A study performed in Japan and published in 1995 noted a stunning comparison of singletons, twins, triplets, and quadruplets per 1000 births.  Cerebral palsy rates for singletons 2.5, twins 9, triplets 31, and quadruplets 111.  In another study, twins weighing at least 5.5 pounds at birth exhibited cerebral palsy 2.7 times more than a single birth.

Birth order is a risk factor for children of a multiple birth.  The later born is more vulnerable to cerebral palsy.  There are also complications during childbirth peculiar to multiple births that can manifest in fetal distress and possible brain damage.  There are specific forms of cerebral palsy that occur more frequently in multiple births, including spastic hemiplegia and spastic diplegia.  In addition, twins are more vulnerable to the effects of shortened gestational period than singletons.  Although cerebral palsy appears far more frequently in multiple births, there is no evidence that the severity of impairment is more extreme than in the case of a single birth. 

Hart, Hilary M., ed.  Clinics in Developmental Medicine.  London:  Mac Keith Press.   Alberman, Eva; Blair, Eve; and Stanley, Fiona.  Cerebral Palsies: Epidemiology and Causal Pathways.  London:  Cambridge University Press, 2000.  (The book is part of a series of hardcover monographs published by Mac Keith Press.  Four new ones are published each year.  The distributor is Cambridge University Press.)