That’s because it takes approximately that long for the fertilized egg to divide for the first time. During the first 2 days, the fertilized egg grows to 2, then 4 cells and by day 3, it’s an 8-celled zygote, containing 23 chromosomes from the man’s sperm and the 23 from the woman’s egg. These chromosomes will determine eye color, hair color, and sex. The cells continue to divide as they’re pushed along the fallopian tube, a kind of preembryonic dervish dance. Approximately four days after fertilization there are about 100 cells, and the zygote is now called a blastocyst. By day 6, the blastocyst is comfortably implanted in the uterine wall. And the rest, as they say, is history.