I am not sure about the wording, but the term basically indicates employees loss of right for maternity leave when the reason (事由) that made them eligible for the leave ceases to exist - which may not necessarily mean birth of the baby and his turning one, but also termination of pregnancy, miscarriage or death of the baby.

I once did a small research on the subject and, if I remember it correctly, according to Japanese law, female employees in Japan should get back to work from their maternity leave no later than after their child turns one (or, one may say, they can stay on maternity leave until their child turns one); but in case the child dies before turning one, the mother is supposed to get back to work earlier even if there is (=would have been) some time of that one yaer left, and she is not allowed to stay on maternity leave for one year after the date of birth of the child if the child dies before turning one. To put it bluntly, no child = no right for maternity leave, even if it's been less than one year from the delivery.
Does this make sense in your context?