How Pregnancy Due Dates Are Calculated
Most US doctors calculate your due date using Naegele's rule โ counting 40 weeks (280 days) from the first day of your last menstrual period (LMP). This is the same method used by the American College of Obstetricians and Gynecologists (ACOG).
What This Calculator Shows
- Your due date โ 40 weeks from LMP
- Current gestational age โ how many weeks and days pregnant you are
- Trimester dates โ first, second, and third trimester start/end dates
- Conception date โ estimated date of ovulation/fertilization
- Milestone dates โ end of first trimester, anatomy scan window, viability, full term
Why Your Due Date Might Change
An early ultrasound (before 14 weeks) is the most accurate way to date a pregnancy. If the ultrasound dating differs from your LMP date by more than 5โ7 days, your doctor may adjust the due date. Only about 4% of babies are born on their exact due date โ most arrive between 38 and 42 weeks.
Trimester Timeline
- First trimester: Weeks 1โ13
- Second trimester: Weeks 14โ27
- Third trimester: Weeks 28โbirth