When I went to a different hospital for a test, they had a system where by when you went to do a urine sample, the sample was handed directly through a hatch to the testing department from the bathroom, and a sticker could be attached to the slip which identified which patient the sample came from. Doing so would cause the midwife to create an opportunity to speak to you privately, presumably by fabricating the need for privacy for an examination, test in another room or something.
It was reassuring to me as a woman that had I had a problem there was a way of telling someone about it secretly.
But I agree that one of the biggest fears pregnant women in a DV situation must have is that her baby will be taken from her (sometimes her partner will have told her this and that she is/will be a bad mum etc. Emphasising that its about keeping the mother safe with her baby might help.
Also as others have said, talking more generally about controlling behaviour, or unhappiness in the relationship rather than or as well as "violence" because many people don't realise that emotional abuse, financial abuse, controlling etc is covered by domestic violence label. I've seen posters of a woman dressed up for going out with the headline "I'm not letting you go out like that" or similar for helpline/information which was perhaps a good thing about the controlling behaviour thing.
I think that at some points the question has to be asked directly so that there is no misunderstanding, but also a more generally being approachable - 'if there is anything you need to talk to me about in private' is important too. And educating about how abuse can start or escalate in pregnancy or after the baby is born, and how to get help/what help is available.
Thank you for trying to protect pregnant women, new mums and babies from abuse.