She may have not wanted the baby but didn't have money or means to do anything about it. Casey was 18, right? I'm not 100% sure about that, but if she was, then that would mean that CA's insurance at work would no longer allow CA to carry Casey. You know ICA, she probably thought she could fool her parents long enough to give birth and give the child up for adoption. By all accounts, she only had to fool them for a couple of more months.
She could have found out about her own pregnancy too late to terminate it.
Kiomarie was right on the money when she told LE that she thought they ought to the check the woods at the back of the school. Unless she and ICA had a bad falling out, she wouldn't have any reason to lie to LE about the conversation she said she had with ICA about adopting Caylee.
So far, nothing has been released that would discredit Kiomarie, on the other hand, you could fill up a blank copy of 'War and Peace' with ICA's lies.
And...to keep it on topic..the struggle of CA trying to keep ICA focused on being a mother instead of growing up to be a shot girl at Fusion caused friction, and that in turn kept them going at each other over Caylee.