Female foetuses don't 'turn into' male foetuses.
Female foetuses are female from the moment of conception, when they reproduce their XX chromosomes into, first 2, then 4 cells and away they go.
Male foetuses are male from the moment of conception, when they reproduce their XY chromosomes into, first 2, then 4 cells and away THEY go.
The whole, everyone is a female until 'x' point in development is what Terry Pratchett calls 'lies for children'. It's an easy way of describing something, a shorthand, if you will.
And, before you leap on my post to tell me about people with different sex chromosome set ups, I know. But the XX XY binary is by far the most common, and so, for illustrative purposes, I used that.
I have female genitalia, I have no clue how to 'feel like a woman' most of the time, I feel like a bit of a loose cog. I reject cis as a label.
TERF is also a load of bollocks. It's not radfems deleting and silencing transpeople, it's not radfems killing Transpeople, it's certainly not radfems who make budgetary decisions about Transpeople or policy decisions. The people you want to tackle about inequalities are men. My personal opinion is that it's way easier for the type of Transactivist who tweets that women should die in fires and be raped don't actually understand very much about the world as it is, and are picking on soft targets, the low hanging fruit, if you will, because radfems are so easy to pick on.