The zygote begins dividing as it travels down the fallopian tube toward the uterus over the next several days. By the time it reaches the uterus (usually 5-7 days later), it has become a ball of cells called a blastocyst, which then implants into the uterine lining. This implantation is when pregnancy is officially established.
About six days after fertilization, the developing embryo (now a blastocyst) implants into the uterine lining,triggering hormone production that maintains the pregnancy and proceeds to embryogenesis.