Some causes could be the mother smoking, drinking, doing drugs etc. Or even simply her body doesn’t fully accept the baby growing inside her so it acts out against it. Sounds strange but it happens. Also if the mother is sick with Zika or another virus, it can harm the baby’s growth.
Clonidine Off Label that patient has Tourette's Syndrome
I think that the answer is 12 billion. So sorry if I am not right.