Is your baby developing normally?

The WHO Motor Development Study (n=816) found that the normal window for independent walking spans from 8.2 months (1st percentile) to 17.6 months (99th percentile), with a median of 12.1 months. A baby walking at 15 months is well within the normal range. Not walking by 18 months is the CDC red flag threshold. Walking age has no correlation with future athletic ability or intelligence.

Yes. Developmental milestones should be assessed against corrected age for premature babies until at least 24 months. A baby born at 32 weeks (8 weeks early) who is now 4 months old chronologically has a corrected age of approximately 2 months. This calculator applies corrected age automatically when you enter premature birth details.

The social smile, meaning a smile in response to a face or voice rather than a reflexive grimace, typically appears between 6 and 8 weeks of age. The CDC (2022 revision) lists social smiling as expected by 2 months, and the absence of social smiling by 3 months is considered a red flag warranting discussion with a paediatrician. Social smiling is one of the earliest and most important milestones because it indicates the baby is beginning to recognise faces and engage in reciprocal social interaction.

No. Crawling is not considered a required developmental milestone. The CDC removed crawling from its milestone checklists in the 2022 revision because a significant minority of typically developing babies never crawl in the traditional hands-and-knees pattern. Some babies bum-shuffle, army-crawl, roll, or go directly from sitting to pulling up and cruising. What matters is that the baby is finding a way to move independently and progressing toward standing and walking.

Most babies produce their first recognisable words between 10 and 14 months, with the CDC listing first words as expected by 12 months. Before words come, babies typically babble with consonant sounds starting around 6 to 9 months. By 18 months, most children have 5 to 20 words. By 24 months, most have 50 or more words and are combining two words into phrases. The absence of any words by 15 months or no two-word phrases by 24 months are the thresholds that trigger speech and language referral.

Most babies roll from front to back before back to front. Front-to-back rolling typically occurs between 3 and 5 months, while back-to-front rolling follows at around 4 to 6 months. The normal developmental window is wider than many parents expect: some babies roll early (as early as 2 months) while others do not roll until 6 to 7 months. What matters is that rolling appears within the expected developmental window and that the baby has adequate tummy time to develop the neck and core strength needed for rolling.

Most babies sit without support between 4 and 7 months. The CDC milestone for sitting without support is typically listed at around 6 months, but the normal range spans from approximately 4 to 9 months. Sitting requires core strength and balance that develops progressively: babies first hold a tripod position (leaning forward on hands), then sit upright briefly, then sustain sitting with increasing stability. Not sitting independently by 9 months is a threshold that warrants developmental assessment.

Differences are modest and largely irrelevant at the individual level. On average, girls tend to develop language skills slightly earlier than boys, and boys may show slightly earlier gross motor development in some studies. However, the within-group variation (between different children of the same sex) is far larger than the between-group difference, meaning a boy developing language early and a girl developing it late both represent normal human variation. The CDC 2022 milestone checklists are not sex-specific, reflecting the clinical judgement that the differences are not large enough to warrant separate thresholds.