Cats - behavior

Question
I have a 3 year old cat named Pumpkin. We had her spayed at about 7 months old.  She has a habit of spontaneously biting me and my husband.  We have not noticed any discernible pattern or triggers.  During the last attack, I was petting her head and scratching her ears as she loves for me to do.
We have tried saying no in a firm voice and giving her two pops on the behind right after the behavior occurs.  Sometimes it is difficult to implement this because she runs and hides. She continues the biting and scratching regardless.
   Our concern is that I am 7 months pregnant and this is the 3rd time she has broken skin since my pregnancy.  We are also concerned that when our baby is old enought to be mobile, she will attack the baby for no reason.

HELP!

Answer
Physical punishment and telling her "NO" will not work because she is a cat.  Best thing to do is squirt her with a water bottle so she doesn't associate punishment with you and then become more aggressive.  

Maybe when you are petting her you are over-stimulating her as some cats are hyperasthetic and can not tolerate a lot of petting.

I would recommend FELIWAY diffusers in the house to help reduce anxiety as well as adding RESCUE REMEDY to her water.  

Make sure to play with her at least 15 minutes a day using a pole toy and NEVER your hands.  She needs some extra TLC.

You can also try to put Bitter Apple Spray or Tabasco on your hands when she bites to get a dis-taste in her mouth.  

As far as the scratching you could place soft paws on her claws or have her declawed.  

I doubt she will attack the baby for no reason, but obviously you will have to monitor her.