I have colleagues who did this - a lot of things and decisions change when you are looking after a child. Their infant may be really needy, they may be uncomfortable going back to work, they may get depression, their life goals may change. How about thinking about this like - you can use the income, and you may yet change your mind when you start juggling your new mummy duties (what if you decide you cannot stand it and want to work instead), it's fine to try out your new lifestyle first before you make the final decision to throw letter.