Here's the general rule as it appears in a CDC report on how to complete a birth certificate in regards to place of birth:
If this birth occurred en route, that is, in a moving conveyance, enter the city, town, village, or location where the child was first removed from the conveyance.
If the birth occurred in international airspace or waters, enter plane or boat.
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If the birth occurred in international waters or airspace, enter the location where the infant was first removed from the boat or plane.
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Thanks for the link.