The fusion creates pronuclei containing genetic material from each parent. These pronuclei migrate toward each other, decompress, expand, replicate their DNA, and then intermingle when their nuclear envelopes disintegrate. This completes fertilization, creating a diploid zygote with 46 chromosomes—23 from each parent.
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, marking the true beginning of pregnancy and triggering hormone production that maintains the pregnancy.