summary: Babies born at or before 34 weeks gestation tend to have lower scores on math, language and IQ tests as teens than those born full term.
Premature birth, before 34 weeks of gestation, is associated with lower scores on math and language tests as adolescents than those born at 40 weeks of gestation, according to a large Danish population study published today in BMJ.
However, the study found no significant difference in late (cognitive) brain function in babies born between 34-39 weeks and those born at 40 weeks. The researchers acknowledge that cognitive outcomes are not predetermined at birth but are strongly influenced by social conditions.
It is estimated that around 15 million babies are born prematurely, before 37 weeks of gestation, worldwide each year. The last weeks of pregnancy are important for fetal brain development, and preterm labor and preterm labor are believed to have a negative impact on later brain function.
However, previous studies were relatively small, limited primarily to a single measure, or did not adequately adjust for other factors that could influence results.
To more precisely determine the effect of gestational age — the duration of pregnancy in weeks — at birth on long-term cognitive function, the researchers analyzed data for all siblings born in Denmark from January 1, 1986 to December 31, 2003.
A total of 1.2 million children were born in this period, of whom 792,724 had at least one full sibling born in the same period, which allowed the researchers to account for genetic factors such as the mother’s intelligence.
Using national registration information, the researchers analyzed gestational age at birth, along with the results of their exams in Danish written language and mathematics at ages 15-16, and, separately, the results of intelligence tests taken by 227,403 siblings, around the age of 18, on compulsory military conscription.
Potential influencing factors, including gender, birth weight, age of parents, educational level at birth, number of older siblings, and family factors shared between siblings were also taken into account in the analysis.
The researchers calculated how far above or below the average score the test score was, and compared that score for siblings at each gestational age to the score for siblings born in the classroom.
Overall, 44,322 (5.6%) of the 792,724 infants were born before 37 weeks. Of these, only those born before 34 weeks had math scores that were significantly lower than average than those born at 40 weeks, and the scores declined progressively as prematurity increased.
As for written language, babies born at or below 27 weeks showed a significantly lower score than average.
An analysis of military recruitment intelligence test scores, measured in IQ points, also showed significantly lower test scores for those born before 34 weeks.
For those born after 34 weeks, there was less than a one-point drop in IQ, compared to those born after 40 weeks. But there was a 2.4-point drop in IQ for those born between 32-33 weeks, a 3.8-point drop for 28-31 weeks, and a 4.2-point drop for those born at or before 27 weeks.
This is an observational study so the cause cannot be determined and the researchers also acknowledge some limitations. For example, smoking during pregnancy was not recorded before 1991, and test results may differ from real-life results such as lifetime income.
But they say the study has the advantage of a large sample size and that the sibling comparison design likely explains other factors such as the mother’s smoking. The results were also similar after further analyses, such as the inclusion of children who did not take exams, indicating that the results hold up to scrutiny.
While the reasons behind these findings remain unclear, the researchers suggest that because lower cognitive ability is associated with lower quality of life and early death, their findings “underscore the need for further research on how to prevent these negative outcomes.”
They add, “Cognitive outcomes are not predetermined at birth but are strongly influenced by social and care conditions, which is why early intervention is necessary for children born prematurely.”
In a related editorial, Canadian researchers acknowledge that cognitive deficits in early life can have a lifelong impact on a person’s ability and abilities.
However, they say that although parents and clinicians should be aware of the potential learning and cognitive difficulties associated with preterm birth, “parents should be reassured that the magnitude of these differences is not always significant, especially for those born in late gestation.”
They suggested that sibling comparisons have caveats and that because the causes of premature birth are complex and poorly understood, “efforts to identify and improve other social and environmental factors could be a more successful approach to mitigate any associated neurological deficits.”
About this research on cognition and neurodevelopment news
author: press office
Contact: Press Office – BMJ
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Original search: open access.
“Gestational age at birth and cognitive outcomes in adolescence: a full population-based cohort studyBy A Husby et al. BMJ
Gestational age at birth and cognitive outcomes in adolescence: a full population-based cohort study
To investigate the relationship between gestational age at birth and cognitive outcomes in adolescence.
A complete, nationwide population-based cohort study.
1.2 million children were born between January 1, 1986 and December 31, 2003, of whom 792,724 were born in the same period of one or more siblings.
Main outcome measures
Scores in written language (Danish) and mathematics exams as graded by masked assessors at the end of compulsory education (9th grade, ages 15-16), as well as an intelligence test score in the military recruitment (mostly 18-year-old) group A nested subset of male adolescents. School scores were standardized as z-scores according to year of examination, and intelligence test scores were standardized as z-scores according to year of birth.
Of the 792,724 full siblings in the cohort, 44,322 (5.6%) were born before 37 + 0 weeks’ gestation. After adjusting for multiple confounding factors (sex, birth weight, abnormalities, parents’ age at birth, parents’ educational level, number of older siblings) and family factors shared by siblings, only children born at less than 34 weeks gestation showed a lower mean score. In written language (z-score difference −0.10 (95% confidence interval 0.20 to −0.01) for 27 weeks of gestation) and mathematics (0.05 (0.08 to −0.01) for 32–33 weeks of gestation, −0.13 (0.17 to − (0.09) for 28–31 weeks of gestation, and −0.23 (−0.32 to −15.15) for 27 weeks of gestation), compared with children born at 40 weeks of gestation. In a nested subgroup of full siblings with IQ test scores, those born at 32-33, 28-31, and 27 weeks gestation showed decreases in IQ scores of 2.4 (95% confidence interval 1.1 to 3.6), and 3.8 ( 2.3 to 5.3) and 4.2 (0.8 to 7.5), respectively, while babies born at 34-39 weeks of age showed a decrease in IQ of less than 1 IQ point, compared to babies born at 40 weeks of gestation.
Cognitive outcomes in adolescence did not differ between those born at 34-39 weeks of gestation and those born at 40 weeks of gestation, while those at <34 weeks of gestation showed significant deficits in multiple cognitive domains.