I found this online (not a reliable medical source but helpful in a general sense) and thought it was good info so I wanted to share. I was curious about how some get super early BFP and others get them late.

hCG doubles every 48 hours and you can have a level of 1 - 5 in a non-pregnant state. Implantation could be later than you think. If a woman implants at 8dpo, with a 'non-pregnant' hCG level of 1 [Which is totally normal] it would rise as follows:

8dpo - 1miu hcg

10dpo - 2miu

12dpo - 4miu

14dpo [Day AF is typically due] - 8miu

16dpo - 16miu

So this lady wouldn't find out until 17 or dpo on a very sensitive 20miu pregnancy test. Obviously if she implants LATER than 8dpo [which is normal] she will find out even later than that.

Now, Take a lady who implants at 8dpo, but has a 'non-pregnant' hCG level of 5:

8dpo - 5miu

10dpo - 10miu

12dpo - 20miu - (positive on internet cheapie or FRER)

14dpo - 40miu (day period is typically due/most tests would be positive)

Even though implantation is on the same day for both ladies, 1 finds out MUCH sooner.