Born Early, Grown Up: How Babies Born Too Soon Do on a Thinking-Skills Test at Age Twenty

A plain-language guide to a Finnish study of nearly 193,000 young men, linking birth records with a test taken during military service

A study of nearly 193,000 young men in Finland found that those born very early or very small scored somewhat lower, on average, on a thinking-skills test taken during military service at about age 20. The differences were mostly in puzzles involving shapes and numbers rather than words, and men born only a few weeks early scored almost the same as those born on time.

If your baby was born early, you have probably wondered what life will look like years from now, long after the hospital stay is a memory. Will school be harder? Will your child grow into an adult who thinks and learns like everyone else? These questions are hard to answer, because most research follows children only until school age. This article explains what a large new study tells us about the grown-up years, and what it does not.

Why doctors have struggled to answer questions about adulthood

The study, led by Jesper Perälä and colleagues in Finland, looked at how being born early relates to thinking skills in a whole country's worth of young men [1]. A baby is called "preterm" when born before 37 weeks of pregnancy; babies born from 37 weeks onward are called "full term". Around the world, about 13.4 million babies were born preterm in 2020, roughly one in every ten births [2]. Being born too early is the leading cause of death in children under 5 years, yet the number of newborn deaths linked to prematurity worldwide fell by almost half between 1990 and 2019 [3].

As more babies survived, doctors began to ask how they fared as they grew. Two leading experts summed up what was known: the earlier a baby is born, the higher the chance of later problems, but no group is completely free of risk, some difficulties with learning or behaviour only show up at school age, and yet most children born very early grow into adults who manage well [4]. Studies of adults born very early (before 32 weeks) or very small (under 1500 g, about 3.3 pounds) found that, on average, they scored lower on intelligence tests in their mid-twenties than adults born on time [5]. A long-running study in southern Germany found that this gap, seen at age 6, was still there at age 26 [6]. Researchers combining four decades of studies also found that, even though more very preterm babies now survive, the gap in childhood test scores has not shrunk [7]. But these studies mostly involved a few hundred people born very early, and they said little about the much larger number of babies born just a few weeks early.

A test taken by most young men in Finland

Finland gave researchers an unusual chance to look at a whole population. Most young men there do compulsory military service, and during their first weeks everyone who serves sits the same written test of thinking skills, called the "b-test". A similar approach had been used before in Norway, where birth records were linked with test results from military service, and showed that scores tended to rise with longer pregnancies and larger birth size [8]. Finnish researchers had also used the b-test to study young men who had had head injuries as children [9].

In this study, the team, based at the University of Eastern Finland and Kuopio University Hospital, with colleagues from the Finnish Defense Forces and hospitals in Finland and France, used national records to find all 350,617 boys born in Finland from 1987 to 1998 [1]. They then matched each boy's birth details, such as how many weeks early he was born and how much he weighed, with his later test result. Some had died in childhood and many had not done military service, so 192,767 young men had a test score to study. Of these, 10,016 had been born preterm. They were usually tested at 19 or 20 years old.

The b-test has three parts: one with words, one with numbers, and one with shapes and patterns (called "visuospatial", meaning the ability to picture and work with shapes in your mind). Scores run on a 9-point scale where the average for everyone is about 5. It is important to know that this is not an IQ (intelligence quotient) test of the kind psychologists use, so its points cannot be turned into IQ points. The researchers also looked at whether a baby was "small for gestational age", meaning much smaller than expected for the number of weeks of pregnancy, using Finnish growth charts [10].

What the researchers found

The overall pattern was a gentle staircase. The earlier a baby had been born, the lower the average score at about age 20, and the difference faded as pregnancy length increased, until it had essentially disappeared at about 36 to 38 weeks [1].

