Whether you're stowing, picking, diverting, or working the dock... you literally just grab someone to cover your position for a few minutes. That's it. There are process assistants present who can literally do that if they have to. There's no reason to deny a pregnant worker this except "This is making my day harder because I can't run my aisles at an absolute minimum head count and still stand around with my cart pretending to do useful work".

The only real case I could see where grabbing someone to cover you while you take a bathroom break might not work is, for example, someone working as a yard marshal... since that's not something everyone gets trained for, so the pool of replacements is smaller (including among process assistants).

So unless this worker was working the yard... there's not a lot of reason to deny them bathroom breaks... and that goes whether they're pregnant or not honestly.