What is an OB wheel, and how do clinicians use it?
The pregnancy dating wheel that clinicians spin between patients — what it is, how it works, and what an online version does better.
ob wheel
OB wheel calculator
Pregnancy dating in seconds, not scrolls.
An online OB wheel for clinicians and expectant parents. Enter one date and get the estimated due date, current gestational age and every key milestone — instantly, and privately.
- Six dating methods: LMP, known EDD, ultrasound, conception, IVF, plus reverse look-up.
- Instant results — estimated due date, gestational age, trimester and milestone dates.
- Runs entirely in your browser. No accounts, no tracking, nothing sent anywhere.
How the OB wheel works
A pregnancy wheel is a simple dating tool: line up one known date and read off the rest. This calculator does the same calendar arithmetic a paper wheel does, but exactly — accounting for leap years and month lengths — and lets you re-date from an ultrasound, a conception date, or an IVF transfer. Learn the formulas on the how it’s calculated page.
Read up on pregnancy dating
How to read a pregnancy wheel (the circle chart), step by step
Line up one date, read off the rest. A plain-language walkthrough of the pregnancy circle chart and the mistakes to avoid.
pregnancy circle chartNaegele's rule: calculating a due date from your last period
The 280-day rule behind every pregnancy wheel — where it comes from, how cycle length changes it, and when to trust a scan instead.
ob wheel pregnancyCrown-rump length (CRL) and first-trimester ultrasound dating
Why the first-trimester scan is the gold standard for dating, how crown-rump length becomes a due date, and when it overrides the last period.
crown rump length calculatorGestational age vs. fetal age: what's the difference?
Why “how many weeks pregnant” is counted from your last period, not from conception — and the two-week gap that confuses everyone.
gestational age vs fetal ageHow accurate is your due date, really?
Few babies arrive on the exact date. What the estimate really means, why methods disagree, and when the scan should win.
due date accuracy