Interactive Obstetric (OB) Wheel & Due Date Calculator
Clinical-grade calculator for GA by LMP, Scan Dating, and Week-by-Week Progress.
Calculate Gestational Age via Last Menstrual Period
Digital equivalent of a physical plastic obstetric wheel (often called an OB wheel or obs wheel). Enter your LMP and cycle length below for immediate, ad-free due date and milestone results.
Calculate Gestational Age via Ultrasound Scan
Back-calculate EDD and LMP from ultrasound dating. First-trimester CRL (7–12 weeks) is more accurate than LMP for irregular cycles.
Calculate Gestational Age on a Specific Date
Find exact gestational age on a specific date — for OT notes, discharge summaries, referral letters, or medico-legal documentation.
Reverse-Calculate LMP from EDD
Reverse pregnancy wheel — calculate LMP and conception date when only the EDD is known. Useful when reviewing records or referral notes.
All four modes above use the same clinical formula — Naegele's Rule adjusted for cycle length — so results stay consistent whether you're dating from LMP, an ultrasound scan, or working backward from a known due date.
Pregnancy Wheel — Week-by-Week Milestones
The OB wheel tracks gestation through 40 weeks across three trimesters. Use this table alongside the pregnancy wheel calculator above to understand each week's clinical significance.
| Week | Trimester | Key Milestone / Clinical Event | Approx. EFW |
|---|---|---|---|
| 4–6w | 1st | Implantation, hCG rise, gestational sac visible on USS | — |
| 7–8w | 1st | Fetal heart activity visible; best window for dating scan (CRL) | ~1–2 g |
| 11–13w | 1st | NT scan; combined first-trimester screening (PAPP-A, free β-hCG) | ~20–40 g |
| 13w | 1st | End of 1st trimester; miscarriage risk drops sharply | ~65 g |
| 16–18w | 2nd | Quickening felt; quad screen window (AFP, hCG, uE3, inhibin-A) | ~150–200 g |
| 18–20w | 2nd | Anomaly scan (Level 2 USS); detailed fetal anatomy survey | ~200–300 g |
| 24–28w | 2nd | Universal GDM screening (DIPSI 75g GTT); Anti-D for Rh-negative | ~600–1000 g |
| 32–34w | 3rd | Growth scan; high-risk GDM rescreen; GBS swab consideration | ~1700–2150 g |
| 36–37w | 3rd | Late-preterm; assess presentation, Bishop score, induction planning | ~2600–2900 g |
| 38–40w | 3rd | Term; EDD at 40 weeks; induction discussion if post-dates | ~3100–3500 g |
| 41–42w | 3rd | Post-dates; CTG monitoring; induction recommended by 42 weeks | ~3400–3700 g |
OB Wheel Calculator — Clinical Reference Guide
What is an OB Wheel (Obstetric Wheel)?
An OB wheel (also called a pregnancy wheel, obstetric wheel, or obs wheel) is a circular slide-rule tool used by obstetricians, midwives, and general practitioners to calculate gestational age and estimated due date (EDD) from the last menstrual period (LMP). It applies Naegele's Rule: EDD = LMP + 280 days, with adjustment for non-standard cycle lengths. This digital OB wheel replicates the physical wheel and extends it with additional clinical tools used in obstetric practice — Bishop score, VBAC eligibility, Hadlock EFW, Apgar score, GDM screening, and PPH risk assessment.
How to Use the Online Pregnancy Wheel
Enter the first day of your last menstrual period (LMP) in the "GA by LMP" tab and select your cycle length. The pregnancy wheel immediately calculates your estimated due date (EDD), current gestational age in weeks and days, which trimester you are in, and your approximate conception date. For non-standard cycles, the wheel adjusts Naegele's Rule by the number of days the cycle differs from 28 days.
If an ultrasound scan has already been performed, use the "GA by Scan" tab to back-calculate EDD from the ultrasound dating. For operation theatre notes or referral letters, use "GA on Date" to find exact gestational age on any calendar date. The "EDD→LMP" reverse wheel is useful when reviewing referral notes where only the due date is recorded.
