Pregnancy Due Date Calculator
Estimate your pregnancy due date and how many weeks pregnant you are. Supports last menstrual period (LMP), conception date, and ultrasound dates.
What this calculates
Due-date math anchors a pregnancy. The estimated due date (EDD) is when you'll be 40 weeks pregnant — the median full-term length. Calculations work from your last menstrual period (LMP), the date of conception, or an early-ultrasound dating, with ultrasound being the most accurate before 12 weeks. Only about 4 % of babies actually arrive on the exact due date; most appear within 2 weeks either side.
Formula & how it works
Naegele's rule: EDD = LMP + 280 days (40 weeks). If you know your conception date instead: EDD = conception + 266 days. Current pregnancy age = today − LMP, expressed in completed weeks and days. This assumes a regular 28-day cycle with ovulation on day 14. Adjust by ±the cycle-length difference if your cycle is shorter or longer. Early-ultrasound dating, when available, replaces both methods.
Worked example
LMP = 2025-09-01. Due date = 2025-09-01 + 280 days = 2026-06-08. If today is 2026-05-15, you've been pregnant for 256 days = 36 weeks and 4 days — third trimester, approaching full term. From a known conception date of 2025-09-15: EDD = 2025-09-15 + 266 days = 2026-06-08 (same result if cycle is regular).
Frequently asked questions
How accurate is the due date?
Modestly. Only 4 % of births land on the exact EDD. Most happen within ±2 weeks. Ultrasound dating before 12 weeks is the gold standard — within 5–7 days. After 20 weeks, ultrasound dating becomes less reliable because growth rates diverge.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle. For longer cycles, add the difference (35-day cycle → add 7 days to EDD). For shorter cycles, subtract. Better yet, use ultrasound dating which is cycle-independent.
When does each trimester start?
First trimester: week 1 through end of week 13. Second trimester: week 14 through end of week 27. Third trimester: week 28 to delivery. Pre-term is before 37 weeks; full-term is 37–42 weeks; post-term is after 42 weeks.
When can I confirm pregnancy?
Home tests detect hCG hormone, typically reliable from the first day of a missed period (~4 weeks LMP). Blood tests at a clinic can detect pregnancy slightly earlier. Ultrasound usually confirms a heartbeat at 6–7 weeks.
Sources
Disclaimer: An estimate, not a medical prediction. Always confirm pregnancy dating with your healthcare provider.