Recent studies suggest that higher relative birth weight provides evidence of improved cognitive performance in children and adolescents, even when comparing twins who share the same genetic and environmental background. Published in Journal of Child Psychology and Psychiatryresearch shows that in-utero conditions can have long-term effects on a person’s brain development. These findings demonstrate that early prenatal growth is important for outcomes over the first three decades of life.
Previous research has shown that babies born with certain risk factors tend to have lower scores on subsequent cognitive tests. Two common risk factors are low birth weight and small head circumference. Low relative birth weight means that the baby is smaller than expected for the specific number of weeks it has spent developing in the womb. This early physical size is often used as a marker of how well the fetus was nourished during pregnancy.
“There are a number of studies comparing the developmental outcomes of babies of different birth weights and head circumferences,” says study author Robert Eves, a postdoctoral researcher at Bielefeld University. “However, these studies typically compare babies from very different families who differ in many other important factors, such as genetics and family factors (such as poverty levels).”
Because family income, neighborhood safety, and parental education influence brain development during childhood, comparing children from widely different backgrounds makes it difficult to pinpoint the exact causes of differences in later cognitive abilities. To get around this problem, scientists turn to twins. Twins share the same environment in the womb and grow up in the same household. Identical twins share exactly the same genetic code.
“We therefore wanted to test whether differences in birth weight and head circumference between twins were important for later cognitive performance,” Eves said. “This is interesting because differences in birth weight and head circumference between twins are thought to be caused by differences in how each baby’s placenta works, rather than genetic factors.”
Examining these specific differences between twins will allow researchers to better understand how in-utero conditions independently shape long-term cognitive development. The authors wanted to test whether differences in birth weight and head circumference between twins actually predicted differences in later intelligence test scores. They also investigated whether this relationship might fade as children grow older. Another aim was to see if head size served as a better proxy for brain development than general body weight.
To answer these questions, the research team analyzed data from the German TwinLife study. This is an ongoing project that tracks the psychological and social development of same-sex twins over time. The particular sample for this analysis included 4,196 people, making up exactly 2,098 twin pairs in the first wave of testing. These pairs include both identical and fraternal twins and represent a variety of ages and backgrounds.
Scientists used official medical records to determine each twin’s exact birth weight in grams and head circumference in centimeters. They then adjusted these numbers based on the baby’s biological sex and the exact number of weeks it spent in the womb. To do this, the researchers used international growth curves, which determine the average size of a baby for each week of pregnancy.
Standardizing the scores allows the authors to see whether a baby is smaller or larger than the average for a particular developmental stage. Comparing a baby born at 32 weeks to a baby born at 40 weeks is not an apples-to-apples comparison. Using relative scores helps researchers measure the actual deviation from a baby’s natural growth potential.
Cognitive ability was measured using a standardized intelligence assessment called the Culture Fair Test. This particular assessment does not rely on language or mathematical skills. Instead, we ask participants to solve visual puzzles, classify shapes, and complete complex pattern matrices. The tests were conducted at two different points approximately six years apart.
The median age at the first wave of testing was 12 years. The median age at the second wave of testing was 18 years, but some participants were older. The researchers adjusted the test scores to account for the different ages of the participants at the time they took the assessment.
The authors found that within twin pairs, differences in relative birth weight were associated with differences in cognitive performance at the first wave of testing. Specifically, twins born at higher relative birth weights tended to have slightly higher scores on visual intelligence tests. This association was present in both identical and dizygotic twins.
Commenting on the main findings of the study, Eves said: “Small twins are associated with poorer cognitive performance 12 years later. However, this was only found for birth weight, not head circumference. We thought this was surprising.”
The positive effect of higher birth weight was seen even in genetically identical twins, suggesting that common genetics and family environment do not fully explain the association between birth weight and cognition. The physical environment in utero appears to have independent effects on long-term brain function.
“This provides evidence that intrauterine conditions are important for our subsequent development,” Eves told Cypost. “It also suggests that interventions that improve prenatal health outcomes may have many benefits, both in the short and long term.”
The researchers also examined whether this effect changed as the twins grew older. They analyzed a small group of twins who completed cognitive tests at both Wave 1 and Wave 2. In this small group, the association between birth weight and cognitive scores was no longer statistically significant. The authors noted that this lack of significance may simply be due to the decreasing number of participants available for longitudinal analysis.
When the scientists looked at head circumference, they found no significant association with later cognitive ability at any age. Differences in head size between twins did not predict who would score higher on intelligence tests. The researchers noted that measuring head circumference at birth as an estimated indicator of brain growth is routinely performed in pediatric practice. However, based on these results, typical birth weight may be a more reliable indicator for predicting long-term cognitive outcomes.
The scientists also checked for complex mathematical relationships in the data. They found that the effect of birth weight differences was highly dependent on the twins’ overall size. If both twins are born with relatively high birth weights, differences between the twins will be less important for subsequent cognitive scores. The cognitive benefits of being a heavier twin appear to diminish once a certain healthy weight threshold is reached.
There was also evidence that differences in birth weight may have a greater effect in twins born early in pregnancy. When researchers looked at longitudinal data, they found an interaction between birth weight and gestational age. When twins were born very preterm, differences in birth weight tended to predict significantly larger differences in subsequent cognitive performance. If a baby is born small for gestational age, the risk of having a baby born prematurely also increases.
Although this study included a large number of participants, there are some potential misconceptions and limitations to consider. One of the major problems was missing data. Approximately 60 percent of participants did not have head circumference recorded at birth in available medical records. This significant lack of information reduces the statistical power of our analysis and makes it difficult to draw firm conclusions about how early head size relates to brain development.
Additionally, the study lacked detailed information about the specific nature of the pregnancy. For example, researchers did not know whether identical twins shared a single placenta or had separate placentas. Twins sharing one placenta often have unequal blood flow, which directly causes birth weight discrepancies. This specific medical detail can help researchers understand exactly why one twin grew larger than the other.
It is also important to remember that twins usually weigh less than a single baby born at the exact same stage of pregnancy. Because of this natural size difference, it is difficult to know whether the results apply to singletons in exactly the same way. Single babies often have much larger birth weights, which may result in different cognitive associations than those seen in twin populations.
Future research may focus on collecting more complete medical records at birth. Tracking the precise placement of the placenta and consistently measuring head circumference can help scientists better understand these early developmental pathways. By pooling data from multiple twin studies from different countries, researchers can build even larger datasets.
A larger sample would provide the necessary mathematical power to ascertain whether the cognitive benefits of higher birth weight eventually fade after adulthood. Continued research in this area will help physicians identify which infants are most likely to benefit from early educational support and targeted health interventions. Improving prenatal conditions can have lifelong cognitive benefits for vulnerable infants.
The study, “Anthropometric differences between twins at birth and their association with later cognitive performance,” was authored by Robert Eves, Marco Deppe, Christian Kandler, Bastian Menkedik, and Sakari Remola.
