Hello,, urine
pregnancy test starts detect the b-hcg levels in urine from 21st day from last day of your last menstrual cycle, if you are
pregnant and from 28th day almost all test come positive if you are pregnant. Serial estimation of
beta hcg is need to check viable pregnancy (increase trend in hcg levels till twelve weeks), much easier is get obstetric scan which can tell you cardiac activity in fetus if it is viable.