What Is NEXPLANON® (etonogestrel implant) 68 mg Radiopaque? WHAT IS NEXPLANON? NEXPLANON is a birth control implant that’s placed just under the skin of the inner, non-dominant upper arm This flexible implant measures 1 6 inches in length (4 centimeters) and is approximately 2 mm in diameter A trained healthcare professional will insert the implant into place in your inner non-dominant upper arm