If you consider ‘mother’ to be the right word for you then use it freely, as is your right.
One of the big 'harms'/issues has been with women who have stillbirth, have a child die, and women who die in childbirth. The former two often have a complicated relationship with that word and it's been shown, over and over, that for many the importance of being recognized by others as a mother, of a mother of that child, is major. We're a social species and in horrible situations, we often look to others on what is our 'right' no matter how we feel.
For the latter, it's on those left behind. I lost a friend to a pregnancy-related complication. Obviously, she wouldn't benefit from a Maternity Bill, but I think culturally it's important that she and those like her are recognized as mothers and how the cultural idea of mothers and a mother's sacrifice and how women's health issues are disregarded contributed to her death. You don't get that with pregnant person. We barely as a society recognize the risks of pregnancy and childbirth as it is, we know demographics affects how people are treated in medical care, and I think we need to be more clear about these factors, not less, especially in the law.
“Mother” is a sexist term derived from “woman” anyway, it should be “parent or expectant parent”
I've never seen anywhere say it's derived from woman, in fact most think the basis for mother was among the first words, well before woman. It's largely discussed in etymology as likely having derived from baby talk - mama / ama - possibly from sounds while breastfeeding which may be why more words derived from mama mean female parent compared to papa, though some language switch or use different baby sounds.
It’s called Swyer Syndrome where the person has 46, XY DNA, or male DNA but all the organs and appearance of a female.
And if you read on the topic, they talk about girls and women with Swyer's, because while chromosomes causes development, it's not the defining factor, we sex biologically on gametes and the structures that support those gametes even if someone cannot produce them and we sex socially on phenotype - the latter can be altered somewhat but not fundamentally.
We can see this in plants and all other animals, humans are not unique in this - like all other mammals, we can only support and develop one if any kind of gametes. Someone who can develop and support eggs and an embryo is female. Someone who has a female phenotype is female whether or not she can produce eggs. Ovotestes don't actually mean both structures are there in a way that would function, just that both tissues are present.
Caster Semaya does not have Swyer's, but 5-ARD, which is a very different condition that include male structures. Someone with 5-ARD wouldn't be able to support eggs or an embryo - the structures aren't there for it. It does no benefit to try to blur all DSDs together and ignore the very important biological differences between them and while there is debate in sports, I think each type of DSD needs to be considered with their own traits.
Yes, there has long been a fight for sex neutral language for legislation and government writing (though him as neutral is still found), as well as in other fields - but not for things that are sex-specific.
When I talk about deaths in teenagers, I need to divide the sexes. I know there are a lot of different branches of feminism, but none I've met yet - even gender abolitionists - think I should arrange data, for the world we live in now, in a way that would make deaths by maternal conditions seem less serious when it's the number cause of death for girls and women 15-19. I also think it would be ridiculous to blur both sexes together and not to point out boys and young men of those ages are at significantly higher rate of vehicle-related deaths and interpersonal violence compared to their female counterparts.
When it comes to aspirations, we can often be sex-neutral, but when it comes to risks and what's being given to counteract those risks and difficulties like maternity protection, it does a disservice to ignore sex and it does many women and their families that have gone through the worst to not use mother in legal literature around those who are pregnant and give birth.