Assessing developmental milestones is a core part of any paediatric assessment, providing a valuable insight into the health and growth of a child. They can reassure us that a child is doing well, or prompt us to investigate further, and also fuel subtle competitive comparison between parents.
As I have checked off these milestones in my own small human over the past year on maternity leave, I’ve enjoyed watching their sequential progression unfold, just as it says in the textbooks. However, it has made me think about who wrote these rules and the influences on them.
History of milestones
Although babies have always developed into adults, the sequence has not always been formalised. The standardisation of developmental milestones occurred through the integration of observation, research and the creation of formal assessment tools over the 20th and early 21st centuries.
Early work in developmental psychology was largely observational. A key player was Arnold Gesell, whose research in the 1920s described development as the inevitable unfolding of events determined by internal genetic forces.
Jean Piaget was also very influential. He proposed that children learn through active interaction with their environment, rather than as passive recipients of environmental forces. He described a child’s development as a progression through four distinct stages, each one presenting an increasingly complex mental structure through which they understand the world. It is still a stepwise progression, but one that the child ‘actively climbs’.
As child health services expanded, there was a growing need for standardised assessment tools. Building on previous research, Ruth Griffiths published the Griffiths Mental Development Scale in 1954, which assessed multiple domains and provided normative comparisons.
Neligan and Prudham’s 1969 work on the Newcastle Thousand Families Study provided further population-based data on developmental norms and highlighted the influence of environmental and social factors. Other tools created include the Bayley Scales of Infant Development, the Alberta Infant Motor Scale, and the Denver Developmental Screening Test (later revised as DENVER II).
One of the major criticisms of these frameworks was that they were based predominantly on Western high-income populations. The World Health Organisation sought to address this through its Multicentre Growth Reference Study (MGRS), which examined healthy children across multiple countries and cultural contexts. Between 1997 and 2003, data on growth and the acquisition of key motor milestones were collected from approximately 8,500 children from Brazil, Ghana, India, Norway, Oman, and the USA. Their data on the age of milestone attainment were comparable to those in existing frameworks. Looking at variables, they found no significant difference between the sexes but did observe some variation between sites, likely reflecting culture-specific care behaviours. WHO pooled data across sites to develop ‘windows of achievement’ based on the 1st and 99th percentiles of age at milestone attainment, producing an international standard that reflects normal variation in healthy children worldwide.
Although developmental milestones are now standardised, they should continue to be calibrated.
It is well documented that average IQ scores have gradually increased over time, a phenomenon called the ‘Flynn effect’, often explained by improvements in factors such as nutrition, health and education across subsequent generations.
This has raised the question of whether similar generational changes might also be seen in developmental milestones. In 2024, Fuschlberger et al. compared the ages at which children reached the 50th percentile on a standardised milestone assessment tool between the 1970s and 2018. They found no significant difference between the groups with no meaningful shift toward earlier attainment. This suggests that, at least in this cohort and using these measures, there is relative stability in milestone attainment over time despite broader societal change.
Influencing factors
Whilst variation in development is normal, it is interesting to consider where this variation comes from and what it can tell us.
One of the main reasons we assess milestones is that we know if a child develops outside of the typical range, there is an increased risk of a developmental disorder. However, research suggests that even within the normal range, the timing of childhood milestone attainment can predict outcomes in adulthood. For example, earlier attainment of developmental milestones has been associated with higher intelligence and cognitive function later in life, whereas later motor development has been linked to increased anxiety and depressive symptoms, as well as psychiatric disorders, particularly schizophrenia and alcohol use disorders.
Flensborg-Madsen et al. examined a range of pre- and postnatal factors and their influence on milestone attainment. They found that during the first year of life, gestational age and birth weight accounted for much of the observed variation. In the second and third years, other factors became more relevant, including parental socio-economic status, paternal age, and sex. However, the strongest predictor of milestone attainment at this stage was the rate of development in the first year of life, supporting the concept of developmental continuity, in which early development predicts later achievements.
Overall, their model explained 16.2% of the variance in milestone timing, highlighting the influence of family background, pregnancy and delivery, postnatal growth, and prior development. The remaining 83.8%, however, reminds us just how much we still have to learn about the complex, innumerable ways in which genetics and environment interact to shape a child’s development.
My observations
I collected data from some friends with babies born around the same time as mine to find our own ‘windows of achievement’. Although not the most heterogeneous group, there is a mix of sexes and ethnicities. As none of our babies has walked yet, and some have not stood alone, these milestones haven’t been included in the table.
I instead asked about rolling (not one of the WHO windows), which was compared to the Denver II scale. Our impressive sample size of four may limit the significance of this study, but it is reassuring to see that we fit within other established frameworks. I didn’t ask about birth weight, head circumference or socio-economic status, so we can’t draw any conclusions about influencing factors, though it would be interesting to compare their attainment with their GCSE results in 15 years’ time!
| Milestone | WHO windows of achievement (1st – 99th percentiles) | WHO mean | Denver II (25th – 90th percentile range) | Our ‘windows of achievement’ (range) | Our mean |
| Rolling | (not included in WHO MGRS) | 3 – 5 months | 3.5 – 5 months | 4 | |
| Sitting without support | 3.8 – 9.2 | 6 | 4.5-8.5 | 6.1 | |
| Standing with assistance | 4.8 – 11.4 | 7.6 | 6 – 8 | 7.2 | |
| Crawling | 5.2 – 13.5 | 8.5 | 6.5 – 8 | 7.3 | |
| Walking with assistance | 5.9 – 13.7 | 9.2 | 8 – 10 | 9 | |
Conclusion
Overall, developmental milestones provide a useful framework for understanding early childhood development, offering reassurance when children follow expected patterns and prompting further assessment when they do not.
Our understanding of milestones has been refined over the past century as we have learned more about their sequence, and continues to evolve as we gain insight into the influences that shape them. I find it both remarkable and reassuring how consistent the broad pattern of development is across generations, sexes, ethnicities and backgrounds. The interest, however, lies in the individual variation and nuances, where we still have much to learn.
References
Thomas A, Lewis A. Child development: From birth to 8 years. An interdisciplinary approach. London: SAGE Publications; 2023.
Radesky J, Kistin C. Encounters with children: Pediatric behavior and development. 5th ed. Philadelphia: Elsevier; 2026.
World Health Organization Multicentre Growth Reference Study Group. WHO motor development study: windows of achievement for six gross motor development milestones. Acta Paediatr Suppl. 2006;95(S450):86–95.
Fuschlberger T, Leitz E, Voigt F, Esser G, Schmid RG. Stability of developmental milestones: insights from a 44-year analysis. Infant Behav Dev. 2024;74:101898. doi:10.1016/j.infbeh.2023.101898
Flensborg-Madsen T, Mortensen EL. Predictors of motor developmental milestones during the first year of life. Eur J Pediatr. 2017;176(1):109–119. doi:10.1007/s00431-016-2817-4
Flensborg-Madsen T, Grønkjær M, Mortensen EL. Predictors of early life milestones: results from the Copenhagen Perinatal Cohort. BMC Pediatr. 2019;19(1):420. doi:10.1186/s12887-019-1778-y
