Estimate your baby's due date, current gestational age, and trimester based on your last period, conception, or IVF transfer date.
Quick Answer:Using Naegele's rule in 2026, your due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period. Most babies arrive between 38-42 weeks, with only 5% born on their exact due date.
Estimated Due Date
Calculating... 40 weeks gestation
Current Gestational Age
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Current Trimester
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Days Remaining
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Estimated Conception
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While Naegele's rule remains the standard for estimating due dates, research shows that first-time mothers tend to deliver about 1-2 days later on average. A first-trimester ultrasound (6-9 weeks) is considered the gold standard for dating, accurate to within 5 days. If your ultrasound date differs from your LMP-based date by more than 7 days, your healthcare provider will typically adjust your due date. Remember, a "due date" is really a "due window" -- healthy full-term births occur anywhere from 37 to 42 weeks.
Due date calculators based on Naegele's rule are accurate to within about 2 weeks for most pregnancies. Only about 5% of babies are born on their exact due date. Most babies arrive between 38 and 42 weeks of gestation. Factors like irregular cycle lengths, uncertain LMP dates, and individual variation can affect accuracy. An early ultrasound (before 12 weeks) is considered the most accurate method for dating a pregnancy.
Naegele's rule is the standard method used to calculate an estimated due date. It works by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. If your cycle is longer or shorter than 28 days, the calculation is adjusted by adding or subtracting the difference. For example, a 30-day cycle would add 2 extra days to the due date.
Pregnancy is divided into three trimesters. The first trimester spans weeks 0-13 and includes early development, organ formation, and the highest risk period for miscarriage. The second trimester covers weeks 14-27 and is often considered the most comfortable period, with the baby growing rapidly and movements becoming noticeable. The third trimester runs from week 28 to delivery (around week 40), during which the baby gains weight rapidly and prepares for birth.