Estimate your baby due date from the first day of your last menstrual period using Naegele rule, the standard method that adds 280 days (40 weeks) to that date. Enter your last period date and, if you know it, your typical cycle length so the estimate can be adjusted, and the calculator returns your estimated due date along with how many weeks and days pregnant you are today and which trimester that falls in. Remember that only about one in twenty babies actually arrives on the estimated date, and this figure is an estimate, not a diagnosis. An ultrasound dating scan from your care provider is more precise. All of the calculation happens locally in your browser and no health information is stored or sent.