Perinatal and neonatal risk factors

While ASD is not diagnosed until well after birth, there has been accumulating evidence that prenatal and neonatal factors may cause ASD. While genetic mutations remain a large risk factor it is important to keep in mind environmental risk factors. A recent meta-analysis (an analysis of published studies) identifies about 60 perinatal and neonatal risk factors. Factors associated with an increased risk of ASD were abnormal presentation, umbilical-cord complications, fetal distress, birth injury or trauma, multiple birth, maternal hemorrhage, summer birth, low birth weight, small for gestational age, congenital malformation, feeding difficulties, neonatal anemia, Rh incompatibility, and hyperbilirubinemia (jaundice). And those not associated with an increased risk were assisted vaginal delivery, postterm birth, high birth weight, and head circumference. When reading these results it is important to bear in mind a number of things: (1) most of the studies investigated by the group showed inconsistent results with only 30% of studies having sufficient power to allow risk factor correlation, (2) inconsistent results were often due to different study designs, (3) there was no evidence to suggest that only one factor was implicated in ASD, but instead, increased risk resulted from multiple factors, (4) a lot of the factors are related; for example cesarian delivery is often undertaken in cases of foetal distress, hyperbilirubinemia is associated with hypoxia, and feeding difficulties can be associated with any number of preceding factors, (5) these factors are not unique to ASD, but can result in other pervasive developmental disorders, children with lower IQ without ASD, etc, (6) similarly, it is not clear how many of the ASD cases reported also had intellectual disabilities, and importantly, co-morbidities such as Down syndrome, (7) the significant risk factors may be a result of an underlying cause of ASD, which leads to the shared-risk hypothesis, (8) ASD subtypes and symptoms were not distinguishable from the published studies and finally, (9) no maternal factors were considered such as smoking or medication, only foetal or neonatal events. Interestingly, head size was not a risk factor of ASD. This would appear to contradict reports that a subtype of ASD has an enlarged brain, but scientists currently believe that a large head size develops after birth, and hence would not necessarily be picked up at birth, and therefore not included in this study.
Read the full report here.
Randal