When I had my doula, she didn't have kids. At first, I wasn't sure she could really understand what I was going through.... Boy was I wrong! She's the greatest! She was still a student doula when she attended my birth. (Believe my birth was #7 for her.) She was caring, understanding, and very knowledgeable. Just what a doula should be! Plus, she didn't have kids. She didn't have to say "Oh, you're in labor.... wait, let me get a babysitter and I'll be over ASAP." She was able to drop things on a minutes notice. Another good thing about a doula without kids is they've usually experienced birth many different ways.... They don't bring their own birth experiences with them (positive or negative). (If a doula only had positive homebirth experiences.... it would make her job a bit more challenging to support a woman who wants a medical birth.) I know I bring my personal birth experiences with me.... even though it's not my birth.... I find it more difficult when a woman is faced with the same circumstances I was in... and makes a different choice than I did. (Ex. smoking during pregnancy)
Also, how many OBs are male. How are they qualified to be at a birth? Yet, so many women never question them. Yes, they've seen birth, but they've never experienced birth first hand. They don't even have the equipment to experience birth. How can these male doctors even understand what cramps feel like? You may not have experienced birth first hand, but as a woman you understand the female body better than any male doctor. Plus, doulas don't just show up the last 30 minutes... you experience and put a lot more effort and time into a pregnancy, labor and birth. Caring and supporting women and their choices is what being a doula is about.... go for it!

Forgot to mention: If you are planning on TTC, let your future clients know if it might affect them. I was worried that my doula was going to succeed in becoming pregnant and then not be available to me when I needed her. (Most likely she would have had morning sickness when I gave birth. I didn't want her throwing up while she was attending my birth....)