So i had sex w my partner my whole fertile window and on my ovulation day i slept w someone else but he didnt nut in me. is my partner the father?

The erect penis is secreting sperm containing semen continuously. It only takes one. In the cited scenario, there is only speculation on who the dad could be. A NIPPS test test done on maternal blood as early as 8 weeks can find cell free fetal DNA that can be used to define paternity. This test holds up in court. It is best never to be surprised at delivery & often obvious when you guess wrong.