Pregnancy Due Date Calculator — ACOG #700 Ultrasound Redating | Vectobox
Calculate your due date 5 ways — LMP, ultrasound (Robinson-Fleming CRL), IVF transfer, conception, or reverse — with ACOG Committee Opinion 700 redating thresholds applied automatically. 100% client-side, no tracking.
ACOG #700 ultrasound redating
ACOG Committee Opinion 700 (2017, reaffirmed 2022) defines a 5-tier discordance table for replacing an LMP-based EDD with an ultrasound EDD: ≤8w6d → 5d, 9w0d–15w6d → 7d, 16w0d–21w6d → 10d, 22w0d–27w6d → 14d, ≥28w0d → 21d. We apply the test strictly: only when |LMP-GA − US-GA| exceeds the threshold for the GA at scan do we redate.
Robinson-Fleming CRL → GA
For CRL between 5 and 84 mm we compute gestational age with the original Robinson-Fleming formula (BJOG 1975;82:702-710) plus the BMUS 1.037 correction factor: GA(days) = round(8.052·√(CRL·1.037) + 23.73). Above 84 mm we refuse the calculation — ACOG #700 mandates BPD/HC/AC/FL biometry past 14 weeks.
Naegele's rule and Mittendorf adjustment
Naegele's rule (LMP + 280d) is a statistical median, not a deadline. Mittendorf-Williams (Obstet Gynecol 1990) found nulliparous medians of 288d and multiparous medians of 283d. The WHO classifies 37–42w as full-term. The Mittendorf chip lets you preview that adjustment without overriding the canonical EDD.
Trimester boundaries
ACOG defines the trimesters as 0w0d–13w6d (1st), 14w0d–27w6d (2nd), and 28w0d–delivery (3rd). 'Fetal age' is GA − 14d (fertilization age), which is what biology textbooks count from; obstetric records always use GA.
FAQ
- Should I use my LMP or ultrasound to date my pregnancy?
- ACOG #700 says: when an early ultrasound's GA differs from LMP-based GA by more than the threshold for that gestational age (5d ≤8w6d, 7d through 15w6d, 10d through 21w6d, 14d through 27w6d, 21d beyond 28w), use the ultrasound. Otherwise stick with LMP. This tool runs that decision automatically.
- How accurate is Naegele's rule?
- Naegele's 280d is a statistical median. Mittendorf-Williams (1990) found nulliparous median pregnancy length of 288d, multiparous 283d; Smith (BMJ 2001) reported mean 283d. ±5-day variance from EDD is normal. WHO considers 37–42 weeks full-term.
- How do I calculate an IVF due date?
- EDD = transfer date + (266 − embryo age in days). So Day 3 → +263d, Day 5 (blastocyst) → +261d, Day 6 → +260d. Fresh vs frozen transfer doesn't change the math (ACOG #700, §Use of ART; CDC/SART reporting standard).
- What is the Robinson-Fleming formula for CRL?
- GA(days) = 8.052·√(CRL_mm · 1.037) + 23.73. Robinson & Fleming published the original in BJOG 1975;82:702-710; the BMUS/FASP 1.037 multiplier is the modern correction. Valid for CRL 5–84 mm only; beyond ~14 weeks, BPD/HC/AC/FL biometry replaces CRL.
- What's the difference between gestational age and fetal age?
- Gestational age is counted from the first day of the last menstrual period (LMP), so it includes ~14 days before conception. Fetal age (also called conceptional or embryonic age) is counted from fertilization, so fetal age = GA − 14d. Obstetric care always uses GA; biology textbooks usually use fetal age.
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