Home pregnancy tests work as soon as two days after implantation, according to Church & Dwight's First Response page. The most reliable time to test is after the first missed period, which is actually eight days after implantation, according to WebMD.
Conception and implantation are unique to each woman and her particular cycle. Generally, the egg implants within six days of fertilization, which is eight days before menstruation for a woman with a 28-day cycle, notes WebMD. Early-detection home pregnancy tests can test the pregnancy hormone (HCG) as early as six days before the period should start. HCG doubles every two to three days, so if an HPT is negative six days before a woman's period, it is possible that she is pregnant but simply does not have enough of the pregnancy hormone in her system. WebMD recommends her to retest closer to her period for more accurate results.