Last period July 17th

Significant other and I had an accident Aug 1st. This is when I was ovulating. My last period was July 17th. I took a test Aug 18th and then a few days after. Both negative. Would a pregnant show up 18 days after conception? My breaks are sensitive. Other than that no other signs. Before I buy a test again as now it's been a month since my last test. What do you guys think?