I was due on Monday yet here I am, sporting quite long pointy acrylics.
I was in the hospital for a check where they put the monitor on me and the bit around the finger had an opening so my nail went through it nicely. There are different kinds of monitors and some have the clamp kind of a thing for the finger but with a more reasonable length of a nail you can fit in it just fine (I have had long nails with all my pregnancies and been hooked to different machines). Beyond the length of the nail it shouldn't be a problem as the thingamabob on your finger takes your pulse from your fingertip and the material or thickness of your nail doesn't really affect that.
But asking the doctor is a good idea in any case just to make sure but I have had doctors tell me I have to clip my nails before I'm due and when asking for the reason they tell me because they are just too long and how do I plan on taking care of the baby with things like that on my fingers and no real reason whatsoever...