OB Wheel vs Basic Due Date Calculator
| Feature | Physical OB Wheel | Basic Online | ObsCalc OB Wheel |
|---|---|---|---|
| Due date from LMP | ✓ | ✓ | ✓ + cycle adj. |
| GA from ultrasound | ✓ | ✗ | ✓ |
| GA on any specific date | ✓ | ✗ | ✓ |
| Reverse EDD → LMP | ✓ | Rarely | ✓ |
| Fetal weight (Hadlock) | Some | ✗ | ✓ |
| Bishop score / VBAC / PPH | ✗ | ✗ | ✓ |
| GDM screening dates | ✗ | ✗ | ✓ DIPSI/FOGSI |
| Works offline / no login | ✓ | Mostly | ✓ |
Gestational Age Calculation — How the Pregnancy Wheel Works
Gestational age (GA) is measured in weeks and days from the first day of the last menstrual period. For a standard 28-day cycle, the total pregnancy duration is 280 days (40 weeks). For non-standard cycles, Naegele's Rule is adjusted: EDD = LMP + (280 + (cycle length − 28)) days. For example, for a 32-day cycle, EDD = LMP + 284 days. This cycle-length adjustment is built into the ObsCalc OB wheel — simply select your cycle length from the dropdown.
Obstetric Wheel Terminology — Glossary
OB wheel / obs wheel / obstetric wheel: All refer to the same tool — a gestational dating wheel used in clinical obstetric practice. EDD (Estimated Due Date): Also called EDC (Expected Date of Confinement) — the date delivery is expected, calculated as LMP + 280 days. LMP: Last Menstrual Period — the first day of the last normal period before conception. GA: Gestational Age — weeks and days since LMP. CRL: Crown-Rump Length — the most accurate dating parameter in the first trimester. EFW: Expected Fetal Weight by ultrasound biometry. DIPSI: Diabetes in Pregnancy Study Group India — the GDM screening protocol used in India.
Frequently Asked Questions
What is an OB wheel pregnancy calculator?
An OB wheel is a circular slide-rule used by doctors to calculate gestational age and estimated due date from the last menstrual period. This digital OB wheel does the same calculation online — enter LMP, get EDD and GA instantly — and adds several more obstetric tools used in clinical practice.
How do I use the pregnancy wheel online calculator?
Enter the first day of your last menstrual period (LMP) in the GA by LMP tab, select your cycle length (default 28 days), and press Calculate. You immediately get EDD, current gestational age in weeks and days, trimester, conception date, and a Google Calendar link for your due date.
How many weeks pregnant am I?
Enter your LMP in the pregnancy wheel calculator above. Gestational age is counted from the first day of your last period — not from conception. A 28-day cycle gives 40 weeks (280 days) total. The wheel shows exact gestational age in weeks and days from today.
Is this the same as an obstetric wheel calculator or obs wheel?
Yes. OB wheel, obstetric wheel, obs wheel, and pregnancy wheel are all names for the same tool — a gestational dating calculator based on Naegele's Rule. This page is a digital version of that physical wheel, with added obstetric tools for clinical use.
What is the Hadlock formula for EFW?
The Hadlock 1985 formula is the gold standard for expected fetal weight (EFW) in Indian obstetric practice. The 4-parameter formula (BPD+HC+AC+FL) is preferred; 3-parameter (BPD+AC+FL) is used when head circumference is unavailable. EFW has ±15% inherent variability.
What Bishop score is needed for labour induction?
Bishop score ≥9: highly favourable — direct oxytocin induction. Score 6–8: favourable — proceed with monitoring. Score below 6: unfavourable — cervical ripening with prostaglandins (PGE2) or Foley catheter required before oxytocin.
What is the DIPSI GDM screening protocol?
DIPSI recommends universal non-fasting 75g OGTT for all pregnant Indian women. Blood glucose ≥140 mg/dL at 2 hours = GDM diagnosis. FOGSI mandates screening at first ANC visit and 24–28 weeks; high-risk women get an additional screen at 32–34 weeks. The GDM Screening tool above personalises these dates for your LMP.
What does an obs calculator do?
An obs calculator (obstetric calculator) computes the clinical numbers used throughout pregnancy care — due date, gestational age, fetal weight, and labour-readiness scores — from a few patient inputs. This obs calculator covers LMP/EDD dating, Hadlock EFW, Bishop score, VBAC eligibility, Apgar, GDM screening, and PPH risk in one free tool.
What is "obs calc" or "ObsCalc"?
ObsCalc (obs calc) is the name of this obstetric calculator suite by RxMedCalc — a free set of clinical tools for Indian obstetricians including a digital OB wheel, EFW Hadlock calculator, Bishop score, VBAC eligibility checker, Apgar score, GDM screening scheduler, and PPH risk assessment. No login required.