Every time I write the following paragraph in new editions of my textbooks, I try to picture what can go wrong during those first cell divisions.
“The birth of a healthy baby is against the odds. Of every 100 eggs exposed to sperm, 84 are fertilized. Of these 84, 69 implant in the uterus, 42 survive one week or longer, 37 survive 6 weeks or longer, and only 31 are born alive. Of the fertilized ova that do not survive, about half have chromosomal abnormalities that cause problems too severe for development to proceed.” (more…)