Hello Doc, I Am 28 Years Old. I Am A

hello doc, I am 28 years old. I am a pregnant of 20 weeks.doc advised me to get tiffa scan done on my 22 or 23 week.can u tell me why it has to be done?this is my second pregnancy .during my first issue i have not undergone tiffa scan before 3 years. What is this tiffa scan ? What is the procedure.i am not aware of this.