How to take pregnancy test for my gf to ensure that she isn't pregnant after taking plan b ?

If period is missed: Conception, as defined by the generation of a fertilized ovum, occurs within day or so of the sex. However, it takes about two weeks for the embryo to make enough HCG to produce a positive pregnancy test. Blood test may be done a day after a missed period. Home pregnancy tests should be done 4-5 days after the missed period. Use first morning urine and follow instructions for the test carefully.