How can you recognize delayed or abnormal development in an infant?
Applies to age: 0–18 months
In short
In the WHO study, sitting without support appeared between 3.8 and 9.2 months, and walking alone between 8.2 and 17.6 months (1st–99th percentile of healthy children). However, not sitting at 8 months or not walking at 18 months is sometimes listed among the possible symptoms of cerebral palsy (this is not a diagnosis), so it is worth discussing with a specialist.
What is normal
All claims: international norm
- In the WHO study (Multicentre Growth Reference Study), the 1st and 99th percentiles of the age at which the skill "sitting without support" was achieved were 3.8 and 9.2 months, respectively.
Source: WHO – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
The estimated 1st and 99th percentiles in months are: 3.8, 9.2 (sitting without support)
WHO · open the source · accessed: 2026-10-12 · license [1]
- In the same WHO study, the 1st and 99th percentiles of the age of walking alone were 8.2 and 17.6 months, respectively.
Source: WHO – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
The estimated 1st and 99th percentiles in months are: 3.8, 9.2 (sitting without support), 4.8, 11.4 (standing with assistance), 5.2, 13.5 (hands-and-knees crawling), 5.9, 13.7 (walking with assistance), 6.9, 16.9 (standing alone) and 8.2, 17.6 (walking alone).
WHO · open the source · accessed: 2026-10-12 · license [1]
- The authors of the WHO study write that the age windows they set reflect normal variation in the age at which healthy children reach milestones.
Source: WHO – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
The windows represent normal variation in ages of milestone achievement among healthy children.
WHO · open the source · accessed: 2026-10-12 · license [1]
- In the WHO study, about 90% of children reached five of the six milestones in the usual order, and 4.3% of children did not crawl on hands and knees.
Source: WHO – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
About 90% of children achieved five of the milestones following a common sequence, and 4.3% did not exhibit hands-and-knees crawling.
WHO · open the source · accessed: 2026-10-12 · license [1]
- The WHO data come from a longitudinal study of children aged 4 to 24 months in Ghana, India, Norway, Oman and the USA (not from Poland).
Source: WHO – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
As part of the WHO Multicentre Growth Reference Study, longitudinal data were collected to describe the attainment of six gross motor milestones by children aged 4 to 24 mo in Ghana, India, Norway, Oman and the USA.
WHO · open the source · accessed: 2026-10-12 · license [1]
When to worry
All claims: international norm
- The WHO age windows are recommended, among other things, to signal the need for appropriate screening when individual children appear to be late in reaching milestones.
Source: WHO – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
They are recommended for descriptive comparisons among populations, to signal the need for appropriate screening when individual children appear to be late in achieving the milestones, and to raise awareness about the importance of overall development in child health.
WHO · open the source · accessed: 2026-10-12 · license [1]
- Delays in reaching developmental milestones, for example not sitting at 8 months or not walking at 18 months, are sometimes listed among the possible symptoms of cerebral palsy. This is a possible symptom, not a diagnosis.
Source: NHS – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
delays in reaching development milestones – for example, not sitting by 8 months or not walking by 18 months
NHS · open the source · accessed: 2026-10-12 · license [2]
- Among the possible symptoms of cerebral palsy is also the impression that a child is too stiff or too floppy.
Source: NHS – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
seeming too stiff or too floppy
NHS · open the source · accessed: 2026-10-12 · license [2]
- Weak arms or legs are also among the possible symptoms of cerebral palsy.
Source: NHS – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
weak arms or legs
NHS · open the source · accessed: 2026-10-12 · license [2]
- Symptoms similar to those of cerebral palsy can have different causes and do not necessarily point to anything serious.
Source: NHS – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
Symptoms like those of cerebral palsy can have a number of different causes and are not necessarily a sign of anything serious.
NHS · open the source · accessed: 2026-10-12 · license [2]
Worth discussing with a doctor:
- Any concerns about a child's health or development are worth discussing with a doctor.
Source: NHS – show the original wording (EN)
Summarized and translated by the editors, not an official translation. Verbatim quote from the source, in its original wording:
Speak to your health visitor or a GP if you have any concerns about your child's health or development.
