Pregnancy Dating (LMP / CRL / EDD)
Two pregnancy-dating calculations in one tool.
Open the calculator → Runs in your browser. Data leaves only if you deliberately send a problem report from the interactive tool.
Example
- LMP date
- 2025-12-23
- Ultrasound CRL
- 50
- Ultrasound date
- 2026-03-12
Result: LMP-derived EDD 2026-09-29; CRL-derived GA at ultrasound ~11w 5d; small T1 discordance (~3 days), within accepted limit.
The tool opens with these values already filled in. Replace them with your own.
What you enter
- LMP date (YYYY-MM-DD)
- Ultrasound CRL (mm)
- Ultrasound date (YYYY-MM-DD)
What this is
Naegele's rule estimates an estimated due date (EDD) from the first day of the last menstrual period (LMP + 280 days, or LMP - 3 months + 7 days). Robinson-Fleming uses first-trimester crown-rump length (CRL in mm) to compute gestational age in weeks and days, with a corresponding EDD. Sophie Well returns both, flags any discordance between LMP-based and CRL-based dating, and labels whether the discordance exceeds the ACOG redating thresholds (7 days in T1, 14 days in T2, 21 days in T3).
When to use it
Use this at the new-OB intake visit and any subsequent visit where dating is in question - certainty of LMP is the central issue (irregular cycles, recent hormonal contraception, lactational amenorrhea, conception under fertility treatment all undermine LMP reliability). The CRL window is roughly 6-14 weeks; outside that window use second-trimester biometry (BPD, HC, FL, AC) instead, which this tool does not cover. The ACOG redating rule is a one-way ratchet: once an EDD has been established by the earliest available reliable ultrasound, it is not revised by later scans even if they disagree.
How this is calculated
Naegele rule (LMP+280); Robinson-Fleming CRL formula; ACOG Practice Bulletin redating thresholds (T1: 7d; T2: 14d; T3: 21d).
A reference and educational tool. Not medical, legal, or financial advice, and not a substitute for clinician judgment.
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