🤰 OB Wheel Calculator
Professional Obstetric Wheel & Pregnancy Dating Tool
Medical Standard • Accurate Dating
📅 OB Wheel: Gestational Age from LMP
Enter your last menstrual period (LMP) to calculate gestational age, due date, and pregnancy week using the traditional obstetric wheel method.
ℹ️ About OB Wheel: The obstetric wheel (pregnancy wheel) is the standard medical tool used by obstetricians and midwives to calculate pregnancy dates. It's based on a 280-day (40-week) pregnancy from LMP.
🔄 Compare Multiple Dating Methods
Calculate gestational age using different methods: LMP, Conception Date, IVF Transfer, or Ultrasound.
👶 Estimated Fetal Size by Gestational Age
Enter gestational age to see estimated fetal measurements and fruit/vegetable size comparisons.
Understanding the OB Wheel
The obstetric wheel (or obstetrical wheel, pregnancy wheel, gestation wheel) is a circular calculator used by healthcare professionals to determine key pregnancy dates and gestational ages. It's one of the most reliable and widely used tools in obstetrics and gynecology.
⭐ Why OB Wheel? The OB wheel provides instant, accurate calculations for multiple pregnancy parameters. It accounts for the 40-week pregnancy duration and handles date calculations that would otherwise be complex and time-consuming.
OB Wheel Calculation Methods
1. Last Menstrual Period (LMP) Method
EDD = LMP + 280 days (40 weeks)
Most common method. Pregnancy is dated from the first day of the last menstrual period, regardless of when conception actually occurred.
2. Conception Date Method
EDD = Conception Date + 266 days (38 weeks)
Used when conception date is known precisely (IVF, known ovulation date). Conception typically occurs 14 days after LMP.
3. Ultrasound Dating
Gestational Age = Ultrasound Measurements (crown-rump length, biometry)
Based on fetal measurements. Most accurate in first trimester (±3-5 days), less accurate in later trimesters.
Fetal Size and Development Timeline
| Gestational Weeks | Size Comparison | Crown-Rump Length (CRL) | Weight (approx) | Key Developments |
|---|---|---|---|---|
| 8 weeks | Raspberry | ~14 mm | 1 g | Heartbeat visible on ultrasound |
| 12 weeks | Plum | ~51 mm | 14 g | First trimester screening, major organs formed |
| 16 weeks | Avocado | ~116 mm | 100 g | Limbs moving, facial features forming |
| 20 weeks | Banana | ~160 mm | 300 g | Anatomy scan, movements felt by mother |
| 24 weeks | Corn | ~210 mm | 650 g | Viability threshold, eyelids opening |
| 28 weeks | Eggplant | ~250 mm | 1000 g (1 kg) | Third trimester begins, rapid weight gain |
| 32 weeks | Jicama | ~280 mm | 1800 g | Lungs maturing, fingernails forming |
| 36 weeks | Romaine Lettuce | ~310 mm | 2700 g | Most babies head down, ready for birth |
| 40 weeks | Watermelon | ~360 mm | 3500 g | Term pregnancy, ready for delivery |
OB Wheel Features & Advantages
- Quick Calculations: Get multiple dates instantly without manual calculations
- Multiple Methods: Compare different dating methods to reconcile dates
- Accurate for Most Pregnancies: When LMP is known accurately and cycles are regular
- Leap Year Compatible: Digital wheels handle leap years correctly
- Pregnancy Milestones: Shows key screening dates and appointments
- Fetal Development: Displays baby size and development stage
- Professional Standard: Used in hospitals, clinics, and by healthcare providers worldwide
Accuracy of Different Dating Methods
| Dating Method | Accuracy Range | Best Used When |
|---|---|---|
| LMP (regular cycles) | ±3-5 days | 28-day cycles, known LMP date |
| Conception date (known) | ±3-5 days | IVF or precisely known ovulation |
| First trimester ultrasound (11-14w) | ±3-5 days | Most accurate method available |
| Second trimester ultrasound (15-20w) | ±5-7 days | Anatomy scan, good accuracy |
| Third trimester ultrasound (>20w) | ±2-3 weeks | Less accurate, use previous dating if available |
Important OB Wheel Notes
- Pregnancy Duration: Average 280 days from LMP, but normal range is 266-294 days (38-42 weeks)
- Due Date vs Delivery: Only 5% of babies deliver on exact due date; 90% deliver within 2 weeks of due date
- Irregular Cycles: OB wheel less accurate with irregular menstrual cycles; ultrasound dating more reliable
- IVF Pregnancies: Use conception date (egg retrieval or embryo transfer) + 266 days
- Multiple Gestation: Twin pregnancies may have different EDD calculations
- Dating Discrepancies: If ultrasound EDD differs >1-2 weeks from LMP EDD, ultrasound typically preferred
Frequently Asked Questions
Why is it called the "OB Wheel"?
