Fentanyl is an opiate that can decrease the intensity of contractions but will not numb the feeling entirely as an epidural could. Opiates will cause mild to moderate sedation and go to work very quickly, but they do not last for an extended period. Fentanyl takes 3-5 minutes to be effective and lasts anywhere from 20-45 minutes. Any opiate does cross the placenta, and as with any medication, there are potential side effects to you and your baby. This episode answers a question about using fentanyl during labor and the risks and benefits associated with the medication.