According to a large Danish population study published in The BMJ, preterm delivery before 34 weeks of pregnancy is connected with inferior performance in arithmetic and language examinations as a teenager compared to those born at 40 weeks. However, the study revealed no significant difference in subsequent brain (cognitive) function between kids born at 34–39 weeks and those delivered at 40 weeks. Furthermore, the researchers admit that cognitive results are highly impacted by social conditions rather than being predetermined at birth. Every year, an estimated 15 million children are delivered prematurely, before 37 weeks of pregnancy.
Preterm and early-term delivery are known to have a deleterious influence on subsequent brain function, and the final weeks of pregnancy are critical for fontal brain development. Previous studies, on the other hand, were small, mostly confined to one measure, or did not adequately account for other factors that may have impacted the results. The researchers examined data for all full siblings born in Denmark between 1 January 1986 and 31 December 2003 to better establish the influence of gestational age—duration of pregnancy in weeks—at birth on long-term cognitive performance.
A total of 1.2 million children were born during this time period, with 792,724 having at least one full sibling born during the same time period, allowing the researchers to account for genetic characteristics such as mother IQ. Using statewide registry data, the researchers examined gestational age at birth, as well as exam results in written Danish language and math at ages 15–16, and separately, the results of IQ tests done by 227,403 brothers around the age of 18, during required military conscription. The investigation took into account potentially important factors such as sex, birth weight, parental age and educational level at birth, number of older siblings, and shared familial characteristics amongst siblings.
The researchers determined how far a test result was above or below the average grade and compared it to the score for siblings delivered at term at each gestational age. In total, 44,322 (5.6%) of the 792,724 children were born before the age of 37 weeks. Only those born before 34 weeks showed considerably worse average arithmetic scores than those born at 40 weeks, and grades declined gradually as prematurity increased.
Only children born at or fewer than 27 weeks had a considerably lower than average grade in written language.
Analysis of military conscription intelligence test scores, assessed in IQ points, revealed that individuals born before 34 weeks had significantly lower test scores.
When compared to those born at 40 weeks, those born after 34 weeks had an IQ drop of less than one point. However, those born between 32 and 33 weeks had a 2.4-point IQ loss, those born between 28 and 31 weeks had a 3.8-point reduction, and those born at or before 27 weeks had a 4.2-point reduction. Because this is an observational study, no causality can be established, and the researchers admit several limitations. Smoking during pregnancy, for example, was not documented prior to 1991, and test findings may differ from real-life effects such as lifetime income. However, they claim that the study has a large sample size and that its sibling comparison design compensates for other variables such as maternal smoking.
Further studies, such as including children who did not take tests, yielded comparable results, indicating that the findings may be trusted. While the underlying causes of these findings are unknown, the researchers argue that because low cognitive ability is associated with poor quality of life and premature mortality, their findings “emphasise the need for greater study into how these unfavorable effects might be averted.”
They add: “Cognitive outcomes are not, however, predetermined at birth but are heavily influenced by social circumstances and nurturing, and this is why early intervention is warranted for children born early preterm.”
In a linked editorial, Canadian researchers acknowledge that cognitive deficits in early life could have a lifelong influence on a person’s capacity and capabilities.
Although parents and physicians should be informed of potential educational and cognitive challenges associated with preterm delivery, they add, “parents should be comforted that the size of these abnormalities is not usually large, particularly for those delivered at later gestations.”
They also propose that because the reasons of preterm birth are complicated and poorly understood, “efforts to discover and improve on other socio-environmental variables might be a more beneficial strategy to minimizing any related neurocognitive deficiencies.”