Due Date Calculator

Due date accuracy depends on the method used. IVF transfer dates are most accurate (±3-5 days), followed by first trimester ultrasound (±7 days), and LMP-based calculations (±14 days). Research shows that only about 5% of babies arrive exactly on their due date, but 95% are born within 2 weeks of the estimated date.

Use the most accurate information you have: IVF transfer date if applicable, conception date if you know it precisely, or last menstrual period (LMP) as the most common method. LMP works best with regular 28-day cycles. If you have irregular cycles, conception date or ultrasound dating may be more accurate.

Naegele's rule, established in 1838, calculates due dates by adding 280 days (40 weeks) to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14. While simple and widely used, modern research shows natural pregnancy length varies significantly, with factors like maternal age and parity affecting timing.

IVF due dates are typically more accurate because the exact embryo transfer date is known. Day 3 embryo transfers add 263 days, while day 5 (blastocyst) transfers add 261 days to calculate the due date. This precision makes IVF-calculated due dates accurate within ±3-5 days compared to ±14 days for LMP-based calculations.

Many factors influence actual delivery timing: first pregnancies tend to go 5 days past due date on average, while subsequent pregnancies average 3 days past. Maternal age over 35 may extend pregnancy by 1-2 days. Genetics, stress levels, and baby's individual development also play roles. Remember, 'term' pregnancy is anywhere from 37-42 weeks.