It's true there was no way of knowing if the baby would have aborted early or not. However, if it had aborted naturally she could have then received treatment. But as long as the baby was alive and growing, this true Mother who truly lived up to the name of "Mother" (unlike so many women of today), did what was right: she left the decision for life and death in God's hands, rather than taking it into her own. He could have preserved her life also, or caused the baby to abort naturally, but He didn't.
If both had died, what would it have served? That's not for us to answer. It is ours to be obedient to Him and leave the consequences to Him, whatever they may be. Death sometimes serves His purpsoes more than life does, especially when we realize that physical death is not the end of one's life.