Well I wouldn't strictly say transition but put it this way 24 weeks is the earliest can be born and the earliest a you can know your pregnant is now forgive me if my biology is wrong but you can't tell if your pregnant until six days post conception.
As for the matter on when an embryo can have a capacity to feel pain that is also at 24 weeks and now the research gets a little bit iffy here because it is an emerging thing the fetus cannot consciously feel pain till 35 weeks forgive me if that research turns out to be wrong.
With all of that being said there is still alot of grey area here not everyone will agree and I think that's good you need opposing sides to scrutinise eachother it stops developing echo chambers and all of that.
But to answer your question I would say the transitional period occurs from 24-35 weeks.
(God it's wierd been three years since I took my biology course yet i remember most of this)
