Our 9-month-old baby girl was born full-
For reference only. Not medical advice.
Question
Our 9-month-old baby girl was born full-term. She rolls both ways easily, reaches for toys, and babbles 'ba-ba' and 'da-da', but she cannot sit without support yet. When we place her in a sitting position on the playmat, she slumps forward or flops onto her side after a second or two unless propped up with pillows.
Our in-laws keep telling us that babies must sit independently by 6 months and that something is wrong.
Can you read this against the CDC 9-month milestone checklist and WHO motor development windows, explain what these windows actually mean, and give us a dated next step and checklist to bring to her upcoming 9-month pediatrician visit?
Answer
Not the act-now lane — no seizure, no breathing trouble, no lost skills, no blue/gray colour. She is full-term and 9 months, so we use the CDC 9-month card straight, no corrected age.
What the 9-month card says vs. what she does
| 9-month CDC item | She |
|---|---|
| Makes sounds like "mamamama" / "bababababa" | ✓ babbles ba-ba, da-da |
| eaches / moves things between hands | ✓ reaches for toys |
| olls both ways | ✓ both ways |
| Gets to a sitting position by herself | ? (you only described being placed) |
| Sits without support | ✗ slumps forward or flops after a second or two |
Source: https://www.cdc.gov/act-early/milestones/9-months.html (accessed 2026-09-02)
"Sits without support" is on the 9-month card, and she is not there yet. One missed item on the matching card is not a diagnosis — but it is the kind of thing CDC and AAP say you raise with her doctor, not something to wait out.
What the WHO window actually means
For sitting without support, the WHO window is 3.8–9.2 months (1st–99th percentile of healthy children). Source: https://www.who.int/tools/child-growth-standards/standards/motor-development-milestones (accessed 2026-09-02).
That band is population variation, not your daughter's percentile and not a pass/fail line. She sits at the very upper end of that band, not past it. The band also overlaps the other motor windows — children do not hit the six gross-motor skills in lockstep. None of that turns "doesn't sit yet" into "nothing to ask about".
About the in-laws' "must sit by 6 months"
That is not what CDC or WHO say. The CDC 6-month card lists *"leans on hands to support herself when sitting"* — that is propped sitting, not independent sitting. "Sits without support" first appears on the 9-month card, and the WHO band reaches 9.2 months. So 6 months is a folk line, not a clinical one.
Today's two rows
| When | What |
|---|---|
| Today, 2026-09-17 | Write down: "9 months, full-term. Does not sit without support — slumps or flops after ~2 s when placed. olls both ways, reaches, babbles ba-ba/da-da." Note which items she can and cannot do on the 9-month card. |
| At her upcoming 9-month visit | Ask the doctor for a formal developmental screening, not just a look in the room. A screening is a standardised tool, separate from the CDC checklist. Source: https://www.cdc.gov/act-early/about/developmental-monitoring-and-screening.html (accessed 2026-09-02) |
A visit already on the calendar is where that request gets made — not a reason to wait for a later one.
Visit checklist to bring
Dated, from the CDC "share with the doctor" prompts:
- Your concern, in one line. "She does not sit without support at 9 months."
- What she used to do / what she does now. olls both ways, reaches, babbles ba-ba/da-da; placed in sitting → slumps or flops after ~2 s.
- Lost any skills? No. (If yes at any point, tell them the same day.)
- Born prematurely? No, full-term.
- Hearing: does she turn when you call her name in a quiet room? date.
- Language: sounds/word-attempts in every language used at home; date.
- Gestures: pointing, showing, waving — yes/no, date.
- Eye contact / showing: does she bring a toy to show you or look to check you are close? date.
- Motor: sitting — yes/no, new stiffness or floppiness — yes/no, uses both sides — yes/no, date.
- equest: formal developmental screening at this visit, and a referral if anything is still unclear afterwards.
Source: https://www.cdc.gov/act-early/milestones/9-months.html (accessed 2026-09-02) · https://www.cdc.gov/act-early/about/developmental-monitoring-and-screening.html (accessed 2026-09-02)
If you want, I can also write this as a one-page PDF to hand to the doctor — say the word.
Does she ever get into a sitting position by herself (rolling or pushing up from tummy), or only ever end up there when you place her?