When to take hpt after implantation? hpt measures 25 hcg. what if negative?

After missed period: 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. A negative result from a test too early is not reliable.