When does the 3rd Trimester Start?
Most healthcare pros use the 12 weeks and 28 weeks as the shift from first->second->third trimesters, but Jess707 is right, it's not an exact science. You can go by whatever you want in conversation, because none of that shifting stuff really matters in the end; it's the weeks that are really critical to most professionals.
I've seen it as:
27-30 weeks - 7 months
31-35 - 8 months
36-Birth - 9 months
Some say you're not 9 months until your due date, though you're IN you 9th month at 36 weeks. Maybe it's just semantics.