Pregnancy Dating Tool (LMP-Based)
Use this obstetric wheel calculator to estimate due date, gestational age, and key timeline milestones from your last menstrual period (LMP).
What is an obstetric wheel calculator?
An obstetric wheel calculator is a digital version of the circular pregnancy wheel used in clinics. It helps estimate the expected due date (EDD) and current gestational age based on the first day of the last menstrual period (LMP). It can also map important milestones, such as trimester transitions and typical screening windows.
How this calculator works
1) Start with LMP
Traditional obstetric dating starts from the first day of the last menstrual period, not the day of conception. This is why gestational age is usually about two weeks greater than fetal age in early pregnancy.
2) Apply Naegele-style due date math
The classic approach estimates due date as LMP + 280 days (40 weeks). If your average cycle is not 28 days, this calculator applies a simple cycle-length adjustment to shift the timeline forward or backward.
3) Report pregnancy timeline outputs
- Estimated due date (EDD)
- Estimated conception date
- Gestational age on a specific date
- Trimester and screening milestone dates
How to read your results
Your output includes both calendar dates and practical interpretation:
- EDD: The estimated day you reach 40 weeks.
- Gestational age: Displayed as weeks + days (for example, 24w 3d).
- Time to due date: Shows days remaining, due today, or overdue days.
- Milestones: Helpful checkpoints for routine prenatal planning discussions.
Important pregnancy milestones (quick reference)
Trimesters
- First trimester: 0w0d to 13w6d
- Second trimester: 14w0d to 27w6d
- Third trimester: 28w0d onward
Common clinical windows
- Nuchal translucency scan: 11w0d to 13w6d
- Anatomy scan: 18w0d to 22w0d
- Term begins: 37w0d (full term often discussed around 39w0d)
LMP dating vs ultrasound dating
LMP-based dating is widely used at the start. However, early ultrasound (especially first trimester) may refine dating when cycle history is uncertain, periods are irregular, or LMP is unknown. If ultrasound-based dating differs significantly, clinicians may use the ultrasound estimate as the official obstetric date.
FAQ
Is this a medical diagnosis tool?
No. This is an educational estimator and planning aid.
What if my cycles are irregular?
LMP-based predictions can be less accurate with irregular cycles. In those cases, early ultrasound dating is usually more reliable.
Can due dates change?
Yes. Due dates are estimates, and many pregnancies naturally deliver before or after the estimated date.
Medical disclaimer
This obstetric wheel calculator does not replace professional medical care. Always review your dates, ultrasound findings, and prenatal plan with a qualified clinician.