Hi... The symptoms of pregnancy is because of the hormone HCG.. The same hormone is use for the detection of pregnancy.. And it take atleast about 2 weeks following the fertilisation of the ova for the hormones to be released and cause the symptoms and for the tests to come positive.. Hence it is mostly the missed period which is the first symptoms to appear in pregnancy and rarely are the other symptoms.. So very unusual to predict pregnancy before the next due date..
There is no harm in continuing sex till the next due date even if you have conceived.. There is an extremely rare chances of repeat conception, I mean for twins to develop with continued sexual intercourse, else no other problems..