I don't think Anakin would have killed her outright. Even with Obi-Wan's appearance, he wasn't in such a rage that he killed her. I do think he might have attacked and held her captive. Whether Padmé would have been able to get through to him or eventually killed, though, is more up in the air. One problem they might encounter, though, with turning Anakin back is that if Padmé survived the birth, Anakin might attribute that to his turn to the Dark Side.