More than 10% of all babies are born preterm every year, and prematurity is a well-established risk factor for cognitive impairment at some level.
Prematurity affects working memory in particular
In a recent German study involving 1326 8-year-old children, it was found that being born preterm specifically affected the ability to solve tasks with a high cognitive load (i.e. greater demands on working memory), whereas tasks with a low load were largely unaffected.
These findings are consistent with other research suggesting that prematurity is associated in particular with difficulties in math, in complex problem-solving, and in simultaneous processing (such as occurs in recognition of spatial patterns).
There was also a clear dividing line, with deficits disproportionally higher for children born before the 34th week of pregnancy compared with children born after week 33.
Rate of cognitive impairment in premature infants
A Swedish study of 491 toddlers (2 ½ years) who had been born extremely preterm (less than 27 gestational weeks) found that 42% of them had no disability (compared with 78% of controls), 31% had mild disability, 16% had moderate disability, and 11% had severe disability. Unsurprisingly, there was an increase in moderate or severe disabilities with greater prematurity. There was no gender difference.
Cognitive impairment in premies linked to smaller brain tissue in specific regions
Why are some individuals affected by prematurity, why others aren’t? An analysis of brain imaging data of 97 adolescents who had very low birth weights, and whose academic progress has been followed, found that more than half of the babies that weighed less than 1.66 pounds and more than 30% of those less than 3.31 pounds at birth later had academic deficits. Academic deficits were linked to smaller brain volumes, and in particular to reduced volume of the caudate and corpus callosum, which are involved in connectivity, executive attention and motor control.
J. Jäkel, N. Baumann, D. Wolke (2013): Effects of gestational age at birth on cognitive performance: a function of cognitive workload demands, PLOS ONE, http://dx.plos.org/10.1371/journal.pone.0065219
 Neurodevelopmental outcome in extremely preterm infants at 2.5 years after active perinatal care in sweden. JAMA. 309(17), 1810 - 1820.(2013).
 Relation of neural structure to persistently low academic achievement: A longitudinal study of children with differing birth weights. Neuropsychology. 27(3), 364 - 377.(2013).