OP, the issue is, when pregnant you can suddenly feel exhausted and weak and there's no warning when that will happen. You say you can just "turn back", but if you start to feel rough at the top of the summit it's a bloody long way down and you will be very slow getting down due to feeling ill.
As I said in my previous post on this thread, this is what happened to me on Scafell pike, and because the weather had also suddenly turned nasty (at high altitudes you can get sudden rain even if it's a fine day at the bottom of the mountain) it was very, very scary for both me and my DH.
It was the kind of walk I could do standing on my head pre-pregnancy and I felt fine, just like normal, for the first hour or two of the walk, but when I started to get unwell I was very high up and pretty much screwed, as there was no option but to continue walking when feeling ill/weak.
The descent down was me crying the whole way in fear/exhaustion, walking at a snail's pace, and my poor DH trying to keep me walking.
Before I had this experience, I, like you, would have scoffed at anyone telling me that I couldn't do such a straightforward hike while pregnant.