated due date anchors all subsequent gestational age milestones and screening test scheduling throughout the pregnancy.

How Due Date is Calculated

Naegele's Rule: EDD = LMP + 280 days (40 weeks)
Conception method: EDD = Conception date + 266 days (38 weeks)
IVF Day 5: EDD = Transfer date + 261 days (37w 2d)
IVF Day 3: EDD = Transfer date + 263 days (37w 4d)

How the Due Date Calculator Works

Formula, assumptions, and calculation steps for this health tool.

Methodology

Health calculators use published screening formulas and common planning rules to estimate body, nutrition, pregnancy, or fitness metrics from user inputs.

Calculation Steps

  1. Enter the personal measurements requested by the tool.
  2. Convert height, weight, age, dates, or activity inputs to standard units.
  3. Apply the health or fitness formula for the selected metric.
  4. Show the estimate with practical ranges or interpretation where available.

Assumptions and Limits

  • Results are educational estimates, not diagnosis or medical advice.
  • Individual factors such as medication, pregnancy, and medical history can change interpretation.
  • Consult a clinician for personal health decisions.

Reference basis: Common public-health and sports-science screening formulas.

Frequently Asked Questions

Only about 5% of babies are born on their exact due date. Full term is considered 37–42 weeks. The EDD is a midpoint estimate — most births occur within 2 weeks before or after. Early ultrasound (8–12 weeks) is the most accurate way to confirm dating.

Naegele's Rule estimates the due date as LMP + 280 days (40 weeks). It assumes a 28-day menstrual cycle with ovulation on day 14. Women with irregular cycles may need adjustment. This is the method used in this calculator.

First trimester: weeks 1–13 (organ development). Second trimester: weeks 14–26 (rapid growth, movement). Third trimester: weeks 27–40 (weight gain, lung maturation). Each trimester brings distinct developmental milestones.

For IVF, the embryo's age at transfer is known. A day 5 blastocyst transfer subtracts 5 days from the conception equivalent, giving EDD = transfer date + 261 days. Day 3 embryo: EDD = transfer + 263 days.

An early ultrasound (ideally at 8–12 weeks, called a dating scan) provides the most accurate EDD. If the ultrasound date differs from LMP date by more than 7–14 days, the ultrasound date is typically used.

Real-World Applications

🩺
Prenatal Appointment Scheduling
The EDD anchors the entire prenatal care calendar — nuchal translucency screening (week 11), anatomy scan (week 20), and GBS test (week 36).
💼
Parental Leave Planning
Calculate the earliest date to notify an employer and when maternity/paternity leave begins relative to the due date.
🎓
Childbirth Class Enrolment
Classes are typically recommended to start at week 28–32 — count backwards from the EDD to enrol on time.
🛏️
Nursery Preparation
Plan furniture delivery and room readiness to be complete by week 36, giving a 4-week buffer before the due date.
🏫
Childcare Waitlist
Many childcare centres have 12+ month waitlists — register as soon as the EDD is known to secure a place.
🛡️
Insurance Coverage Timing
Open enrolment windows and insurance effective dates need to align with the expected birth date for newborn coverage.

Common Mistakes

1
Using cycle date instead of LMP
The calculation uses the first day of the last menstrual period — not the date of ovulation, conception, or positive test.
2
Ignoring cycle length variation
Naegele's Rule assumes a 28-day cycle. Women with longer (35-day) or shorter (21-day) cycles should use conception date for accuracy.
3
Not adjusting for the week-8 ultrasound
Clinical EDD is often adjusted at the first scan based on CRL measurement — treat the ultrasound date as the most authoritative.
4
Treating the EDD as a deadline
Only ~5% of babies arrive on the exact EDD — a 2-week window either side is normal and clinically expected.
5
Confusing gestational age and developmental age
Gestational age is measured from LMP (starts at week 0); developmental age is from conception (2 weeks later). Healthcare providers use gestational age.

Key Pregnancy Milestones by Week

Week Milestone / Screening Notes
Week 8–10 First prenatal visit; blood panel EDD confirmed by dating scan
Week 11–13 Nuchal translucency (NT) scan; NIPT Down syndrome screening window
Week 18–20 Anatomy scan (20-week scan) Gender visible; anatomy checked
Week 24–28 Glucose tolerance test (GDM screen) Gestational diabetes screening
Week 32–36 GBS swab; birth plan discussion Birth plan finalised
Week 39–41 EDD window; induction discussion Post-40 weeks: monitoring increases

References

  1. American College of Obstetricians and Gynecologists. Methods for Estimating the Due Date. ACOG Committee Opinion 700, 2017.
  2. National Health Service. Your Pregnancy and Baby Guide. NHS UK, 2024.
  3. Naegele, F.K. Lehrbuch der Geburtshülfe. Heidelberg, 1830.
  4. Hadlock, F.P. et al. "Sonographic Estimation of Gestational Age." American Journal of Roentgenology, 1984.
  5. World Health Organization. WHO Recommendations on Antenatal Care for a Positive Pregnancy Experience. WHO, 2016.

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