Possibly six weeks

So, my Mema called the health department today and asked about me coming in to find out if I'm pregnant. I guess the lady asked when I had my last period and my Mema said three weeks approximately (which is true but Idk how she figured that lol) and so the lady said I'd probably be six weeks pregnant if I am. I've taken many hpts and they all came back negative. What's the latest you found out you were pregnant? My friend didn't know she was until she was seven months with her son.