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It may sometimes occur that the Usg does not show fetal pole
till a few days after urine pregtest positive.
Also ther are some conditions like ectopic pregnancy
which may not b seen on scan but in your case its too early to comment anything.
All you can do is to wait for atleast a week to get a repeat scan done.
Another test to determine pregnancy is blood beta HCG
So do not worry. Consult your Gynaecologist and follow her advice.