My boyfriend finished in me , i'm on nexplanon . chances of pregnancy?

Very low chance.: Nexplanon has a 0.5% failure rate, so the most accurate answer is, Yes, you COULD be pregnant but it's extremely unlikely. 0.5% means that for every 200 intercourse events without a condom, there will be 1 pregnancy. Additional use of a condom improves your chance to be pregnancy-free, AND it protect you and your partner from STDs, (which the implant does not).