I found the same thing happened to me. My genre hasn't necessarily changed (I write contemporary women's fiction) but I feel the maturity and subject matter/content has changed since becoming a mother.
During my pregnancy with DS1, I journaled about the pregnancy but that was it. I just wasn't motivated/inpsired/feeling creative/insert blank here. I just couldn't. I would sit there with a blank document, or pencil poised, and... nada. I found it frustrating, and as the PP mentioned, pregnancy brain was a major problem. For a long time I thought I was done with writing.
I remember dabbling a little after he was born, but probably not until he was close to a year old and I was feeling a little more like myself again. The first serious writing I did since the pregnancy was NaNoWriMo in November 2007, when DS was 18mths old. I hadn't written much of anything since my BFP in August 2005. I dove into it and had passion again.
Go figure, I went through the same thing again when we conceived the following month (granted, we managed two major moves 3 weeks apart). I lost that baby, but found writing after my miscarriage was therapeutic, and then we conceived Jasper, who is 3 weeks old, and the same thing happened.
I am attempting NaNo again this year but I am forcing myself to churn out the words because I want to prove to myself that I CAN do it with a toddler and a newborn, but I am not very inspired, which bugs me because I want to be. It's just hard to focus.
write@home mama to big boy (04-06) and little boy (10-08)
and someone new in november