Parents often wait anxiously for the “right week” to deliver, weighing advice from doctors and their own instincts. For decades, gestational age, whether a baby arrives at 37 weeks or closer to 41, has been the focus of those conversations.

But a sweeping new study led by the University of Warwick suggests that size at birth may carry far more weight for a child’s long-term development than the exact week of arrival.

Researchers tracked more than 92,000 term-born children from infancy to adolescence, examining early language and motor skills and later school performance in math, reading, and English. All were born between 37 and 41 weeks, the “term” period.

The results revealed two distinct stories:

Timing within term matters briefly: Babies born at 37–38 weeks showed slightly poorer language and movement skills in early childhood compared to those born at 39–41 weeks. But by age five, those differences had largely disappeared.

More babies were born on weekdays than on weekends or holidays

Children who were born smaller than average for their gestational age will continue to have lower academic performance in several areas of education through their teenage years. These children tend to score below the average on standardized tests of mathematics and reading; these performance gaps can persist until at least age 15.

Researcher Xuan Zhao reflected on the surprising contrast: “Differences linked to being born slightly earlier within the term period largely washed out by age five, while those linked to being smaller than expected were still apparent in school performance at 14–15 years of age.”

Professor Siobhan Quenby MBE emphasized the reassurance for many families: “If your baby is normal size or large for its stage in pregnancy, there are no long-term effects of being delivered a few weeks before your due date.”

“Not every baby born smaller than expected will experience difficulties. However, the study underlines the importance of monitoring a baby’s growth and ensuring that children receive appropriate support as they develop.”

Professor Dieter Wolke added a hopeful note: “For babies born at term, being small for their stage of pregnancy may indicate adverse effects on the brain that affect learning and last into adolescence. However, the long-term effects of being born small may be altered by parenting home stimulation and school support.”

Skin color and neurodevelopment are not linked

The results show that gestational age and fetal size are two different, non-substitutable characteristics. Being born a little earlier within the term period may briefly affect early milestones; fetal growth restriction has a more enduring impact on academic performance.

The fact that monitoring the development of a fetus is as vital to the parent and the clinician as determining the length of pregnancy (gestational age) suggests that early assistance to those born small-for-gestational-age may be able to lessen long-term problems.

Shared decision-making about when to deliver a baby must weigh both short-term risks to the newborn against the fact that the size of the fetus can have much larger long-term impacts.

Journal Reference:

Xuan Zhao, Siobhan Quenby, Ragnhild E. Brandlistuen et al. The relationship between gestational age at delivery and fetal size, and neurodevelopmental outcomes from infancy to adolescence. American Journal of Obstetrics & Gynecology. DOI: 10.1016/j.ajog.2026.08.025