Frequently asked questions
Which method gives the most accurate due date?
IVF transfer dates are the most precise because the exact fertilization timing is known. Conception-date methods are next. LMP-based calculations are least precise because they assume a 14-day follicular phase. A first-trimester ultrasound (7 to 13 weeks) is generally the clinical gold standard.
Why is IVF due date calculated differently for Day-3 vs Day-5 embryos?
A Day-5 embryo (blastocyst) is 2 days more developed at transfer than a Day-3 embryo, so the due date is 2 days earlier. Day-5 transfer: due date = transfer + 261 days. Day-3 transfer: due date = transfer + 263 days. The difference accounts for those 2 extra days of lab culture.
How does cycle length affect the LMP-based due date?
The standard calculation assumes ovulation on day 14 of a 28-day cycle. If your cycle is longer (e.g. 35 days), ovulation likely occurred on day 21 instead, pushing the due date 7 days later. This calculator adjusts by adding (cycle length minus 28) days.
Can the due date change during pregnancy?
Yes. If a dating ultrasound in the first trimester shows the fetus measuring differently from what the LMP predicts (by more than 5 to 7 days), providers typically adjust the due date to match the ultrasound. Later ultrasounds are less reliable for dating.
What is the difference between gestational age and fetal age?
Gestational age counts from the first day of the last menstrual period and is about 2 weeks longer than fetal age, which counts from conception. Medical care uses gestational age, so a '40-week' pregnancy is really 38 weeks from conception.
Is a due date of 40 weeks or 38 weeks from conception?
Both are the same thing stated differently. Full-term pregnancy is 40 weeks gestational age (from LMP) or 38 weeks from conception. The 2-week difference is the assumed time from LMP to ovulation. The due date marks 40 completed gestational weeks.