The research, which tracked nearly 2,500 two-year-olds, is the latest piece of evidence in the debate about how much screen time is safe for kids.
In Canada and the US, experts say children should not use screens before they are at least 18 months old.
But UK guidelines set no such limit.
The Royal College of Paediatrics and Child Health says there is not enough evidence, even when you include this new study, for a "direct toxic effect".
Mums were surveyed (between 2011 and 2016) about screen use, and filled out questionnaires about their child's skills and development at ages two, three and five.
Screen time included watching TV programmes, films or videos, gaming, and using a computer, tablet, phone or any other screen-based device.
At the age of two, the children were clocking up around 17 hours of screen time per week.
This increased to around 25 hours a week by the age of three but dropped to around 11 hours a week at the age of five, when the children started primary school.
The findings, published in the JAMA Paediatrics, suggest increased viewing begins before any delay in development can be seen, rather than children with poor developmental performance then going on to have more screen time.
But it is not clear whether screen time - including how much or what type - is directly to blame. Screen use might just go hand-in-hand with other things linked to delayed development, such as upbringing and how a child's remaining leisure time is spent.
What do the researchers think ?
When young children are observing screens, they may be missing important opportunities to practise and master other important skills.
In theory, it could get in the way of social interactions and may limit how much time young children spend running, climbing and practising other physical skills - although they may still eventually catch up.
Even without solid proof of harm, Dr Sheri Madigan and colleagues say it still makes sense to moderate children's screen time and make sure it doesn't interfere with "face-to-face interactions or family time".
They also said that, with hindsight, perhaps they should have followed the children from an even younger age because it is becoming increasingly common for 12-month-old babies to be watching and using screens.
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