When I was on X during the Olympics, it frustrated me no end that Emma Hilton wouldn't categorically say whether Swyer was a female or male DSD. She just described it as "a DSD".
Eventually someone helped me out and explained the equivalent of the diagram that has been posted above by flowchartnotrequired
I now also see Swyer's as a female DSD because of the impact of the missing SRY. I think I can also see why Emma Hilton doesn't categorically say male or female, despite her saying that every DSD is sex-specific. I'm assuming it's similar to my pronoun avoidance in that it's going to provoke a reaction either way: if she said it's a female DSD there will be people who disagree (because the body hasn't been organised around the production of eggs) and if she says it's a male DSD there will be people who disagree (because the body hasn't been organised around the production of sperm).
As I understand it, it's the only DSD which comes close to making sense of the false statement that "all foetuses start off as female". The default pathway is to develop female reproductive organs if there is no active SRY. (Unfortunately it ruins Mr Menno's brilliant song "Y chromosome" because being male is about the action of the SRY gene, that sits on the Y chromosome the vast majority of times).
Unlike 5-ARD, with Swyer there will be no testosterone boost at any time. Meaning there will be no development of any male secondary sex characteristics.
If I put myself in the shoes of a person with Swyer (or a child, pre-puberty, with 5-ARD) I can fully understand why they say they are female. I do too. However, I would have been mistaken regarding the child with 5-ARD as when puberty hits, this child's actual (male) sex would become clear. The testicles were there all along and the penis will naturally grow at this point. That must be a horrible experience to go through. But for someone with Swyer this will never happen and their physical development pathway will remain as it started. To develop any secondary sex characteristics, they'll need to take hormones. Logically, anyone who has grown up with Swyer will take oestrogen. Whether I'm "correct" that someone with Swyer is female becomes a matter of opinion, which will be divided.
Swyer probably also helps to explain Kirsty Blackman's ridiculous statement in parliament that she didn't know her chromosomes but was "probably XY". She has children and she has breasts. More importantly, if she had Swyer, she would have been told her chromosomes were XY at the time of diagnosis and that in order to develop breasts she would need to take oestrogen (and she would need to have IVF if she wanted to try for children). Surely she'd remember going through all that 🤦♀️ So of course she knows her chromosomes... although her grasp on biology is evidently very shaky, even without the complexity that Swyer adds, so maybe not. That doesn't stop the rest of us knowing them though from the facts that we know about her.
Until I saw the diagram again, I had forgotten that people with Swyer had a uterus, so I'll take back my earlier suggestion of spotting this DSD on an ultrasound. D'oh. (But it would still be possible to identify 5-ARD as I suggested above, because there would be no uterus and no penis/scrotum, which should flag as an anomaly).
Edited for clarity.