Legs over the shoulders...

[RawDog]

The Os is the mouth or surface of the cervix.

 

[NinjaStallman]

Yeah, according to the girl in question if I went into the posterior fornix (the small pouch like bit just under the cervix) it was uncomfortable, if I touched the cervical opening (ie the external os) head on it wasn't painful, just odd.

To confuse matters the opening of the uterus is called the cervix, but sometimes the the small bit above the opening (correctly the anterior fornix) is too.

There's your female genital anatomy lesson for today

Edit: Here's the plate from Gray's anatomy (20th american edition) showing what you're bumping into.

 

[mellisa1983]

I luv this position!! it works well 4 me 2 get big guys room to work!

 

[redbear52]

If the woman is not flexible enough to put her legs up over the guy's shoulders, a useful approximation is for her to lie on her back with her ass right at the edge of the bed. The man enters her standing by the bed and holds her legs up. He can them hold them up as high as her flexibility comfortably allows and still get pretty deep, if that is the goal.

For me, even with my relatively modest length of just over 7 inches, this position has been uncomfortable for most of the women I have tried it with (too deep).

 

[TX_Cowboy]

This is one of the reasons I love my wife. She was a gymnast growing up and it extremely flexible. Plus she loves it when I bump her cervix. The harder I hit it the more she likes it. She really likes riding me because she can keep the head of my cock rubbing against her cervix.