3 weeks - Once in the uterus, the developing embryo, called a blastocyst, searches for a nice place to implant, where it actually burrows beneath the surface of the uterus. The yolk sac, shown on the left, produces blood cells during the early weeks of life. The unborn child is only one-sixth of an inch long, but is rapidly developing. The backbone, spinal column, and nervous system are forming. The kidneys, liver, and intestines are taking shape.