I thought it was confirmed somewhere that Regina did intend to kill the baby. She could not harm Snow (but note, that might not have covered Charming). If the baby died before it might not have broken it before cast, but the baby had to be alive to break it after. So then the curse never could have been broken.
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“If I had a world of my own, everything would be nonsense. Nothing would be what it is, because everything would be what it isn't. And contrary wise, what is, it wouldn't be. And what it wouldn't be, it would. You see?” -- Lewis Carroll, Alice's Adventures in Wonderland & Through the Looking-Glass