Differences in birthweight between twins predict later intelligence test scores

A recent study suggests that a higher relative birthweight provides evidence for better cognitive performance in children and young adults, even when comparing twins who share the same genetic and environmental backgrounds. Published in the Journal of Child Psychology and Psychiatry, the research indicates that conditions in the womb can have long-lasting effects on how a person’s brain develops. These findings point to the importance of early prenatal growth for outcomes spanning the first three decades of life.

Past studies indicate that babies born with specific risk factors tend to have lower cognitive test scores later in life. Two common risk factors are a low birthweight and a smaller head circumference. A lower relative birthweight means a baby is smaller than expected for the specific number of weeks they spent developing in the womb. This early physical size is often used as a marker for how well a fetus was nourished during pregnancy.

“There are many studies comparing babies of differing birthweights or head circumferences with later developmental outcomes,” said study author Robert Eves, a postdoctoral researcher at Bielefeld University. “However, these studies usually compare babies from vastly different families who are different on lots of other important factors, such as genetics and family factors (e.g., poverty levels).”

Because family income, neighborhood safety, and parental education play a role in childhood brain development, comparing children from vastly different backgrounds makes it hard to isolate the exact cause of later cognitive differences. To get around this problem, scientists look at twins. Twins share the same environment in the womb and grow up in the same household. Identical twins also share the exact same genetic code.

“We therefore wanted to test whether birthweight and head circumference differences between twins are important for later cognitive performance,” Eves said. “This is interesting because differences in birthweight or head circumference between twins are thought to come about due to differences in how the placenta works for each baby, rather than genetic factors.”

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By looking at these specific differences between twins, researchers can more accurately see how conditions in the womb might independently shape long-term cognitive development. The authors wanted to test whether differences in birthweight and head circumference between twins actually predicted later differences in their intelligence test scores. They also looked at whether this relationship might fade as children grow older. Another goal was to see if head size might act 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 tracking the psychological and social development of same-sex twins over time. The specific sample for this analysis included 4,196 individuals, making up exactly 2,098 twin pairs at the first wave of testing. These pairs included both identical twins and fraternal twins, representing a wide variety of ages and backgrounds.

The scientists used official medical health records to find each twin’s exact birthweight in grams and head circumference in centimeters. They then adjusted these numbers based on the baby’s biological sex and exactly how many weeks they spent in the womb. To do this, they used an international growth chart that identifies the average size of a baby at every single week of gestation.

Standardizing the scores allows the authors to see if a baby was smaller or larger than average for their specific stage of development. Comparing a baby born at 32 weeks to one born at 40 weeks is not an equal comparison. Using relative scores helps the researchers measure the actual deviation from a baby’s natural growth potential.

Cognitive performance was measured using a standardized intelligence assessment called the Culture Fair Test. This specific assessment does not rely on language or mathematical skills. Instead, it asks participants to solve visual puzzles, classify shapes, and complete complex pattern matrices. The tests were given at two different time points separated by about six years.

The median age for the first wave of testing was 12 years old. The median age for the second wave of testing was 18 years old, though some participants were older. The researchers adjusted the test scores to account for the varying ages of the participants at the time they took the assessment.

The authors found that within twin pairs, differences in relative birthweight were associated with differences in cognitive performance during the first wave of testing. Specifically, the twin born with a higher relative birthweight tended to score slightly higher on the visual intelligence tests. This association was present for both identical and fraternal twins.

Regarding the study’s main takeaway, Eves said, “That being the smaller twin is associated with lower cognitive performance 12 years later. However, this was only found regarding birthweight but not for head circumference, which I did find surprising.”

Because the positive effect of a higher birthweight was seen even in genetically identical twins, it suggests that shared genetics and family environments do not completely explain the link between birthweight and cognition. The physical environment inside the womb appears to have an independent impact on long-term brain function.

“This provides evidence that conditions in the womb are important for our later development,” Eves told PsyPost. “It also suggests that interventions that improve prenatal health may have a plethora of both short-term and long-term benefits.”

The researchers also looked at whether this effect changed as the twins grew older. They analyzed a smaller group of twins who completed the cognitive tests at both the first and second waves. In this smaller group, the association between birthweight and cognitive scores was no longer statistically significant. The authors noted that this lack of significance might simply be due to the reduced number of participants available for the long-term analysis.

When looking at head circumference, the scientists did not find any significant association with later cognitive performance at any age. The difference in head size between twins did not predict who would score higher on the intelligence tests. The researchers noted that measuring head circumference at birth is routine in pediatric care as a presumed indicator of brain growth. However, based on these results, general birthweight might be a more reliable metric for predicting long-term cognitive outcomes.

The scientists also checked for complex mathematical relationships in the data. They found that the impact of birthweight differences depends heavily on the overall size of the twins. If both twins were born with a relatively high birthweight, the gap between them mattered much less for their later cognitive scores. The cognitive advantage of being the heavier twin seems to diminish once a certain healthy weight threshold is reached.

There was also evidence that birthweight differences might be more impactful for twins born earlier in a pregnancy. When looking at the longitudinal data, the researchers found an interaction between birthweight and gestational age. If the twins were born very prematurely, a difference in their birthweights tended to predict a noticeably larger gap in their later cognitive performance. Being born small for gestational age appears to carry a higher risk when the baby is also born prematurely.

While the study included a large number of participants, there are some potential misinterpretations and limitations to consider. One major issue was missing data. About 60 percent of the participants did not have their head circumference recorded at birth in the available medical records. This substantial lack of information reduces the statistical power of the analysis and makes it harder to draw firm conclusions about how early head size relates to brain development.

Additionally, the study did not have detailed information about the specific nature of the pregnancies. For example, the researchers did not know whether the identical twins shared a single placenta or had separate ones. Twins sharing a single placenta often experience unequal blood flow, which directly causes discrepancies in their birthweights. Having this specific medical detail would help researchers better understand exactly why one twin grew larger than the other.

It is also important to remember that twins generally weigh less than single babies born at the exact same stage of pregnancy. Because of this natural size difference, it is difficult to know if the findings apply exactly the same way to singletons. Single babies often reach much higher birthweights, which might result in different cognitive associations than those seen in twin populations.

Future research could focus on gathering more complete medical records at birth. Tracking exact placental arrangements and ensuring consistent measurements of head circumference would help scientists better understand these early developmental pathways. By pooling data from multiple twin studies across different countries, researchers could build even larger datasets.

Larger samples would provide the mathematical power needed to confirm whether the cognitive benefits of a higher birthweight eventually fade in later adulthood. Continued research in this area can help doctors identify which infants might benefit the most from early educational support or targeted health interventions. Promoting better prenatal conditions could offer 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 Mönkediek, and Sakari Lemola.