Six weeks after fertilization, a human embryo is roughly 1.2 centimetres (0.5 inches) from crown to rump, and there is nothing about its lower body that would let anyone tell one from another. There is a ridge of tissue that could become a testis or an ovary. There is a small bud. There are two…

At six weeks in, the blueprint is identical after fertilization, a human embryo is roughly 10 to 14 mm long (about 0.4 to 0.6 in) — the length of a coffee bean. Along the back wall of its abdominal cavity sit two small ridges of tissue.