Not an expert on that but I'd say that's a little far out for it to be real effective. If the heifer is that age that would put her being bred at somewhere between 12-14 months. Honestly if it were mine, I wouldn't worry much about it and just watch her close when she calves. It's probably not the accepted thing to do on here, but we sometimes go on and turn 12 and 13 month olds with the bull rather than holding a few odd aged calves separate. Most of our heifers are 14 months when bred. Unless this calf is just small or happens to have been bred by a bull that throws monstrous calves there's a good chance every thing will be ok
I'm not a fan of lutalysing them unless it's right at first. I'll always believe I killed a good heifer once with it when she tried to have the calf not sure how far along she was probably farther than that but after that I'll just take the chance on getting a calf out of the deal.