I safely bred the majority of my 2 year old mares, but they had to have a good foaling hip, and show more of a maturity in their manners and handling. There were some that just didn't fit the bill, so I didn't breed them until they were older. I was also very careful to breed the maidens to stallions that wouldn't produce "chunky" foals for a first time momma -- chose stallions that I knew produced smaller, more "stream-lined" offspring. Then I fed the mommas carefully, making sure they had good nutrition, but kept them a little "lean", and made sure they got good exercise during their last trimester, to build up their strength for the hard job ahead.
I'm excited to see we're having babies!!! Can't wait!