How soon after having sex can you start to see signs of pregnancy, and what symptoms would you see first?

About 14 days. Sperm can fertilize an egg in as little as 30 minutes and as long as 5 days post-coitus. It takes about 14 days for an egg to be fertilized and to implant in the uterus. The first symptom you would most likely notice is the loss of your period. A home pregnancy test can tell you whether you are pregnant or not 14 days after (possible) conception.
Signs of Pregnancy. Ivf has taught us a lot about when everything happens to become pregnant. We know that the egg can be fertilized for only about 18 hours, that it implants about 5-6 days later and that the first time that HCG is measurable in the blood stream is about 11 days after ovulation (or egg retrieval). Once the HCG is in your blood then symptoms can start. Sperm lives in you about 2-3d but can be longer.