ACOG reverse gestational mechanics
According to the American College of Obstetricians and Gynecologists, natural embryonic gestation lasts 38 weeks (266 days).
While patients often wish to pinpoint the specific day of conception, fertilization can take place up to 5 days following sexual intercourse due to sperm longevity in the female reproductive tract.
Coupled with implantation taking an additional 6 to 12 days, gestational age calculations always maintain a biological window of confidence.
Worked example: Due date reverse calculation
For an EDD of October 10th established by a first-trimester crown-rump length ultrasound, subtracting 266 days gives an estimated conception date of January 17th.
The intercourse window that resulted in pregnancy spanned January 14th through January 18th.
US conception inputs
Due Date
Enter your doctor-confirmed estimated due date.
CALCULATOR QUESTIONS
US conception date & intercourse window calculator questions
Can an ultrasound determine the exact day of conception?
No. First-trimester ultrasound has an accuracy margin of ±5 to 7 days, which is why it confirms gestational age rather than an exact single day of intercourse.
What is the fertile window?
The fertile window encompasses the 5 days before ovulation plus the day of ovulation itself, representing the only time sexual intercourse can result in conception.