Mother in Law and my pregnancy

My background: I grew up with always having dogs. I know how much attention they need and the messes they make. I haven't had dogs or any animals in my house for 10 years now and I love the freedom of being able to go to work, workout and run errands without having to worry about dog feelings. I am also a part time dog sitter for the past 15 years.  

My husbands background: never owned dogs.

My dilemma: I don't know the best and nice way to say No to my mother in law. Being six weeks pregnant with a full time job, im exhausted at the end of the day. I can barely keep our apartment clean, fridge full and laundry done...let alone taking care of dogs. Plus I don't want the dirtness of her long haired chiuahahs all over or new carpet and furnture. She did mention she would come in our house several times a day to take the dogs for a walk but I feel like she is intruding in our home. We have only been married a year, I don't want my mother in law barging in whenever she wants. 

Help!