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Birth is a time of peril for the human brain, especially in pre-term infants. For vulnerable preemies biochemical signs of reduced blood oxygen levels soon after birth are associated with lower IQs and language skills.

Now, new research has linked pre-term birth, risk for birth hypoxia, and cognitive problems and reveals how the risk threshold for brain damage in preemies could be lower than thought (Neuropsychology, January 2003).¹ This research bears strong consideration, since premature babies comprised 12 percent of U.S. births in 2001, the highest level in 20 years, due in part to more multiple pregnancies, induced labor and older mothers.

Psychologists at Wayne State University, in Detroit, MI; the University of Memphis, in Tennessee; and Baptist Memorial Hospital, in Memphis, studied 52 children who had been born at or before 36 weeks. During or immediately following birth, half of the group was at slight to moderate risk for hypoxia. The other half had no such risk, although the babies resembled the risk group on other early risk factors and on socio-demographic characteristics. Despite the relatively small difference between the groups in the degree of risk, the two groups "diverged significantly" in their development, the authors reported.

Researchers Tracy Hopkins-Golightly, PhD; Sarah Raz, PhD; and Craig J. Sander, MD, tested the 5- and 6-year-olds on intelligence and receptive and expressive language skills. There was a significant relationship between blood pH soon after birth and later cognitive and language skills. For example, the pre-term group, which had mild to moderate acidosis, scored approximately 10 to 11 points lower on verbal and visuospatial tests than the low-risk pre-term group—a large discrepancy.

Such data, noted the authors, reveal that even a minor risk for hypoxia around birth may have a "discernible influence on the course of cognitive development."

The relationship between mild to moderate birth hypoxia and later cognitive abilities contradicts established wisdom that regards severe oxygen deprivation as the threshold for brain damage in an all-or-nothing manner. The findings add to other recent evidence of a continuum of brain injury due to asphyxia around birth.

"Most neonatologists would probably not expect to find a statistically significant relationship between degree of acidosis measured soon after birth and performance on cognitive tests in preschool and early school-age children, when acidosis is only mild to moderate," stated Dr. Raz, of Wayne State.

Although it is well-known that preemies tend to have more cognitive problems than full-term infants, scientists want to tease out the specific complications, from a host of many, that cause the most trouble. A good way to do that is to compare two groups of pre-term infants who share the risks of pregnancy, delivery and the vulnerable postnatal period but differ in terms of a single risk factor, such as birth hypoxia.

Structural or functional imaging, such as magnetic resonance imaging (MRI), may shed light on which brain areas are the most vulnerable to damage by hypoxia in the pre-term infant. For now the authors speculate that even minor risk in babies born prematurely may be associated with damage to the periventricular white matter, which is deep inside the brain.

By linking birth complications to specific cognitive problems occurring later, scientists hope to understand the vulnerability of the brain to insult during early human development. Furthermore, knowledge of how early risk factors affect cognitive abilities may help doctors evaluate the effectiveness of medical interventions that support pre-term infants during and after birth.

Reference
1. Hopkins-Golightly, T., Raz, S., Sander, C.J. (2003). Influence of slight to moderate risk for birth hypoxia on acquisition of cognitive and language function in the preterm infant: A cross-sectional comparison with pre-term birth controls. Neuropsychology, 17 (1).