If you've ever spent time on a labour ward, you've seen one: a small plastic disc with two dials that an obstetrician spins to work out a due date. It's called an OB wheel (or pregnancy wheel / obstetric wheel), and despite being decades old, it's still standard kit in most Indian clinics — mine included.
I'm a doctor who also writes code, and I got tired of two things:
- Physical wheels drift out of calibration and get lost
- Every "due date calculator" online does one thing (LMP → EDD) and stops, when the actual back of a real OB wheel has a lot more printed on it — fetal biometry by week, a prenatal test schedule, a β-hCG dating chart
So I built a digital one: OB Wheel Calculator — part of RxMedCalc, a free calculator suite I maintain for Indian clinicians.
What it does that a basic due-date calculator doesn't
Most "pregnancy calculators" online are a single form: enter LMP, get a due date. That's maybe a third of what a physical OB wheel actually does. This one has five calculation modes because that's what you actually need across a real antenatal visit:
- GA by LMP — the classic Naegele's Rule calculation, adjusted for cycle length (not everyone has a 28-day cycle, and most calculators quietly assume they do)
- GA by Scan — back-calculate from ultrasound dating, since first-trimester CRL is more reliable than LMP for irregular cycles
- GA by IVF transfer date — day-3 vs day-5 blastocyst transfers shift the math, and this is a case generic calculators almost never handle
- GA on a specific date — for backdating OT notes, discharge summaries, or referral letters
- Reverse EDD → LMP — useful when you're reviewing a referral and only the due date was recorded
All four dating paths funnel through the same Naegele's Rule engine (EDD = LMP + 280 + (cycle length − 28) days), so results stay internally consistent no matter which mode you enter from.
Digitizing the "back of the wheel"
The part I actually enjoyed building was the reference data — the stuff printed on the reverse of a physical wheel that most digital calculators skip entirely:
- Expected fetal biometry (CRL/BPD/HC/AC/FL) by gestational week, from the Hadlock nomograms
- A trimester-wise prenatal test and precautions table (DIPSI GDM screening, Anti-D timing, GBS swab windows)
- A quantitative β-hCG dating chart, since a slow-rising hCG is one of the first flags for a non-viable or ectopic pregnancy
- A bleeding-in-pregnancy differential table, split by trimester
None of this changes the due-date math, but it's the reason a physical OB wheel is genuinely useful beyond one number — and it's why I didn't just clone the fifty other "due date calculator" pages that already exist.
Stack notes, for the dev crowd
Nothing exotic — the whole RxMedCalc site runs on Cloudflare Pages/Workers/D1/KV, static HTML/CSS/JS, no backend framework, no login, no tracking beyond privacy-respecting analytics. Keeping it framework-free has made it trivial to keep every calculator fast on low-end Android phones, which is what a lot of clinicians in India are actually using between patients.
Try it / break it
It's free, no signup: rxmedcalc.com/medical-calculators/obs-calc
If you're a developer who's also clinical staff (or married to one, or just curious about medical calculators as a genre of software), I'd genuinely like feedback on the UX — particularly whether the five-mode tab layout is intuitive on first use, or whether it needs to be simplified.