OB stands for "Obstetrics" (pregnancy and childbirth). The "wheel" refers to its circular design, which allows healthcare providers to quickly rotate the device to match dates and instantly see corresponding pregnancy information.
How accurate is the OB wheel?
When using the LMP method with regular 28-day cycles, the OB wheel is accurate to within ±3-5 days. However, accuracy depends on knowing the LMP accurately and having regular cycles. Ultrasound dating in the first trimester is more accurate.
Can the OB wheel handle irregular periods?
The traditional OB wheel assumes 28-day cycles. For irregular periods, ultrasound dating is more reliable. If your cycles are irregular, inform your healthcare provider so they can adjust your due date accordingly.
What's the difference between LMP and conception dating?
LMP dating assumes conception at day 14 of the cycle. Conception dating starts from the actual conception date. LMP pregnancy age is typically 2 weeks older than fetal age. Conception date + 266 days = EDD, while LMP + 280 days = EDD.
Should I trust my due date?
The due date is an estimate. Only 5% of babies are born on their exact due date. Pregnancy is considered full-term between 37-42 weeks. Your healthcare provider will monitor you throughout pregnancy and may adjust your due date based on ultrasound findings.
How is the OB wheel different from Naegele's Rule?
Both methods use the same principle (LMP + 280 days), but Naegele's Rule is a manual calculation method, while the OB wheel is a physical or digital tool that performs the same calculation instantly by rotating or entering the LMP date.
Can the OB wheel be used for IVF pregnancies?
Yes. For IVF, use the egg retrieval or embryo transfer date and add 266 days, or add 14 days to the conception date to get the equivalent LMP date, then add 280 days. Modern OB wheels and calculators have specific IVF dating options.
Why do doctors sometimes change my due date?
Due dates may be adjusted based on ultrasound findings, which are more accurate than LMP alone. First trimester ultrasounds are most accurate. If your ultrasound EDD differs >1-2 weeks from your LMP EDD, healthcare providers typically use the ultrasound date.
What happens if I go past my due date?
Going 1-2 weeks past your due date is common and normal (pregnancy is 37-42 weeks). Most doctors schedule induction between 41-42 weeks if labor hasn't started. Regular monitoring ensures baby's wellbeing as you approach and pass your due date.
Are there different types of OB wheels?
Yes. Traditional paper/plastic wheels exist, along with modern digital versions (apps, web calculators). Digital OB wheels often include additional features like fetal size estimates, ultrasound calculators, and milestone tracking that paper wheels don't have.
Official OB Wheel & Pregnancy Dating Sources
All obstetric wheel information sourced from official medical and healthcare organizations:
🔄 OB Wheel - Digital Obstetric Wheel Calculator 🏥 Perinatology - Due Date & Gestational Age Calculator 🇬🇧 NHS - Pregnancy Due Date Calculator 🤰 American Pregnancy Association - Pregnancy Calculator 📚 Healthline - How to Calculate Your Due Date 📱 OB Wheel Mobile App - Google Play