If the thoughts are really disturbing and upsetting it might be the result of OCD. OCD generates the kinds of thoughts that you are talking about. EG, holding a baby and then a thought that is just awful comes along like throwing the baby on the ground, something that nobody that is rational would ever consider doing. The more you try to stop thinking about terrible though the more it returns. You should see a therapist and explain the thoughts, if it is OCD there are wonderful methods to treat this illness.