Ectopic pregnancy
From Tardis Wiki, the free Doctor Who reference
Ectopic pregnancy was a complication in human pregnancy which would prevent the foetus from surviving.
Alex's ectopic pregnancy meant she would not birth this child. Along with other factors, this legally meant that she was future-proof, that her death would have little to no impact on the causal nexus. In part because this pregnancy would not be brought to term, Alex was deemed fair game for the Hunt. (AUDIO: Moving Target)