if you had sex using a condom there is no need for testing.
If you did not use a condom, then you will need to test at 12 weeks.
All the best
i used the new i thnk thats ph i got result within few minit
12 weeks is 100% conclusive and reliable.18 days is to early for a conclusive result no matter what test you had.
What modernt test u used?