Estimate your due date and current week of pregnancy based on the first day of your last menstrual period.
This uses Naegele's rule, the standard clinical method of estimating a due date by adding 280 days (40 weeks) to the first day of your last menstrual period.
Naegele's rule gives a standard estimate, but only about 5% of babies arrive exactly on the calculated due date — a range of 1-2 weeks before or after is completely normal. An early ultrasound typically provides a more precise estimate.
The standard method adds 280 days (40 weeks) to the first day of your last menstrual period, assuming a 28-day cycle.
An early ultrasound is generally considered more precise than LMP-based dating alone, especially for people with irregular cycles.