No, I don't think I'm stupid. Unless you define "stupid" as "disagrees with me", which I don't.
Why would I means test maternity services?
(1) Childbirth is predictable, even if conception comes as a surprise it is very rare (though I accept it happens) for anyone to not have some months' warning of the birth. It can - and perhaps should - be paid for by those who can
(2) I would means test the parents, not simply the mother. There would need to be a system to obtain money from unwilling fathers and/or to discount their income where they genuinely have no input (maybe no parental responsibility if you fail to contribute to the costs of medical care?) - but that is administrative detail, not principle
(3) Means testing is not charging everyone, it's charging those who can afford it. Of course nobody should be left without care in pregnancy, childbirth and postnatally. However, means testing wouldn't do that.
(4) If people had to pay for medical care, even from the NHS, this may well result in more private maternity options, other than the uber-expensive options available in London. I would strongly support some form of competition for the NHS (not only in maternity services - see further below), because this should improve service. And by service I mean people not being left with completely avoidable injuries and illnesses because of failures.
Of course, means testing might be administratively unworkable, but maternity services in this country are appalling. Something needs to be done. Some more money, and the fear of losing that money due to competition might assist with that.
FYI, I would means test other things. Joint replacements for one. Possibly some long term treatments (my cousin has just died following years of chemotherapy, which kept her alive - I am not sure that she shouldn't have been in her circumstances expected to contribute to the cost of that. I don't think she would have disagreed either).
Using the NHS is an appalling experience. My worst experiences have been in maternity. I would pay quite a lot to make that better. I would not expect anyone unable to pay to do so, and all emergency treatment should be free to everyone, at point of care and afterwards. But yes, if it resulted in a better system, I would be happy with appropriate means testing for lots of things.