Finding out you're pregnant brings an immediate question: "When is the baby due?" This due date calculator uses three standard medical methods — Last Menstrual Period (LMP), known conception date, and ultrasound measurements — to estimate your Expected Delivery Date (EDD). While only about 5% of babies arrive exactly on their due date, knowing the EDD is essential for prenatal care scheduling
## How Due Date Is Calculated **Naegele's Rule (LMP method)**: The most common method. Add 280 days (40 weeks) to the first day of your last menstrual period. This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the estimate is adjusted. **Conception date**: If you know the exact date of conception, add 266 days (38 weeks). Conception typically occurs 12–24 hours after ovulation. **Ultrasound dating**: First-trimester ultrasound (6–13 weeks) is the most accurate method, measuring the embryo's crown-rump length. Accuracy: ±5 days. Second-trimester ultrasound is accurate to ±1–2 weeks.
## Trimester Timeline **First Trimester (Weeks 1–12)**: The most critical period for organ development. By week 8, all major organs have begun forming. By week 12, the baby is about 2.5 inches long. **Second Trimester (Weeks 13–27)**: Often called the "golden trimester" — morning sickness usually subsides, and energy increases. The baby begins moving (quickening) around week 16–20. **Third Trimester (Weeks 28–40)**: Rapid growth and final development. The baby gains about half its birth weight in the last 7 weeks. Full term is 39–40 weeks; early term is 37–38 weeks.
## When Do Babies Actually Arrive? Only about 5% of babies are born on their exact due date. The normal delivery window spans 37–42 weeks: - **37–38 weeks**: Early term (95% healthy, but slightly higher risk of respiratory issues) - **39–40 weeks**: Full term (optimal outcomes) - **41 weeks**: Late term (monitoring recommended) - **42+ weeks**: Post-term (induction typically discussed) First-time mothers tend to deliver slightly later (average 41 weeks). Second+ pregnancies often arrive 1–3 days earlier than the EDD.
Use this calculator as a starting point for any health or fitness goal that requires a numeric benchmark. The result is an estimate, not a diagnosis — but it provides a concrete figure to track against over time.
The most common mistake is treating the result as a precise measurement rather than an evidence-based estimate. All body metric calculators have margins of error — use the result as a tracking baseline, not a clinical diagnosis.
A personal trainer uses the calculator with a new client to set a measurable starting point. Re-running the calculation at 4-week intervals provides an objective progress metric that supports motivation and programme adjustments.
Naegele's Rule
EDD = LMP + 280 days (40 weeks)
EDD = Conception Date + 266 days (38 weeks)
Based on a 28-day menstrual cycle with ovulation on day 14. Adjust for cycle length: add (cycle length − 28) days to the EDD.
A woman's last menstrual period started on January 1, 2026.
Result: Estimated due date: October 8, 2026 (full term at 39–40 weeks)
LMP-based calculations are accurate to ±2 weeks. First-trimester ultrasound narrows this to ±5 days. Most babies arrive between 38 and 42 weeks.
Add the extra days to your estimated due date. For a 32-day cycle, ovulation likely occurs on day 18 instead of day 14, pushing the EDD 4 days later.
First-trimester ultrasound (6–13 weeks) measuring crown-rump length is the gold standard, accurate to ±5 days.
Yes. If an ultrasound shows the baby measuring significantly larger or smaller than expected, your provider may adjust the EDD.
Full term is defined as 39 weeks 0 days through 40 weeks 6 days. This is when the baby has the best health outcomes.
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