She was a body double.
You can even read in the end credits: "body double for Jennifer Connelly", and it may be mentionned in some books.
Jennifer Connelly had only eleven when she started filming, she had her 12th birthday on the set. She hadn't kissed a boy before and she couldn't stop crying (I read that in an interview of her). So SL saw he couldn't shoot the scene and he said" Well, maybe we could do that tomorrow.)
And all these scenes where shot in Rome.
mmm. funny anecdote. thanks.