Due date?

Tabitha

So I just found out yesterday that I'm pregnant. I took a pregnancy test the first week of April and it came back negative. My period was supposed to start this week, but didn't which is why I took the pregnancy test. I did have unprotected sex between after taking the pregnancy test the first time and when my period was supposed to start. How do I know when I conceived to figure out when my due date is? Do I have to talk to my gyno about it?