Cerclage and progesterone

I had a McDonald cervical cerclage put in last Tuesday. The dr wants me to start progesterone vaginally today at 16 weeks. I do not really understand why I am to use this and I am nervous about using it. Directions on bottle say vaginally and dr said vaginally but the directions on the info pages say by mouth and could cause harm to unborn baby. I know I am to take it vaginally. Why would vaginally be ok and by mouth not?  Anyone else using this and can tell me your opinions and experience?  Thanks.