A teenager says he was drunk and upset when he attempted to run off with his former girlfriend's baby at Hawke's Bay Regional Hospital.

The 17-year-old went to see his ex-girlfriend when she was at hospital last month with her sick toddler and two-month-old baby.

The pair argued and the drunk teen became aggressive, assaulting the woman and a nurse, who tried to stop him from taking the baby away.

When security guards intervened, he kicked in a window of the children's ward.

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The teenager yesterday pleaded guilty to charges of assault on a female, common assault and wilful damage.

He'll be sentenced on Friday.