PVL is caused by
lack of blood flow & oxygen
to the white matter
around the ventricles of the brain, an area most susceptible to damage around the end of the 2nd trimester. Severity of impairment varies widely, but isn't seen right away.
Start Early Intervention therapy at 4-6 weeks as "Early Prevention" for optimal outcome. See: http://cerebralpalsy.org/about-cerebral-palsy/cause/periventricular-leukomalacia/