Estimate your due date from last menstrual period, conception date, or ultrasound — plus current week and trimester.
Choose the method that matches the information you have. Last menstrual period (LMP) is the most common method and the one most clinicians default to. If you know your conception date precisely, that method skips the cycle-length assumption. If you've had a dating ultrasound, entering the gestational age it reported gives the most clinically refined estimate, since ultrasound measurements directly observe fetal size.
The standard method (Naegele's Rule) adds 280 days (40 weeks) to the first day of the last menstrual period, adjusted for cycle length if it differs from the 28-day average. Conception-based estimates add 266 days (38 weeks) to the conception date, since conception typically occurs roughly two weeks after the start of the last period. Ultrasound-based estimates work backward from the gestational age measured at the scan.
Only about 5% of babies are born exactly on their estimated due date. Most births happen within a window of one to two weeks before or after the EDD. The due date is a useful planning marker, not a prediction of the exact birth day.
Each method makes different assumptions. LMP-based dating assumes a standard 28-day cycle with ovulation on day 14, which doesn't hold for everyone with irregular cycles. Ultrasound dating measures actual fetal size and is generally considered more accurate, especially when done in the first trimester, which is why doctors often adjust the EDD after an early scan.
Gestational age is counted from the first day of the last menstrual period, while fetal (or conceptional) age is counted from the actual date of conception, roughly two weeks later. Medical providers almost always use gestational age, which is why a pregnancy is described as "40 weeks" even though actual fetal development time is closer to 38 weeks.
The first trimester runs from week 1 through the end of week 13. The second trimester runs from week 14 through the end of week 27. The third trimester runs from week 28 until delivery, typically around week 40.