Impact of birth size on academic performance in adolescence

Does the week a baby is born matter as much as their size? A study following more than 92,000 children reveals that early developmental differences linked to delivery timing largely fade, while associations between smaller birth size and academic performance persist into adolescence.

Study: The relationship between gestational age at delivery and fetal size, and neurodevelopmental outcomes from infancy to adolescence. Image credit: gpointstudio/Shutterstock.com

A recent study in the American Journal of Obstetrics & Gynecology compared the associations of gestational age and fetal size with early childhood neurodevelopment and adolescent academic performance.

How gestational age at birth relates to child development

Term delivery describes childbirth between 37 and 41 weeks of gestation, a period when the fetus is generally mature enough for life outside the womb. Timing is determined collaboratively by the patient and obstetrician, weighing risks such as stillbirth, maternal complications, fetal growth restriction, and neonatal issues from early delivery.

Rates of labor induction and planned cesarean delivery have increased worldwide. In the United States, this has coincided with a shift toward births earlier in the term period, particularly at 37 weeks. While short-term outcomes of early delivery are well documented, the long-term neurodevelopmental consequences remain less certain, emphasizing the need for evidence-based decisions.

Gestational age (37–41 weeks) and fetal size (birth weight for gestational age) are distinct but interrelated factors in fetal development. Lower gestational age within the term window is associated with slightly reduced childhood cognitive outcomes, which tend to improve as gestation approaches 39–41 weeks; birth at 41 weeks may correspond to the lowest rates of special education needs. Since most elective deliveries take place within this window, a large number of children may be affected, but the long-term cognitive impact of early-term birth (37–38 weeks) is still debated.

Fetal size, classified as small (SGA), large (LGA), or appropriate (AGA) for gestational age, also influences risk. SGA is consistently associated with lower cognitive and academic outcomes, sometimes persisting into adulthood, while findings for LGA are mixed. Because fetal brain development is highly sensitive to nutrition, smaller fetal size may reflect adverse conditions affecting neurodevelopment. It is critical to understand whether gestational age or fetal size has a more persistent association with neurodevelopmental outcomes.

How gestational age and fetal size relate to development from infancy to adolescence

This population-based longitudinal study analyzed data from the Norwegian Mother, Father and Child Cohort (MoBa), which included over 114,000 children and their families recruited across Norway between 1999 and 2008. Follow-up surveys were conducted during pregnancy and at child ages 6 months, 18 months, 3, 5, 8, and 14 years. Study data were linked to the Medical Birth Registry for gestational age and birth weight, and to Statistics Norway for national standardized test results. Children born before 37 weeks, after 42 weeks, or with congenital malformations were excluded.

The main exposures were gestational age, determined by ultrasound at 17 to 20 weeks and analyzed both by specific week (37 to 41) and as a continuous variable, and fetal size, measured as birth weight adjusted for gestational age and sex. Birth weight was categorized as small (below the 10th percentile), appropriate (10th to 90th percentile), or large (above the 90th percentile).

Outcomes assessed included language and motor development at ages 6 months, 18 months, 3, and 5 years, national test scores in math, reading, and English at grades 5 and 8, math and reading at grade 9, and parent-reported special educational needs.

Covariates included infant sex, maternal age, birth year, parity, delivery mode, parental income, education, and marital status. The researchers also used statistical methods to account for systematic participant dropout and missing covariate data.

Both gestational age and fetal size are associated with neurodevelopment; patterns vary over time

The current study directly examined whether gestational age or fetal size has a longer-lasting association with neurodevelopment in term-born children. The study included 92,551 term-born children, 49.9% female, with 5.5% born at 37 weeks of gestation.

After adjusting for confounders and correcting for multiple testing, the study found that gestational age at term and fetal size were associated with childhood language and motor skills, as well as adolescent national test scores in math, reading, and English.

Compared to children born at 41 weeks, those born at 37 weeks had poorer language skills from 6 months to 3 years and poorer motor skills from 6 months to 5 years. However, these differences diminished with age and were minimal by around age 5. Larger fetal size was generally linked to better language and motor outcomes in childhood, although not all associations were statistically significant.

In adolescence, gestational age at term had little association with national test scores, except for a small negative association between birth at 37 weeks and English performance at fifth grade. Being small for gestational age was consistently linked to lower math and reading scores from fifth to ninth grade, while being large for gestational age was associated with higher math scores at ninth grade.

Although these differences persisted into adolescence, their magnitude was relatively small. Children born small for gestational age scored approximately 1.8 to 2.3 points lower in math than those of appropriate size, on a standardized scale with an average of 50. Girls showed stronger negative associations between small fetal size and math outcomes than boys.

The researchers also found little evidence that gestational age and fetal size interacted in their associations with developmental outcomes, suggesting that the two measures provide largely independent information. Findings concerning special educational needs were less conclusive, with inconsistent results at ages 8 and 14 and substantial loss to follow-up.

Fetal size shows more persistent links to development

This study found that, among children born at term, fetal size was more persistently associated with neurodevelopmental outcomes than differences in gestational age. Although later gestational age was associated with better early language and motor development, these associations diminished with age and were largely absent for academic performance in adolescence. Conversely, being born small for gestational age was consistently associated with poorer academic performance throughout adolescence.

Clinically, these findings underscore the need to consider gestational age and fetal size separately during counseling and shared decision-making. However, the authors emphasize that the results should inform discussions about delivery timing rather than be interpreted as evidence supporting changes in current obstetric management.

The study has several limitations, including potential selection bias from participant dropout, reliance on birth weight as a measure of fetal size, and the observational design, which prevents causal conclusions. Future randomized trials with longer-term follow-up are needed to better understand how delivery timing may affect child development.