✦ Obstetric Calculator

Estimated Due Date
Calculator

Based on ACOG, AIUM, and NICE guidelines. Gestational age, trimester milestones, and prenatal appointment schedule.

Last Menstrual Period — Naegele's Rule

First Day of Last Menstrual Period Must be a past date — cannot be today or future

Average Cycle Length

28d

Standard 28 days · Range 21–45 days (ACOG). Cycle length modifies the EDD via Naegele's Rule.

Pregnancy Status

Pregnancy Type

Reference Date (today) Defaults to today. Must be ≥ LMP and ≤ today.

Ultrasound Dating — AIUM / ACOG Criteria

Date Ultrasound Was Performed Must be a past date — cannot be today or future

Gestational Age Reported at Scan

From scan report. Minimum 4w0d for reliable dating (CRL ≥3 mm)

Biometric Measurement Used

Pregnancy Type

ℹ️ ACOG Redating Thresholds Redate if LMP–US discrepancy exceeds: >7 d (CRL, ≤13w6d) · >10 d (14–15w6d) · >14 d (16–21w6d) · >21 d (≥22 wks). CRL is the gold standard for 1st-trimester dating.

IVF / Frozen Embryo Transfer Dating

Embryo Transfer Date Date of actual embryo transfer — must be in the past

Embryo Age at Transfer EDD = Transfer date + (266 − embryo day). GA is counted from equivalent LMP.

Pregnancy Type

Reference Date (today) Defaults to today. Must be ≥ transfer date and ≤ today.

Known Conception / Ovulation Date

Conception or Ovulation Date Must be 10–294 days in the past for a clinically meaningful result

Pregnancy Type

ℹ️ Clinical Note GA is counted 2 weeks ahead of fertilisation age. EDD = conception + 266 days (38 weeks). Equivalent LMP = conception − 14 days. Minimum 10 days from conception required for detectable hCG and a clinically meaningful GA estimate.

✦ Estimated Due Date

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Current Gestational Age

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Current Trimester

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Days Until EDD

Pregnancy Progress

1st Trimester

2nd Trimester

3rd Trimester

1st Trimester

Wks 1 – 13+6

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2nd Trimester

Wks 14 – 27+6

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3rd Trimester

Wks 28 – 40+0

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Key Clinical Milestones

Recommended Prenatal Appointments

Clinical Basis & Sources

GA is always counted from the equivalent LMP (not from conception). EDD via Naegele's Rule = LMP + 280 days, adjusted for non-28-day cycles. IVF EDD = transfer date + (266 − embryo day), per ASRM guidelines. Conception EDD = ovulation date + 266 days. Trimester 1 ends at 13w6d; Trimester 2 ends at 27w6d. Multiples adjustment: twins −14 d (~38 wks), triplets −42 d (~34 wks), per ACOG/SMFM. Only ~5% of babies deliver on their exact EDD. Full-term = 39w0d–40w6d (ACOG 2013 redefinition).

ACOG Practice Bulletin 700
AIUM Ultrasound Guidelines 2020
ASRM ART Dating Guidelines
Naegele's Rule (1812)
NICE CG62 (UK)
WHO Reproductive Health
Hadlock Fetal Growth Charts
ACOG/SMFM Obstetric Care Consensus 2013

⚕️

Medical Disclaimer This tool is for informational and educational purposes only. Due dates are estimates — only ~5% of births occur on the exact EDD; actual delivery may occur 2–3 weeks before or after. Always consult your obstetrician, midwife, or registered healthcare provider for personalised clinical decisions. Do not use this tool as a substitute for professional medical advice, diagnosis, or treatment.