What's the baby have to do w/ your fishing schedule. Your Mother-in-Law doesn't need your help. She'll take care of your wife just fine.
13 years ago, when we had our first, 2 weeks after my daughter was born, I left for 10 day to a fishing trip in Onterio. My mother-in law did a great job. I'm still married, and our daughter is 13 and doing great.
Oh, I do hear about it from time to time, but It gives my wife enjoyment to bash me from about it by telling the story.
Go ahead, go trout fishing for a few days and enjoy.