To put real numbers on it: young men born on time scored an average of 4.90 on the 9-point scale. Those born before 28 weeks (called "extremely preterm") scored an average of 4.12, which is about three-quarters of a point lower. Those born at 28 to 31 weeks scored about a third of a point lower than men born on time, and those born at 32 to 33 weeks scored only about a tenth of a point lower. Men born at 34 to 36 weeks (called "late preterm"), who make up by far the largest group of preterm babies, scored almost exactly the same as men born on time. Birth weight showed the same pattern: men who had weighed less than 1000 g at birth (about 2.2 pounds) scored just under a point lower than men born weighing 2500 g or more.

Two other findings stood out. First, the differences were mainly in the shapes-and-patterns and number sections of the test. In the word section, men born early did about as well as everyone else. Second, being small for the length of pregnancy was linked to somewhat lower scores, about a third of a point overall and, unlike prematurity, also in the word section. This disadvantage was seen at every stage of pregnancy and was most noticeable among babies who were also born very early.

How to make sense of these numbers

It helps to remember that these are averages for large groups. Within every group, scores varied widely from person to person, so the scores of men born extremely early and men born on time overlapped a great deal. The researchers describe the gap after extremely preterm birth as small to medium in size. As one way to picture it, the Finnish army asks for a score of 4 to train as a non-commissioned officer (a junior leader) and 5 for officer school; the average for men born extremely early was above the first of these marks.

There are also good reasons to read the results carefully. Only about 1 in 7 of the boys born extremely early in those years ended up in this study (151 of 1019), because some died as babies and others did not take part in military service, perhaps because of health problems. That means the extremely preterm men who were tested may have been the ones doing best, so the true difference for everyone born that early could be larger than the study shows; this is our interpretation, not a finding of the study. The researchers also could not take into account family circumstances such as parents' education or social and economic circumstances, which are known to matter a great deal for thinking skills in adults born preterm [6]. Studies of brothers in Norway have shown that family background can make birth-weight links look different from what they really are [11]. So this study shows a link, not proof that being born early directly causes lower scores. It included only men, all born in one wealthy country with strong health care and schools. If you are interested in how sibling comparisons help untangle these questions, see our article on how gestational age relates to school outcomes when siblings are compared.

What this means for families

If your baby was born at 34 weeks or later, this study is reassuring: on average, young men born late preterm performed almost the same as those born on time. Earlier Finnish research also found that, among young people in mainstream schooling, grades at age 16 differed only a little by length of pregnancy, and those small differences did not seem to lead to differences in education completed by age 25 [12].

If your baby was born much earlier, particularly before 28 weeks or weighing less than 1000 g, the study suggests that, on average, some differences in thinking skills may last into adulthood, mostly in working with shapes and numbers. That knowledge can be useful rather than frightening. Because word skills were the least affected area in this study, a child who talks and reads well may still need help with maths or spatial tasks, so it is worth paying attention to these areas at school and asking teachers or your child's follow-up team for support if difficulties appear. If your baby was born small for the length of pregnancy, mention this to doctors and teachers as your child grows.

Thinking-test scores are also only one part of a person's life. Research on adults born preterm looks at much more, such as relationships and everyday choices; one Finnish study, for example, found that young people born preterm tended to take fewer risks [13]. Our article on how adults born very preterm in Germany are doing in their thirties explores this wider picture.

What researchers are working on next

This study included only men, so researchers will need to find out whether the same pattern is seen in women. The young men in this study were born between 1987 and 1998, and today doctors care for even smaller and earlier babies, some born at just 22 or 23 weeks [14], so newer studies will need to follow these children into adulthood. Researchers will also need to compare brothers and sisters within the same family, and to include information about family circumstances, to understand how much of the difference comes from being born early and how much from life after birth. Finally, it would help to learn whether extra help with shapes, patterns and numbers during childhood could make a difference later on.

In short, this large Finnish study shows that the earlier and smaller a baby is born, the lower, on average, his thinking-test score tends to be at about age 20, with the differences concentrated in shapes and numbers and fading almost completely for babies born just a few weeks early.

