it's possible but blood tests can determine paternity as early as the eighth week of pregnancy so it is possible to find out your gp should be able to help you.
Hi, MASC, if your cycles are super-regular, you have a good chance that you ovulated somewhere around March 18 or 19. When was the last time you and your boyfriend had sex before March 27? (You said "throughout my period," so does that mean your last sex with him in early March would have been around March 9?) Sperm can live 5 or 6 days in your system after sex, but even at the longest life, if your last sex with your boyfriend was March 9, it's asking the sperm to live a long time to make it to March 18 or 19.
Was all the sex unprotected? Also, how did you get your due date? If it was from an ultrasound scan, when was that?
Hello~I am honestly not sure if there is such a test or not, I would certainly ask your gyn, hopefully, he/she will be able to help you.