Fertilization, which happens when the sperm and egg unite, is what most people refer to as “conception,” said Dr. Abigail Cutler, an OB-GYN at UW Health and assistant professor of obstetrics and gynecology at the University of Wisconsin School of Medicine and Public Health.
About five to 10 days after fertilization, the fertilized egg implants in the lining of the uterus. HCG is produced shortly afterward, Cutler said, first in low levels which rise rapidly over time. “The very earliest someone can confirm whether they are pregnant is following implantation, the timing of which varies but can take a week or more,” she said