References

  1. Perälä J, Möttönen J, De Luca D, Kuitunen I. Preterm Birth and Birth Weight and Cognitive Performance Among Males Eligible for Military Service. JAMA Netw Open. 2026;9(10):e2640763. doi:10.1001/jamanetworkopen.2026.40763 ↩
  2. Ohuma EO, Moller AB, Bradley E, et al. National, regional, and global estimates of preterm birth in 2020, with trends from 2010: a systematic analysis. Lancet. 2023;402(10409):1261-1271. doi:10.1016/S0140-6736(23)00878-400878-4) ↩
  3. Cao G, Liu J, Liu M. Global, Regional, and National Incidence and Mortality of Neonatal Preterm Birth, 1990-2019. JAMA Pediatr. 2022;176(8):787-796. doi:10.1001/jamapediatrics.2022.1622 ↩
  4. Saigal S, Doyle LW. An overview of mortality and sequelae of preterm birth from infancy to adulthood. Lancet. 2008;371(9608):261-269. doi:10.1016/S0140-6736(08)60136-160136-1) ↩
  5. Eves R, Mendonça M, Baumann N, et al. Association of Very Preterm Birth or Very Low Birth Weight With Intelligence in Adulthood: An Individual Participant Data Meta-analysis. JAMA Pediatr. 2021;175(8):e211058. doi:10.1001/jamapediatrics.2021.1058 ↩
  6. Eryigit Madzwamuse S, Baumann N, Jaekel J, Bartmann P, Wolke D. Neuro-cognitive performance of very preterm or very low birth weight adults at 26 years. J Child Psychol Psychiatry. 2015;56(8):857-864. doi:10.1111/jcpp.12358 ↩
  7. Behboodi S, Chaimani A, Benhammou V, et al. Trends Over Time in Cognitive Outcomes of Children Born Very Preterm: A Systematic Review and Meta-Analysis. JAMA Pediatr. 2025;179(10):1074-1081. doi:10.1001/jamapediatrics.2025.2221 ↩
  8. Eide MG, Oyen N, Skjaerven R, Bjerkedal T. Associations of birth size, gestational age, and adult size with intellectual performance: evidence from a cohort of Norwegian men. Pediatr Res. 2007;62(5):636-642. doi:10.1203/PDR.0b013e31815586e9 ↩
  9. Möttönen J, Ponkilainen VT, Mattila VM, Kuitunen I. Impact of childhood traumatic brain injury on fitness for service class, length of service period, and cognitive performance during military service in Finland from 1998 to 2018: A retrospective register-based nationwide cohort study. PLoS One. 2024;19(5):e0303851. doi:10.1371/journal.pone.0303851 ↩
  10. Sankilampi U, Hannila ML, Saari A, Gissler M, Dunkel L. New population-based references for birth weight, length, and head circumference in singletons and twins from 23 to 43 gestation weeks. Ann Med. 2013;45(5-6):446-454. doi:10.3109/07853890.2013.803739 ↩
  11. Kristensen P, Susser E, Irgens LM, Mehlum IS, Corbett K, Bjerkedal T. The association of high birth weight with intelligence in young adulthood: a cohort study of male siblings. Am J Epidemiol. 2014;180(9):876-884. doi:10.1093/aje/kwu241 ↩
  12. Alenius S, Kajantie E, Sund R, et al. School grades and educational attainments of adolescents and young adults born preterm. Sci Rep. 2023;13(1):231. doi:10.1038/s41598-022-27295-4 ↩
  13. Alenius S, Kajantie E, Sund R, et al. Risk-Taking Behavior of Adolescents and Young Adults Born Preterm. J Pediatr. 2023;253:135-143.e6. doi:10.1016/j.jpeds.2022.09.032 ↩
  14. Isayama T, Norman M, Kusuda S, et al. Outcomes of Preterm Infants Born at 22 to 23 Weeks' Gestation in 11 International Neonatal Networks. JAMA Pediatr. 2025;179(11):1183-1193. doi:10.1001/jamapediatrics.2025.2958 ↩