hi OP. I'm similar height / weight / BMI to you.
this is my second pregnancy and I weighed a few kilos more at the start of my first, so was normal BMI then, and underweight BMI now. Not a choice to lose weight, I'm just naturally skinny like you, have never been able to put on weight, can eat all the cakes and chips that I want (won't feel very well but won't put on weight!), and in the few years since having my first I have naturally lost a bit of weight as I am rushing around more than before, and have been picking up a growing baby/toddler/pre-schooler and running around at the park, commuting to work etc. I would prefer not to have lost the extra few kilos and have tried everything to put it on, but nothing (including disgusting protein shakes) has worked.
I have been criticised about my weight and been accused of being anorexic many times over the years. Which I think is massively offensive and wrong and I would never and have never criticised anyone for their weight.
This pregnancy, at booking appointment, I was deemed higher risk and told I'd need to see a consultant and get an extra growth scan - simply due to my BMI.
I don't mind an extra scan/appt - and my first DC was 7lb 12oz which is perhaps where we differ. I have been referred to a dietician to see if I need to adjust my diet (but doubt it will make any difference to my weight after everything I've tried).
As some of the nicer PPs have said, be honest with the midwife, ask exactly what the reason for high risk is (e.g. your weight, previous DCs' weights - plus anything else?) and what the extra appointments are for, and whether they are all really necessary. And eat whatever you want, don't listen to others' criticisms and if you do have to have lots of extra appts just try to think of it as nice to have someone taking extra care, rather than a stress-inducing experience. I imagine it must be hard to go to extra appts when you have 4 kids to look after so I hope you have help with childcare to do so.
Good luck and be nice to yourself xx