You choose either last menstrual period (LMP) or conception date as your starting point. If you enter a conception date, the tool subtracts 14 days to estimate the LMP. It then applies Naegele's rule — adding 280 days (40 weeks) to the LMP — to determine your estimated due date. Current gestational age is the number of full weeks from LMP to today. Only about 5% of births occur on the exact due date.
Naegele's rule is a standardized way to estimate gestational age and due date. It uses 280 days (40 weeks) as the standard pregnancy duration from LMP. This accounts for the fact that the LMP typically precedes conception by about 2 weeks, making the full pregnancy gestation from ovulation about 38 weeks (266 days).
Ultrasound dating is typically more accurate, especially in the first and second trimesters. This calculator gives an estimate based on menstrual dates, which can vary due to cycle length and ovulation timing. Ultrasound can narrow the margin of error to ±3–5 days in early pregnancy.
If you're unsure of your LMP, an ultrasound in the first or early second trimester can provide a more accurate due date estimate. Conception date estimates are also helpful if known (your healthcare provider can use this).
Yes. Due dates calculated early in pregnancy (first trimester) are typically accurate to within a few days. Later ultrasounds may adjust the due date slightly, though changes become less common after the first 20 weeks.
Yes. The due date is an estimate, not a guarantee. About 50% of babies arrive within a week of the due date, and it's normal to deliver anywhere from 2 weeks before to 2 weeks after. Consult your healthcare provider about induction timelines.
Estimate only. Results reflect your inputs and standard formulas — they are not financial, tax, legal, health, or investment advice. Verify important decisions with a qualified professional.