NHS · open the source · accessed: 2026-10-12 · license [2]
See a doctor
- Polish care pathway: the Ministry of Health (MZ) regulation on guaranteed services in primary care (POZ) (consolidated text, the Journal of Laws (Dz.U.) 2023 item 1427, annex 1 part III, entries 3 and 4) provides, at 9 and at 12 months of life, a medical examination that takes the pace of psychomotor development into account. These are the dates of the well-child checkup and an indication of who assesses development, not age ranges for skills.
Source: Akty prawne w Dzienniku Ustaw – show the quote
Verbatim quote from the source:
Polish original
Badanie lekarskie podmiotowe i przedmiotowe, z uwzględnieniem tempa rozwoju fizycznego (pomiary: masy i długości ciała, obwodu głowy i klatki piersiowej) i psychomotorycznego.
Akty prawne w Dzienniku Ustaw · open the source · accessed: 2026-10-12 · license [3]
- The Patient Portal (Portal Pacjenta) from the Ministry of Health (MZ) and the National Health Fund (NFZ) describes the child's well-child checkup as an examination that makes it possible to check whether the child is developing properly and to detect developmental defects and diseases early.
Source: Portal Pacjenta – show the quote
Verbatim quote from the source:
Polish original
Pozwala sprawdzić, czy Twoje dziecko rozwija się prawidłowo oraz wcześnie wykryć wady rozwojowe i choroby
Portal Pacjenta · open the source · accessed: 2026-10-12 · license [4]
- According to the Patient Portal (Portal Pacjenta), the child's well-child checkup is carried out by a family doctor, a pediatrician or a nurse.
Source: Portal Pacjenta – show the quote
Verbatim quote from the source:
Polish original
Badanie przeprowadza lekarz rodzinny, pediatra lub pielęgniarka.
Portal Pacjenta · open the source · accessed: 2026-10-12 · license [4]
- The Patient Portal (Portal Pacjenta) from the Ministry of Health (MZ) and the National Health Fund (NFZ) says that if a child develops differently from peers, it is worth discussing with a specialist whatever is worrying you, and using the advice of a midwife, a nurse or a pediatrician. The source does not link this consultation to the date of the well-child checkup.
Source: Portal Pacjenta – show the quote
Verbatim quote from the source:
Polish original
Warto jednak skonsultować ze specjalistą to, co Cię zaniepokoiło i korzystać z rad położnej, pielęgniarki czy pediatry.
Portal Pacjenta · open the source · accessed: 2026-10-12 · license [4]
international norm “International norm” next to a section heading means its claims come from a non-Polish source, because no Polish source in our registry covers this topic. When this applies to only some of the claims, the foreign ones carry the same label next to their source.
Sources
- WHO (who.int)License: CC BY-NC-SA 3.0 IGO. Source: WHO (who.int). Content summarized and translated by the editors; changed relative to the original. WHO does not endorse this site or its content.
- NHS (nhs.uk, Anglia)License: Open Government Licence v3.0. Source: NHS (nhs.uk, Anglia). Content summarized and translated by the editors; changed relative to the original. Contains public sector information licensed under the Open Government Licence v3.0. The summary is ours and is not attributed to the NHS.
- Akty prawne w Dzienniku Ustaw (ELI: eli.gov.pl, api.sejm.gov.pl)License: Akt normatywny, art. 4 pkt 1 ustawy o prawie autorskim (nie podlega ochronie). Source: Akty prawne w Dzienniku Ustaw (ELI: eli.gov.pl, api.sejm.gov.pl).
- Portal Pacjenta (pacjent.gov.pl)
- https://pacjent.gov.pl/zapobiegaj/zaprowadz-dziecko-na-bilans-zdrowia
- https://pacjent.gov.pl/zapobiegaj/jak-dbac-o-zdrowy-rozwoj-psychiczny-dziecka
License: CC BY 3.0 PL. Source: Portal Pacjenta (pacjent.gov.pl). Content summarized and translated by the editors; changed relative to the original.