You don't need "her": It was not until she lost consciousness that she realised she was having a miscarriage. - However, this sentence doesn't really make sense, because if she was unconscious, she couldn't "realise" anything...🤔 Perhaps you could say this: It was only after she had been unconscious that she realised she had had a miscarriage.

It was not until she lost her consciousness that she never thought she was having a miscarriage. There are 2 changes that need to happen in this sentence. 1 minor and 1 major. minor - 'her' is unnecessary, because we can't lose someone ELSE's unconsciousness! major - the word 'never' should not be in this sentence "It was not until she X" == "It was only when she X" this introduces the time of some event She realized she was having a miscarriage when she lost consciousness. 0 thought before that time. But she thinks that when she loses consciousness. If you have the word 'never', it conflicts with the phrase "it was not until", because they are opposing ideas, and you can't introduce a specific time....that never happened! So this sentence should be "It was not until she lost consciousness that she thought she was having a miscarriage. Or something like "...that she realized (that) she might be having..." "...that it occurred to her (that) she might be having..." If you want to use the word NEVER... It never occurred to her that she was having /// might be having a miscarriage until (the moment when) she lost consciousness. Now NEVER THOUGHT is limited to BEFORE, but not all time. I hope that helps!

You could say, "It was not until she lost her consciousness that she thought she was having a miscarriage". You could also use "that she realised" instead of "that she thought".