I was risk assessed the next day after I told them, but withdrawn from normal duties immediately as soon as I let the boss know.
You cannot be expected to do your normal duties if you are not capable. Especially if a doctor has recommended light duties. It's not acceptable.
Can you get your doctor to sign you off instead? I'm sure they will if you explain that the light duties recommendation has not been applied. By the way, I believe that pregnancy related sick live is automatically mitigated so you shouldn't have a problem with it x