For those who believe babies are a blank slate until they're born:

A study of the general population found that PTSD affects 5% of men and 10% of women. Studies also show a greater likelihood of PTSD development in the children of trauma survivors, including data on babies born to women who were pregnant and escaped from the World Trade Center on September 11 suggesting in utero and other developmental effects.

Science Daily: New Research Offers Insight into Posttraumatic Stress Disorder

If these babies experienced and reacted to their mothers' trauma before their bodies were fully developed, it seems likely they'd react similarly to a traumatic birth.