What are my chances getting pregnant on implanon and he came in me on my first day of my period starting?

Less than 1%: Out of 100 women using Implanon in a given year, less than 1 out of 100 will become pregnant. No method other than complete abstinence is 100% effective at preventing pregnancy. Even sterilization still has a chance of pregnancy. If you are happy with your method but don't want to become pregnant, you could consider using condoms as back up method. Hope that helps!