We have a herd that the heifers mature way earlier than the others. If we keep heifers out of this heard we now lute them. In the past we'd get at least one from this herd calve at 15ish months. Sometimes it goes well, sometimes not. I've learned that you can kind of tell whether they will by just size. Most times when they are bred early, they dont grow as big as they would have unbred.
If you catch them when they start labor, watch the clock and if they get nothing out in around an hour, get them to the chute for evaluation. If you feel you can palpate and see whether the heifer is roomy enough to pull the calf and you know how to pull one, try. But if you slip your hand in and cant get it around the head easily, she'll need a c section and get her to the vet before anything comes out. Once the head is out, the only thing you can do is pull or cut out.
If this is going to be heifers you want to keep long term, take the calf away and bottle feed it. Or, leave it on and let her raise it, but know she probably wont breed back and she'll take that year to catch up in size. And if she raises the calf, it wont be like if she were 2, it will be dinky..If you have more 'cows' calving, take the heifers calves and hold them to graft onto cows if you have any lose calves. Then, that calf has a chance to grow like the rest, and the heifer will get bred when the bulls are put out........