The OvuFirst Sensor should be worn on your skin in one of three locations. We recommend you wear it under your arm (as far into the armpit as is comfortable) using one of our specially designed armbands that you can choose free of charge during the checkout process, as that has been shown to produce the most accurate and consistent results. However, you can also wear it on the underside of your wrist using our wristband. Alternatively, you can wear it under a bra strap. We strongly recommend that you do not move your OvuFirst Sensor from one of these 'locations' to another during a cycle as it is likely to provide inaccurate results. If you do want to change 'location' for the OvuFirst Sensor on your skin, wait until your next cycle starts and then switch.