Frequently asked questions
Why is pregnancy dated from the last period, not conception?
Conception date is rarely known precisely, but the start of the last period usually is — using LMP as the reference point gives a consistent, practical starting line for dating a pregnancy, even though conception happens about two weeks later.
When does each trimester actually start?
By convention: first trimester is weeks 1-13, second is weeks 14-27, third is week 28 onward. Your provider may use slightly different week cutoffs.
This calculator provides estimates for general informational purposes only and is not medical advice. Your healthcare provider's dating, especially from an early ultrasound, takes precedence over an LMP-based estimate.