How to Use an OB Wheel to Calculate Pregnancy Due Dates
Step-by-step guide to calculate EDD, gestational age and LMP-based dating — plus when to redate with ultrasound (CRL).
What is an OB wheel?
An OB wheel (pregnancy wheel / gestational calculator) is a two-disc sliding calendar used since the 1950s in obstetrics. The outer ring shows calendar dates; the inner ring shows gestational weeks. Align the LMP arrow with the patient's Last Menstrual Period, and the wheel instantly reveals the EDD, current gestational age and each trimester boundary.
Common terminology: LMP = Last Menstrual Period · EDD = Estimated Due Date · GA = Gestational Age · CRL = Crown-Rump Length · EDC = Estimated Date of Confinement.
5 steps to calculate EDD
Follow this sequence — it works for paper wheels and digital calculators alike.
- 1
Confirm the LMP
Ask the patient for the first day of her last menstrual period. Confirm cycle regularity — a 28-day cycle is the OB wheel's default assumption.
- 2
Align the LMP arrow
Rotate the inner disc until the arrow marked 'LMP' points exactly to the patient's LMP date on the outer calendar.
- 3
Read the EDD
The arrow labelled '40 weeks' (or 'EDD') on the inner disc now points to the estimated due date. Naegele's rule: LMP + 280 days.
- 4
Read current gestational age
Find today's date on the outer ring — the aligned inner ring shows current GA in weeks + days (e.g., 22w 3d).
- 5
Redate with ultrasound if needed
If a first-trimester CRL is available, apply Robinson-Fleming and update the EDD when the discrepancy exceeds 5-7 days (ACOG 2017 / RCOG).
Worked example
Patient LMP: 1 January 2026 · Cycle: 28 days regular · Today: 3 July 2026
Step 1: Align LMP arrow → 1 Jan 2026.
Step 2: EDD arrow → 8 October 2026 (LMP + 280 days).
Step 3: Today (3 July) → GA = 26 weeks 0 days, i.e., early third-trimester ready.
Tip: For a 32-day cycle, add 4 days to the EDD. For a 26-day cycle, subtract 2 days.
Digital OB wheel vs paper wheel
Paper OB wheel
✓ No internet needed
✗ Prone to reading error (±3-5 days)
✗ Doesn't adjust for cycle length or CRL
✗ No printable report for the patient file
SmartClinic OB Wheel
Supports LMP, Ultrasound, CRL and Known-EDD inputs
Auto-adjusts for cycle length (21-45 days)
Robinson-Fleming CRL formula built in
Trimester milestones + printable summary
Frequently asked questions
What is an OB wheel?
An OB wheel (obstetric wheel or pregnancy wheel) is a circular sliding calculator used by obstetricians, gynecologists and sonologists to estimate the Estimated Due Date (EDD), current gestational age and key pregnancy milestones from the Last Menstrual Period (LMP) or an ultrasound measurement.
How is EDD calculated from LMP?
The classic Naegele's rule adds 280 days (40 weeks) to the first day of the LMP — equivalent to adding 1 year, subtracting 3 months and adding 7 days. The OB wheel automates this and adjusts for cycle length.
When should ultrasound override LMP dating?
First-trimester ultrasound (CRL between 7-84 mm) is more accurate than LMP if the difference is more than 5-7 days. From 14-20 weeks, a difference of more than 10 days should prompt redating by ultrasound (ACOG/RCOG guidance).
What is CRL and how is it used?
Crown-Rump Length (CRL) is the straight-line measurement from the top of the fetal head to the bottom of the rump between 7 and 14 weeks. The Robinson-Fleming formula converts CRL (mm) to gestational age with an accuracy of ±3-5 days.
Is the OB wheel calculator free?
Yes. SmartClinic India's OB Wheel is 100% free, works offline in your browser, and never stores patient data on our servers.
Ready to calculate an EDD?
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Disclaimer: This guide is for educational purposes and does not replace clinical judgement. Always correlate with ultrasound and patient history.