I read this story a few years ago.

Kagome is just about ready to deliver her baby (Sesshomaru is the father). I cannot recall if he thought she lost the child or she told him she got rid of it and tried to keep it secret. (Maybe it was a one time thing and he didn't even know she was pregnant.) Inuyasha stands by her and helps to protect her. He warns her that Sesshomaru will know when she goes into labor. So Sango helps pack their things and they head to a cave near the village so Kagome can deliver. They don't want to be near the village in case Sesshomaru shows up and is intent on killing Kagome. Someone (maybe Miroku) puts up a barrier to help keep them safe. Sesshomaru shows up because his heir is being born, but Inuyasha tries to fight him to protect Kagome. I believe they are positive that Sesshomaru will kill the pup. If I'm not mistaken (it has been awhile since I read it) the moon on his forehead disappears and reappears on the pups forehead.

I hope this is enough to help, and that someone remembers this fic. If not, I will do my best to come up with more information. Right now, though, I believe that is about all I can recall.

A Curse of Lust by Squash rated MA
www.dokuga.com/fanfiction/story/4993/1

That was a lovely story.