When does it become positive

A: The earliest you can get a positive result on the most sensitive pregnancy tests is 3-4 days after implantation. Implantation needs to occur before hCG is produced, and implantation generally happens between 6-12 days post ovulation, so the HPT would be positive as early as 9 days and on average 13-14 days after ovulation/fertilization. For this reason, it makes economical sense to wait until 10-12 days after ovulation, but even 10-12 days after ovulation over one-half of pregnant women will still show a negative result.
If you did not have an hCG injection (common in fertility treatment, brands include Profasi, Pregnyl and Novarel), you can believe the positive results, but a negative results does not necessarily exclude a pregnancy. If your period is late, test again. Your hCG levels should rise at least 60% every 2 days and many people won't have a positive HPT until the first day of a missed period or even a few days later