OB Wheel
Pregnancy calculator for EDD, gestational age & trimester milestones — the same engine SmartClinic India gynecologists & USG centres use.
Choose calculation method
Key Milestones
LMP
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Conception
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2nd Trimester
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3rd Trimester
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Want this built into your clinic workflow?
SmartClinic India auto-applies OB Wheel results to USG bookings, prescriptions, and ANC visits — with per-patient history.
About the OB Wheel Pregnancy Calculator
The Obstetric Wheel (commonly called the OB Wheel or pregnancy wheel) is the standard chairside tool used by gynecologists, obstetricians and sonologists to estimate the expected date of delivery (EDD) and the current gestational age (GA) during pregnancy. This free online OB Wheel from SmartClinic India digitizes that same calculation and supports four dating methods — LMP with cycle-length correction, Ultrasound GA, CRL (Robinson-Fleming), and a known EDD — so you can confirm or cross-check dating at any visit.
How the calculations work
- LMP method (Naegele's rule): EDD = LMP + 280 days, adjusted by (cycle length − 28) days when the cycle is irregular.
- Ultrasound GA: EDD = scan date + (280 − GA in days). First-trimester ultrasound is considered the gold standard for redating.
- CRL (Crown-Rump Length): Uses the Robinson-Fleming formula
GA (days) = 8.052 × √(CRL × 1.037) + 23.73, most accurate when CRL is between 7 and 84 mm. - Known EDD: Back-calculates LMP, conception date and trimester milestones directly from the due date.
LMP dating vs CRL (ultrasound) dating
Both methods answer the same question — how many weeks pregnant am I, and when is the baby due — but they rely on very different assumptions. Dating by LMP assumes ovulation on day 14 of a regular 28-day cycle, so a late or early ovulation shifts the EDD by exactly as many days. Dating by CRL measures the embryo itself, which grows at a highly predictable rate in the first trimester, so it is unaffected by cycle irregularity, breastfeeding, recent contraceptive use or PCOS.
| Aspect | LMP (Naegele's rule) | CRL / Ultrasound GA |
|---|---|---|
| Input needed | First day of last period + cycle length | Scan date + CRL in mm (or reported GA) |
| Typical accuracy | ±5 to ±8 days | ±3 to ±5 days (CRL 7–84 mm) |
| Best window | Any time, if LMP is certain | 6–14 weeks (first trimester) |
| Fails when | Irregular cycles, PCOS, unsure LMP, bleeding in early pregnancy | Scan done after 14 weeks, poor image quality |
| Wins a disagreement | Only if difference is within 5–7 days | Yes — redate to the earliest good scan |
Practical rule used in ANC clinics: book dating by the earliest reliable first-trimester scan, then keep that EDD for the rest of the pregnancy. Repeated redating at later scans is a common error — third-trimester biometry can be off by two weeks or more because growth, not age, is what varies at that stage.
Reading your result
- EDD — the 40-week mark. Only about 4% of babies arrive exactly on it; roughly 90% deliver between 37 and 42 weeks.
- GA today — shown as weeks and days (for example 12w 3d), the same format written in ANC cards and scan reports.
- Conception date — approximately LMP + 14 days, useful when the couple wants a probable date of conception.
- Trimester dates — second trimester starts at 14w 0d, third at 28w 0d; the NT scan window is 11w 0d to 13w 6d and the anomaly scan window is 18w 0d to 20w 6d.
- Time Machine — check what the GA was, or will be, on any other date, handy for backdating a scan or planning an LSCS date.
Who uses this OB Wheel?
Used daily inside SmartClinic India by gynecologists, ANC clinics, USG / sonography centres and government hospital OPDs across India. Expectant parents can also use it to plan ANC visits, anomaly scans and delivery dates with confidence.
Frequently Asked Questions
How accurate is the OB Wheel?+
When the LMP is reliable, EDD by Naegele's rule is within ±5 days of the true delivery date. First-trimester ultrasound (CRL 7–84 mm) tightens this to ±3–5 days and is considered the gold standard.
Which dating method should I trust if LMP and ultrasound disagree?+
Per ACOG and FOGSI guidance, first-trimester ultrasound dating should override LMP if the two differ by more than 5–7 days. Always use the earliest accurate scan for final dating.
Is my data stored anywhere?+
No. This OB Wheel runs entirely in your browser. No LMP, EDD, CRL or patient information is sent to any server or saved.
Can I use it on my phone?+
Yes — it is fully responsive and works on Android and iOS browsers, perfect for chairside use during ANC visits or USG scans.
When do the trimesters start?+
First trimester: 0w 0d to 13w 6d. Second trimester: 14w 0d to 27w 6d. Third trimester: 28w 0d to delivery.
More free clinical tools
- BMI & BSA Calculator — body mass index and body surface area for adults and children.
- Pediatric Dose Calculator — weight-based mg/kg drug dosing.
- Prescription Generator — free Rx pad with clinic letterhead.
- Medical Certificate Generator — fitness & sick leave certificates.
Disclaimer: This OB Wheel is provided for educational and clinical reference. It does not replace professional medical advice. Always confirm dating with a qualified obstetrician or sonologist.