Standard intelligence tests showed that 41 percent of children born between 22 and 25 weeks had moderate or severe learning disabilities when compared to the test scores of a group of similar classmates who were born at full-term. It is also shown that higher levels of education were less likely to be obtained with decreasing gestational age at birth. People born prematurely may be more susceptible to developing depression as teenagers. Some of these problems can be described as being within the executive domain and have been speculated to arise due to decreased myelinization of the frontal lobes. Studies of people born premature and investigated later with MRI brain imaging, demonstrate qualitative anomalies of brain structure and grey matter deficits within temporal lobe structures and the cerebellum that persist into adolescence. Throughout life they are more likely to require services provided by physical therapists, occupational therapists, or speech therapists. Despite the neurosensory, mental and educational problems studied in school age and adolescent children born extremely preterm, the majority of preterm survivors born during the early years of neonatal intensive care are found to do well and to live fairly normal lives in young adulthood. Young adults born preterm seem to acknowledge that they have more health problems than their peers, yet feel the same degree of satisfaction with their quality of life. Beyond the neurodevelopmental consequences of prematurity, infants born preterm have a greater risk for many other health problems. For instance, children born prematurely have an increased risk for developing chronic kidney disease. Preterm birth complicates the births of infants worldwide affecting 5% to 18% of births. In Europe and many developed countries the preterm birth rate is generally 5–9%, and in the USA it has even risen to 12–13% in the last decades. As weight is easier to determine than gestational age, the World Health Organization tracks rates of low birth weight (< 2,500 grams), which occurred in 16.5 percent of births in less developed regions in 2000. It is estimated that one third of these low birth weight deliveries are due to preterm delivery. Weight generally correlates to gestational age, however, infants may be underweight for other reasons than a preterm delivery. Neonates of low birth weight (LBW) have a birth weight of less than 2500 g (5 lb 8 oz) and are mostly but not exclusively preterm babies as they also include "small for gestational age" (SGA) babies. Weight-based classification further recognizes "Very Low Birth Weight" (VLBW) which is less than 1,500 g, and "Extremely Low Birth Weight" (ELBW) which is less than 1,000 g.