How many days apart do sexual encounters have to be to determine the father of the baby?

Swimmer survival: Expect the products of one encounter to remain on board for as long as 4-5 days before the swimmers lose strength & fade. That said, the dates you are given early in pregnancy for expected due date and how far along you are are estimates. Encounters that overlap or are closer than a month apart may only be sorted out through dna testing.

This is moot: Ultimately, who the father is will be determined by dna testing. This is the 21st century. If you're among the putative fathers, even one romantic interlude on almost any day of the month makes it possible that you will indeed be a dad.