Work backward from your due date to find your likely conception date, estimated implantation window, and earliest positive test date.
Calculated using the standard 266-day gestation period from fertilization.
Conception (fertilization) occurs approximately 266 days before birth. Our calculator subtracts 266 days from your due date to estimate when fertilization likely occurred. This typically aligns with your fertile window in the cycle before your missed period.
When your doctor says "you are 6 weeks pregnant," they mean 6 weeks from your LMP — you've only been truly pregnant (post-fertilization) for about 4 weeks. This 2-week difference is standard medical convention used worldwide.
Medical Disclaimer: This tool is for educational purposes only and should not replace professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for personalized guidance.
Your conception date is the day a sperm fertilized your egg. Because fertilization happens in the fallopian tube and can't be directly observed, conception dates are always estimates — but they can be calculated with reasonable accuracy using your known dates.
There are two ways to estimate conception: working forward from your last menstrual period (ovulation typically occurs 14 days after the start of your LMP, and fertilization happens within 24 hours of ovulation), or working backward from your due date (subtract 266 days from your EDD to get the estimated conception date).
Conception (fertilization) and implantation are two different events. Conception happens when the sperm fertilizes the egg in the fallopian tube — typically within 24 hours of ovulation. The fertilized egg then travels to the uterus over 3–5 days before implanting in the uterine lining. Implantation typically occurs 6–12 days after ovulation, and hCG production begins only after implantation — which is why pregnancy tests can't detect pregnancy immediately after conception.