How Conception Dates Are Calculated
Pinpointing the exact moment of conception is difficult without medical intervention (like IVF). However, we can estimate it with high accuracy using the "Naegele's Rule" logic in reverse.
A standard pregnancy lasts 40 weeks (280 days) from the first day of the Last Menstrual Period (LMP). However, ovulation and conception typically don't happen until 2 weeks after the period begins. Therefore, the "true" age of the fetus is usually 38 weeks (266 days), not 40.
The Fertile Window
Why do we show a date range instead of a single day? Because conception is a process, not just an instant event.
- Sperm Survival: Healthy sperm can live inside the female reproductive tract for up to 5 days while waiting for an egg.
- Egg Survival: Once ovulation occurs, the egg only survives for 12 to 24 hours.
This means if you had intercourse on Tuesday, but didn't ovulate until Friday, the sperm could still be waiting there to fertilize the egg. This is why the "Conception Window" includes the days leading up to the estimated ovulation date.
Frequently Asked Questions
Is this calculator 100% accurate? ▼
It is an estimate. Every woman's cycle is different. While the 266-day rule is the medical standard for estimation, ovulation can vary by a few days due to stress, diet, or irregular cycles. Ultrasound scans in the first trimester are the most accurate way to date a pregnancy.
Why is it different from my LMP date? ▼
Doctors date pregnancy from the Last Menstrual Period (LMP) because it's an observable date. Conception happens roughly 2 weeks after the LMP. So, if you are "6 weeks pregnant" by doctor's standards, your baby was likely conceived only 4 weeks ago.