Question 1

How accurate are due date calculators?

Accepted Answer

Due date calculators provide a reliable estimate, but only about 4–5% of babies are born on their exact estimated due date. Most births occur within a two-week window around the EDD — between 38 and 42 weeks of gestation. First-time mothers tend to deliver slightly later than their due date on average. Ultrasound dating in the first trimester can refine the estimate and is considered more accurate than LMP-based calculations when there is a discrepancy of more than seven days.

Question 2

What is Naegele’s Rule?

Accepted Answer

Naegele’s Rule is the standard formula used by obstetricians to estimate a due date based on the first day of the last menstrual period. The rule adds 280 days (40 weeks) to the LMP date, assuming a 28-day menstrual cycle with ovulation on day 14. It was formulated by German obstetrician Franz Karl Naegele in 1812 and remains the most widely used method for due date estimation worldwide, though it may be adjusted for women with irregular or non-standard cycle lengths.

Question 3

Does cycle length affect my due date?

Accepted Answer

Yes, cycle length can significantly affect your estimated due date. The standard 280-day calculation assumes a 28-day cycle. If your cycle is longer — say 35 days — ovulation likely occurs around day 21 rather than day 14, pushing your actual conception date (and therefore your due date) about a week later. This calculator accounts for cycle length by adding or subtracting the difference from 28 days. Tracking your cycle length over several months before pregnancy gives the most reliable adjustment.

Question 4

What percentage of babies are born on their due date?

Accepted Answer

Studies consistently show that only about 4% to 5% of babies arrive on their exact estimated due date. Approximately 80% of babies are born between 37 and 42 weeks of gestation, which is considered the normal range. About 11% of births are preterm (before 37 weeks), while around 7% are post-term (after 42 weeks). These statistics underscore that the due date is best understood as the midpoint of a likely delivery window rather than a precise prediction.

Question 5

How is the IVF due date different from a natural conception due date?

Accepted Answer

With IVF, the date of embryo transfer and the age of the embryo at transfer are both known precisely, which eliminates the uncertainty around ovulation timing. For a Day 5 blastocyst transfer, the embryo has already undergone five days of development, so 263 days are added to reach the due date. For a Day 3 transfer, 266 days are added. These calculations tend to be the most accurate of any method because the exact moment of fertilization is controlled and documented.