The stress of the word утро seems to be irregular, since it depends on the absence or presence of a preposition.
According to some sources:
- the stress is always у́тр- without a preposition.
- the stress is always утр-́ after a preposition.
However, it's again утр-́ not necessarily after a preposition. Examples:
- A dictionary of mine cites "в семь часов утра́"
- Some books cite утро́м (or at least рано утро́м) to mean "in the morning".
So, what is the correct stress in each context?