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 accuracyMore pregnancy tools
Beyond the wheel, a few focused calculators — all free and private.
- Due date calculatorA quick due-date estimate from your last period.
- Gestational age calculatorHow far along in weeks and days, from LMP or due date.
- Ovulation & fertile-window calculatorFind your most fertile days from your cycle.
- hCG doubling-time calculatorDoubling time and daily rise from two hCG results.
Frequently asked questions
What is an OB wheel?
An OB wheel (also called a pregnancy wheel or gestational wheel) is a dating tool that turns one known date — your last period, an ultrasound, a conception date or an IVF transfer — into your estimated due date and how far along you are. This site does the same maths digitally and exactly.
How is my due date calculated?
The standard method adds 280 days to the first day of your last menstrual period, adjusted for your cycle length. You can also date from a known due date, an ultrasound, a conception date, or IVF timing. Every formula is shown on the how-it's-calculated page.
How accurate is the due date?
It's an estimate, not a deadline. Most babies arrive within about two weeks either side of the date, and a first-trimester ultrasound is the most accurate way to set it. Only a small share of births land on the exact due date.
Is this OB wheel calculator free and private?
Yes. It's completely free with no sign-up, and every calculation runs in your browser — the dates you enter never leave your device and nothing is stored or tracked.
What's the difference between gestational age and how many weeks pregnant I am?
They're the same thing. “Weeks pregnant” means gestational age, counted from the first day of your last period — which is about two weeks before conception.
Can I use it if I had IVF or IUI?
Yes. IVF mode dates from your transfer or retrieval with day-3 or day-5 precision. For IUI, use conception mode with the insemination date. Both are supported and explained in dedicated guides.
