I can only think of three approaches, none of which is without its risks.
The first is to ask if they used similar erasing language when discussing, fatherhood, and if they would, what would the people we tend to call father now be called? 'Impregnating' people?
It might be possible to frame that more pleasantly, of course.
The second one is to say that many women have an embodied gender identity which is anchored in their experiences of living with a female body, including the experience of pregnancy. When someone writes 'pregnant people,' they mean "pregnant men, pregnant women, and pregnant people who are neither men nor women or at least not all the time."
And that erases the gender identity of all women and men for whom it is embodied. So they are invalidated!! (sorry, I find that funny)
The third one is to refer to the UK MOMA bill where attempts were made to use 'pregnant person' for 'pregnant woman' and that was changed to 'mother.'