Looking at if from the standpoint of how DNA is transmitted from man to woman and the practically infinite possible combinations of DNA, I'd say that any small change (even position during the act) would significantly decrease the likelihood of your existence. So, I'd say it would be highly improbable that the same person would exist if any predecessor made even the slightest change in his daily activities.
You should watch "About Time". It's about a time traveler and it explores this idea a bit.