My opinion is that it is also sexual maturity. Last summer, 3 of my 5 bottle fawns had fawns. The 2 that didn't have fawns were my biggest body ones. The littlest one was the first one to have a fawn and she had him the 2nd week of June. The 2nd one was the last part of June and the last one was August 10th.
We aren't breeding our doe fawns anymore because we don't think it's worth the risk of losing our does. 2 out of the 3 had major problems....tore themselves up in the back end. They took weeks to get back to normal and we had to give them antibiotics for infection. I had to pull one fawn out and they both disowned them because of the pain that it caused them. The other one probably would have taken care of hers, but we took her because she was late born and I wanted her tame since she was out of our Allstar daughter.
I compare it to a young teenager having a baby....most just aren't ready to take on the responsibility and their bodies can't handle it either. My opinions are from our experience.