Thank you for your interesting question that other readers want to know the answer to as well.
In a large study of a lot of patients, the rate of pregnancy in patients receiving Ella ranged from 0.9 to 1.4 percent versus 2.2 to 2.5 percent in patients receiving postinor.
Hence the short answer is 1 in 100. But in life, if you are the one, it is 100% for you.
I would strongly encourage you to try to use other regular methods. Speak to your GP or Gynae.