It really all depends on when you ovulate, or in this case when she ovulates. Since sperm has been known to survive up to 5 days internally (2-3 is the more common life span) if she ovulates really early or really late in her cycle it's possible.
Most women ovulate 14 days before their period though, so like others have said, not really likely.

Granted if DH makes his move a few days before my evil AF is due, she invariably shows up a few days early sending my averages out of whack.

This might be an opportunity to remind her that unprotected activities in this day and age aren't really the safest. Who knows who ex has been doing while they were broken up if you get my drift