Frequently asked questions
Why is there a 3-day range on the conception date?
Ovulation timing and fertilization are not instantaneous events. The egg lives 12 to 24 hours; sperm can fertilize for up to 3 to 5 days after intercourse. A ±3-day range reflects the biological uncertainty in pinpointing the exact moment of fertilization, even with known gestational age.
Is the 'from current week' mode accurate?
It depends on how accurately your gestational age was determined. If based on a first-trimester ultrasound (accurate to ±5 days), the conception estimate is reliable. If based on LMP with irregular cycles, there may be more uncertainty than the ±3 days shown.
What is the difference between this and the conception calculator?
The conception calculator works backward from a due date or birth date. This calculator also works from your current gestational week, which is useful when you know how far along you are (from an ultrasound) but want to trace back to when conception occurred.
Can I determine who the father is from the conception date?
A conception date range narrows the window to about a week, which may help identify or exclude possibilities. However, it is not definitive proof. If paternity is in question, DNA testing after birth (or non-invasive prenatal paternity testing from 7 weeks) is the only reliable method.
How does gestational week relate to conception?
Gestational age is counted from the first day of the last menstrual period, which is about 2 weeks before conception. So at 'gestational week 12,' the embryo is actually about 10 weeks old. This calculator accounts for that 2-week offset when estimating conception.
Does the due date method assume full-term pregnancy?
Yes. The calculation assumes a standard 40-week (280-day) pregnancy from LMP, placing conception at due date minus 266 days. If your due date was adjusted by ultrasound, this is already accounted for. The ±3-day range covers normal biological variation.