Conception Date Calculator
Estimates the date of conception by working backward 266 days (38 weeks) from your known or expected due date. Useful when you know your due date but want to narrow down when fertilisation likely occurred.
About this calculator
Conception occurs approximately 14 days after the start of the last menstrual period in a standard 28-day cycle — or about 266 days (38 weeks) before the due date. Because the due date is set at 280 days from LMP, subtracting 14 days gives the estimated conception window: Conception Date ≈ Due Date − 266 days. In this calculator the input is expressed as days remaining until the due date, so the conception date in days from today is: Days to conception = Days until due date − 266. A positive result means conception already occurred in the past; a negative result would indicate the future, which is not physiologically meaningful in a normal pregnancy context.
How to use
Suppose your due date is 200 days from today. Step 1: Conception = 200 − 266 = −66 days from today, meaning conception occurred approximately 66 days ago. Step 2: If today is May 15, 2025, count back 66 days → conception was around March 10, 2025. Enter 200 in the 'Days until due date' field and the calculator will confirm conception was roughly 66 days in the past.
Frequently asked questions
How does a conception date calculator estimate when fertilisation occurred?
The calculator subtracts 266 days from the due date, because a full-term pregnancy from conception to delivery averages 38 weeks (266 days). Since ovulation and fertilisation typically occur around cycle day 14, this approach places conception in the correct two-week window after the LMP. The result is an estimate — sperm can survive up to 5 days in the reproductive tract, so the actual fertilisation window spans several days. Ultrasound dating remains the most precise way to confirm gestational timing.
Why is the conception date different from the last menstrual period date?
Clinicians date pregnancy from the LMP because it is a known, observable date, whereas the exact moment of ovulation and fertilisation is rarely certain. The LMP typically precedes conception by about 14 days in a standard 28-day cycle. So while a 40-week pregnancy is counted from the LMP, the embryo is only about 38 weeks old at birth. This two-week difference is a built-in offset in all standard pregnancy dating systems.
Can the conception date calculator tell me who the father is?
The conception date calculator provides only a probabilistic window — typically a 5–7 day span — during which fertilisation likely occurred. If intercourse with more than one partner happened within that window, the calculator cannot determine paternity. DNA paternity testing is the only scientifically reliable method to establish biological parenthood. The conception window can, however, help you assess whether timing aligns with a specific encounter, but it should never replace a formal paternity test.