Pregnancy Concerns: Late Period, Unprotected Sex & Negative Tests - Seeking Advice

Question
I am 22 yrs old, i have no kids and my last period was on March 3rd. It is now May 28th, i am concerned because on May 7th I had unproteted sex after he ejaculated in the condom, and then he ejaculated again outside of me.  After this on May 18th i had an annual exam and had a preganacy test that was negative (maybe to soon to find out?) and had a home pregnancy text on may 24th and was also negative.  On May  21st and 22nd I had unprotected sex with my partner of 4 yrs. I am now worried that if I do get pregnant , I wont know who the father is.   I have not had a period since March, is there any way i can find out who can be the father without telling my partner of the situation and getting a paternal test? If I do get pregnant can a blood test detect when i conceived? Can you please help me? Thank you very much!

Answer
If you have a normal 28 day cycle (from the first day of your period until the first day of your next period) ovulation occurs on cycle day #14. You can ONLY become pregnant if you have unprotected sex on cycle day #13, 14, 15, or 16 each month. If you have not had a period since March 3 and a pregnancy test on May 18 and 24 was negative, you are not pregnant. By now, a sonogram would also show if you were pregnant of not pregnant. Since you have not had a period since March 3, there is no way to tell if you had ovulated around May 7, 21, or 22. A pregnancy test will not show a positive result until 14-21 days after conception. If you believe that you may have cnceived on May 21, take a pregnancy test on June 11. If it is negative at that time, it is accurate. If you are, indeed, pregnant, a sonogram performed after 12 weeks gestation can pinpoint the exact day of conception. Don't worry.