Is a wife divorced if she miscarried her fetus due to a quarrel after her husband swore to divorce her thrice, saying: "If the child is aborted again due to a quarrel, then she is no longer bound to me an hour after that," given that the miscarriage was necessary due to birth defects?

If you tied your oath to abortion due to a quarrel, you only break it if the abortion occurs for this specific reason, whether the tying was done verbally or by intention. If the wife aborts for another reason, such as a fetal defect, then you have not broken your oath, and neither divorce nor expiation is required of you. It is also fitting that the oath be by Allah Almighty, not by divorce.