These diagnoses allow families with the same genetic conditions to connect and access support, and help inform better clinical management.
The study led by researchers at the Wellcome Trust Sanger Institute in the UK also accelerates research into disease mechanisms and possible therapies.
Each year, thousands of babies are born who do not develop normally because of errors in their genetic makeup. This can lead to conditions such as intellectual disability, epilepsy, autism or heart defects.
There are over 1,000 recognised genetic causes, however many individual developmental disorders are so rare that the genetic causes are not known.
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Researchers screened all 20,000 human genes from more than 4,000 families, from across the UK and Republic of Ireland, with at least one child affected by a developmental disorder.
The team focused on spontaneous new mutations that arise as DNA is passed on from parents to children.
The children's conditions were also clinically assessed and the team combined the results to match up children with similar disorders to provide diagnoses.
The study team was able to diagnose children who had new mutations in genes already linked to developmental disorders - approximately one quarter of the patients in the study.
"Each of these disorders is incredibly rare, so the large number of patients in this study was crucial to diagnosis," said Jeremy McRae from the Wellcome Trust Sanger Institute.
"An individual doctor may see only one case, but by collaborating with hundreds of NHS staff and researchers we were able to link children from clinics across the British Isles," said McRae.
"This allowed the team to match up children with similar disorders within the project and provide diagnoses for them," McRae added.
"This can help doctors better manage the child's condition, and gives clues for further research into future therapeutics," said FitzPatrick.
"A diagnosis can let parents know what the future holds for their child and the risk of any subsequent pregnancies being affected with the same disorder, which can be an enormous help if they want a larger family," he added.
The study was published in the journal Nature.