Pregnancy Due Date Calculator
Estimate your due date from the first day of your last period, with current week and trimester.
About this tool
The standard due date estimate, known as Naegele's rule, adds 280 days (40 weeks) to the first day of the last menstrual period. This calculator applies it and also adjusts for cycle length, since ovulation occurs later in longer cycles, shifting the estimate by the same number of days.
Alongside the due date it reports how far along the pregnancy is today in weeks and days, the current trimester, and the dates each trimester begins. Only around 4 percent of babies arrive on their exact due date; the estimate is best read as the midpoint of a window of a few weeks.
Frequently asked questions
How accurate is a due date from the last period?
It is a good first estimate, but a first trimester ultrasound is more accurate and clinicians will use it to adjust the date when the two disagree.
What if my cycles are irregular?
Cycle based estimates are less reliable with irregular cycles. Use the calculator as a rough guide and confirm with ultrasound dating.