Naegele's Rule
The LMP method adds 280 days to your last menstrual period. The cycle length adjustment shifts the due date by (cycle length - 28) days for cycles other than 28 days. The conception date method adds 266 days (38 weeks) directly to the conception date.
Trimester Breakdown
Weeks 1-13 = First Trimester. Weeks 14-27 = Second Trimester. Weeks 28+ = Third Trimester. Full term is 37-42 weeks; 40 weeks is the standard estimated due date.
Frequently Asked Questions
The most common method (Naegele's Rule) adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If you know your conception date, the calculator adds 266 days (38 weeks) instead.
The estimated due date is an estimate. Only about 4% of babies are born on their exact due date. Most births occur within two weeks before or after the EDD. Your healthcare provider will refine the estimate using ultrasound measurements, especially a first-trimester ultrasound, which is the most accurate dating method.
The first trimester covers weeks 1-13, the second trimester covers weeks 14-27, and the third trimester covers weeks 28 until delivery (typically week 40). The first trimester is when major organ systems develop and miscarriage risk is highest. The second trimester is when most parents find out the baby's sex via ultrasound.
Gestational age counts from the first day of the last menstrual period, not conception. A pregnancy at week 6 gestational age is typically only 4 weeks post-conception. Gestational age is the standard used by healthcare providers for pregnancy tracking and milestone development.
If your cycle is longer or shorter than 28 days, the LMP-based calculation may be off by several days. Your healthcare provider can use an early ultrasound (at 6-10 weeks) to measure the embryo's crown-rump length, which gives a more accurate gestational age estimate.
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