IUD after 2 c sections

I decided to get a mirena iud and I mentioned it to my midwife weeks ago because I wanted to know when I could. She said I might not be able to because I’ve had more than one c section and because of how my cervix is (I don’t know the word for it but my cervix remained high and didn’t dilate further). I asked my ob at my 6 weeks appointment and she said I can get it at 12 weeks because of the c section. I’ve never heard of such. I’m 8 weeks pp now and ready for long term contraception. Has anyone had any experience with an iud after c sections. Why do they want me to wait so long ? I was told 6 weeks after my 1st c section