I'm new here, but i have just been through everything involved in having a baby with scars and some fresh SI cuts on my thighs and things like picking and scratching bites or scabs until they are open almost all the time... and I just warned my OB and explained that I was in a helpful program... Always better to be forthcoming and not just hope for the best... Them discovering it themselves and not knowing how to properly react would tend to be more stressful for you and the doctor... They are professionals, and most of them qualified to treat and diagnose many psych related issues for postpartum moms (which is what I am currently diagnosed and being treated for now)

He/She should be understanding, and you should feel comfortable letting him or her know... this kind of doctor/patient relationship is very personal, and they are used to that... don't be afraid.