How is the result calculated?
Per Naegele's Rule, adopted by the World Health Organization and the American Pregnancy Association: due date = first day of last period + 280 days (40 weeks), with a slight adjustment if your cycle length differs from 28 days.
Is the result 100% accurate?
No. Only about 80% of women give birth within a two-week window before or after the calculated date — full accuracy is impossible because actual delivery is affected by individual factors. A doctor's first-trimester ultrasound is more accurate than any online calculator.
When should I use "conception date" instead of "last period"?
If you know the conception date precisely (like IVF/ICSI cases or ovulation tracking), this option is more accurate than relying on the last period, especially if your cycle is irregular.
Why is gestational week calculated from the last period and not conception?
Because most women remember their last period date precisely, while it's hard to determine the actual day of conception. So it was medically agreed to use the first day of the last period as a unified starting point, even though actual conception occurs about two weeks after it.
What's the importance of the screening test dates shown in the result?
Every pregnancy screening test has an optimal "time window" where it's most accurate (like the nuchal translucency scan between weeks 11 and 14, for example). Calculating the actual calendar dates for these windows specific to your pregnancy helps you plan your medical appointments instead of manually calculating the weeks every time.
