The exact date of conception is too difficult for anyone to find out, but doctors use the first day of the last menstrual cycle to determine the age of the fetus, according to WebMD. When a woman has regular periods, she can determine how far along in the pregnancy she is by counting the time from the first day of her most previous menstrual cycle.
After approximately 5 to 6 weeks after a woman's menstrual cycle has ended, an ultrasound can be performed to measure the fetus and get a more accurate due date, as noted by American Pregnancy Association. Between the 8th week and 18th week of pregnancy is when the most accurate age of the fetus can be determined.
Most conception dates are guesses based on when a woman had her last period. In certain cases, such as when a woman goes through an artificial insemination procedure, she can know exactly when the baby was conceived. In most circumstances, babies are conceived approximately 11 to 21 days after her last menstrual cycle began, which is when most women are most fertile, according to WebMD. Due dates are calculated by adding 40 weeks from the first day of the last menstrual cycle, but only 5 percent of babies are actually born on their estimated due date.