Due Date Calculator
Enter the first day of your last menstrual period. Adjust cycle length if yours isn't 28 days.
How this works
This calculator uses Naegele's Rule, the same formula every OB/GYN in Tennessee uses at your first prenatal visit: due date = first day of last menstrual period (LMP) + 280 days (40 weeks). The cycle-length adjustment shifts the estimate for cycles longer or shorter than the standard 28 days, since ovulation (and therefore conception) tracks with cycle length, not the calendar.
Your current gestational age is calculated in weeks + days from LMP. Trimester boundaries follow the ACOG convention: weeks 1–12 (T1), 13–26 (T2), 27–40 (T3).
Limitations
LMP-based dating assumes you know the exact first day of your last period and that you have a regular cycle with ovulation around day 14. If either isn't true — or if you conceived on a shorter or longer cycle than usual — your first-trimester ultrasound will be more accurate. Tennessee OB practices standardly re-date if the ultrasound disagrees with LMP by more than 5–7 days in the first trimester.
Only about 4% of pregnancies deliver on the estimated due date. Full term is 37–42 weeks.
Sources: ACOG Committee Opinion on Methods for Estimating the Due Date; Naegele's Rule.
Frequently asked questions
The Naegele-based estimate is a starting point. Only about 4% of pregnancies deliver on the exact estimated due date; roughly 80% deliver within two weeks on either side. First-trimester ultrasound dating is more accurate than LMP dating and will supersede this estimate if the two disagree by more than 5–7 days.
Adjust the cycle-length slider. The calculator adds one day to the due date for each day your cycle exceeds 28, and subtracts one day for each shorter day. For irregular cycles, first-trimester ultrasound is a better dating method — your OB will confirm at the 8–10 week visit.
Trimester 1: weeks 1–12. Trimester 2: weeks 13–26. Trimester 3: weeks 27–40. Some sources use week 13 as the start of the second trimester; the difference is trivial.
Use LMP (first day of your last menstrual period). Obstetric dating in Tennessee — and worldwide — is standardized to LMP, not conception. Even if you know your ovulation date, providers will re-date from your first ultrasound if there's a mismatch.