Do cloth nappies delay a baby's developmental milestones like walking? It's the comparison in 'bulk' between disposable nappies and cloth nappies that usually raises this question.
Cloth nappies are definitely larger - they have to be as they don't have super absorbent polymer crystals. But do cloth nappies cause developmental problems or even a delay because they're larger? And if they do, will rolling, crawling or walking a little later than a child might have otherwise cause ongoing issues?
For experienced cloth nappy users this question can trigger a rolling of the eyeballs and a reminder that cloth nappies have been worn for generations with no obvious detrimental effects and children will sit, stand, walk when they're ready.
But as parents we are naturally concerned about choices that may negatively impact on our children, so it's a valid question.
In my personal experience, my three children wore cloth nappies and they evidenced no delay at all - they rolled when they should, were all crawling at 6 or so months, and were walking at 13 months, 9 months, and 10 months. No real delays there (in fact a delay would have been welcomed for the younger two!). My cloth nappied nieces were walking at 13 and 14 months, which is very average.
Has there ever been a study that compares the development of children who wear disposables with children who wear cloth nappies? Kind of...
There was a study undertaken in 2012, 'Go Naked: Diaper Affect Infant Walking,' that looked at how nappies impacted on the mobility of children and they "compared infants’ walking while naked, wearing a modern, thin disposable diaper, and wearing an old-fashioned, thick, cloth diaper." It's actually quite an interesting article, but not actually very helpful in answering the question as it only examines how children walk wearing different nappies, not the development of the skills themselves.
This study had some problems, too, including this statement "Infants today walk sooner and better than those of previous generations, when all infants wore cloth diapers" that implies disposable nappies have facilitated walking rather than things like changes in parenting, higher birth weights, or more awareness of childhood development. Oh, and the study was partially funded by Proctor & Gamble, the manufacturer of the largest market share of disposables in the US.
The basic finding was that ALL nappies (disposable and cloth) impacted a child's walking compared to being naked, and that more children stumbled while wearing cloth nappies than disposables. While in a cloth nappy, the children's feet were further apart when walking than in a disposable, which were further apart than when the child was walking naked.
The study examined 30 13 month olds and 30 19 month olds (so a very small sample size, plus 'fussiness' from some participants meant a reduction in data) over 90% of whom habitually wore disposable nappies!!! While the authors stated they didn't believe that the 'novelty' of wearing cloth nappies would influence the result, I think that's stretching it, personally. A cloth nappy would feel very different to a child used to disposables, especially when they use a cloth nappy that's "folded for nighttime use, making the crotch 12.3 cm across and 2.8 cm thick." (The disposable was 7.5 cm across by 1.1 cm thick at the crotch.) Yep, they used the NIGHT version of a traditional flat or prefold cloth nappy (they don't state which) optimised for absorbency - not trimness or ease of walking - when they were testing. It's possible that a trimmer cloth nappy would have shown a more similar impact to the disposable nappies, but we won't know.
Another interesting point is that they only tested the nappies while they were dry - they were changed as soon as they were wet. I'm not sure why they didn't add a 'wet' test to the study (possibly those 'fussy' participants again!), since surely that would have an increased effect on the children's gaits: being wet would increase the weight of the nappy, and in the case of the disposable nappy, the size as it expands. Especially since (generally) disposable nappies are changed less often than cloth nappies due to their super absorbency.
The reality is that we don't know for sure whether choosing cloth nappies over disposables will make any difference to when a child learns to crawl or walk or sit up. And, if it does, whether a few weeks will even make any difference in the long run. We do know that nappy-free time is incredibly beneficial for babies' development, so no matter which nappy you use, nappy-free time should be a given - especially as this study indicates that any nappy can impact on how your child walks (though only while they're wearing it). And if you're particularly concerned about it, then perhaps explore Elimination Communication techniques so you don't need to use nappies at all.
Now a word about the hips...
Sometimes parents express concern that in cloth nappies their babies are not as easily able to straighten their legs as babies in disposables and worry that cloth nappies will affect their hip development. In fact, the best position for a baby's legs during the first few months is a flexed and abducted hip positioning - straight legs are not necessarily a good thing, and a reason why cloth nappies have been recommended for babies with hip dysplasia. It's exactly the reason why carriers that support a healthy hip position are recommended over 'crotch danglers.'
"The way infants are positioned in the womb and in the first few months of life determine whether the hip can form properly. It has long been known that breech positioned babies are more prone to hip dysplasia. Similarly, babies whose legs are forced straight either by swaddling or dangling also have an increased risk of hip dysplasia. Straightening the legs places a lever-like force on the femur, encouraging the hip to pop out of the socket."
Naturally, if you have concerns about your child's development, please see your doctor or maternal child health nurse.