How to Calculate Your Pregnancy Due Date
A simple guide to LMP, conception date, and why due dates are estimates.
Most due dates start with a simple estimate: 40 weeks from the first day of your last period.
That number is useful, but it is still a starting point. Your clinician may adjust it based on your cycle, ultrasound measurements, or fertility treatment details.
The common LMP method
LMP means last menstrual period. Pregnancy dating often starts from the first day of your last period, even though conception usually happens later.
The common estimate is:
- Start with the first day of your last period
- Add 280 days
- That gives an estimated due date
This works best for people with fairly regular cycles.
Due date from conception
If you know your conception date, the estimate is slightly different.
Instead of adding 280 days, you usually add 266 days from conception.
That is because conception usually happens around two weeks after the start of the menstrual cycle.
Why due dates can change
Due dates are estimates, not promises. A date may shift because of:
- Irregular cycles
- Ovulation happening earlier or later than usual
- Early ultrasound measurements
- IVF or fertility treatment timing
The simple takeaway
Use BumpCalc to get a quick estimate, then use your clinician's date for medical decisions.
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Final Take
Due date math is simple, but pregnancy dating is personal. Start with a calculator, then confirm your official timeline with your care team.
BumpCalc is for informational purposes only and is not medical advice.