I feel I ought to answer because nobody else has, though I don't know.... But if you're really worried about it, can you plug it in and then go out? Or stay with a friend or relative for a day or so?
My parents had something like that in the garden for moles. I would guess that the baby is well protected by your magnetic field so it should be OK. You could try switching it on when you have time to pay attention and then just see how it feels. I know during pregnancy I'm particularly sensitive to that kind of thing....
Is it a pulse, or is it high frequency sound waves? If it is high-frequency sound waves, you should be fine. Baby's ear bones aren't really developed until 18-20 weeks, I think, so you are on the edge of that (if you are due in early April). But even if baby can "hear," it can not hear the high frequency that rodents can.
If it is some kind of pulse thing, though, I don't have a clue!
I have never found the plug-in devices to work. We bought a whole slew of them last winter, and are still plagued. Save your $. I am puzzled about why the cats don't scare them off, though, have you seen droppings? Maybe you have squirrels in the attic, perhaps you should call someone to check.
I agree with pp, save your money, those things don't work at all. Also, if your problem visitor/s are in the attic, the plug in things won't be anywhere near enough to even hope to work, according to the packages I read.
It could also be bats. I have bats in part of my attic crawl space, and I like them so well I've been negligent about evicting them. But both squirrels and bats will make themselves at home. (Although the bats should be settling down for hibernation.) See if you can investigate what their leavings and droppings actually look llike. -And put the cats in the attic overnight for a while, lol.