The film makes it very clear that the expanding supernova was stopped, and thus the TNG timeframe where Spock Prime came from was saved. And also both the bridge crew and Nero ("You were regarded as a great man - you became Captain of the Enterprise, but that was another life!") pound in the fact that it's an alternate timeline due to the changes made by Nero's arrival in the past.
Any indication of how far along Winona Kirk is in the pregnancy at the time of Nero's attack? If she was in the latter stages, and James was born in Iowa in the pre-Nero timeline, then would that imply that the Kelvin was due to return to Earth shortly at this time, to give Winona the opportunity to have her baby on Earth?
She seems to be giving birth before the Narada attacks the Kelvin, at least when we first see her. There's nothing to indicate the attack disrupted the normal birth cycle as such.