During each menstrual cycle, an oocyte (premature egg) develops in one of a woman’s two ovaries. As it enlarges, it produces hormones that signal the uterus to build up its lining and prepare itself for implantation if conception does occur.  During ovulation, the oocyte is released from the ovary and enters a narrow tube — called the fallopian or uterine tube — that is connected to the uterus. Conception occurs when a viable sperm meets with a viable oocyte in the middle third of the fallopian tube. At this point, the fertilized egg becomes a zygote and begins to divide right away. The zygote continues through the fallopian tube into the uterus. There, it will implant into the thickened lining in the wall of the uterus and at this point, pregnancy begins. This occurs between 6 and 12 days after ovulation.

When the zygote implants, it becomes known as an embryo. It also begins secreting a hormone called human chorionic gonadotropin (hCG). This is the hormone that is detected in pregnancy tests (both those that use blood and those that use urine).