How pregnancy due date calculator works
Your estimated due date (EDD) is calculated from the first day of your last menstrual period (LMP). Naegele’s rule adds 280 days (40 weeks) to the LMP, because ovulation and conception typically happen around day 14 of a 28-day cycle and pregnancy lasts about 266 days from conception. Enter your LMP and cycle length and the tool does the rest.
Cycle length matters. If your cycles are longer or shorter than 28 days, ovulation shifts, and so does your due date. This calculator adjusts the due date by your cycle length minus 28 days, so a 32-day cycle pushes the due date four days later. You can also switch to the conception-date method, which adds 266 days to the date you conceived.
The tool reports your gestational age (how many weeks and days pregnant you are today), your current trimester, the days remaining until your due date, and a milestone timeline — the end of the first trimester (13 weeks), the earliest viability (24 weeks), the start of the third trimester (28 weeks), early term (37 weeks), and full term (40 weeks). This is an estimate for education only and not medical advice; only about 5% of babies arrive on their exact due date.
Frequently asked questions
How is the due date calculated from my last period?
Naegele’s rule adds 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the calculator shifts the due date by the difference (cycle length minus 28 days), because ovulation — and conception — move with cycle length.
Why does my cycle length change the due date?
The luteal phase (the days from ovulation to your next period) is fairly fixed at around 14 days, but the follicular phase before ovulation varies. Longer cycles mean later ovulation, so conception and the due date shift later by the extra days. Shorter cycles shift everything earlier.
What if I know my conception date?
Switch the method to Conception date and enter it. The calculator adds 266 days to your conception date to estimate the due date, and back-calculates an LMP 14 days earlier. This is useful for IVF or when you know the exact date of intercourse or insemination.
How accurate is the due date?
It is a population-based estimate, not a prediction of when your baby will arrive. Only about 5% of babies are born on their exact due date; most arrive within two weeks of it. An early ultrasound (first trimester) gives the most accurate dating. Always confirm your dates with your obstetric provider.
What do the trimester milestones mean?
The first trimester runs from week 1 to week 13, the second from week 14 to week 27, and the third from week 28 to birth. Viability (around 24 weeks) is the earliest age a fetus can potentially survive outside the womb with intensive care. Full term is 39–40 weeks; babies born before 37 weeks are considered premature.
