O Negative blood

So I’m o negative blood type and I believe I will be getting the anti D injection but I’ve been reading things online and people are saying the NHS test the babies blood first before they give the injection. Is this correct? Does anyone know the process? I expected to just go in at 28 weeks and have the injection and then again when the baby is born but I’m not sure what’s happening now.

I’m 24 weeks and got a letter in the post for an appointment when I’ll be 28 weeks but for some reason my trust never say what the apps are for unless it’s a scan 🤦🏼‍♀️