The term "conceived" generally refers to the beginning of pregnancy, which medically includes both fertilization and implantation of the fertilized egg into the uterus. Fertilization occurs when sperm combines with an egg in the fallopian tube, but conception is considered to have occurred only after the fertilized egg implants into the uterine wall. This process usually happens about 11 to 21 days after the first day of a woman's last menstrual period in women with regular cycles. Because of variability in ovulation and menstrual cycles, pinpointing the exact date of conception is difficult, but conception is estimated to occur around ovulation, which is typically about 14 days after the start of the last menstrual period. This date can be estimated more accurately using methods such as ultrasounds or based on an expected due date by subtracting 266 days (38 weeks) from the due date.

In summary, conception is when a fertilized egg implants in the uterus, occurring roughly 2 weeks after the last menstrual period, and marks the start of pregnancy.