The front crease is a pretty reliable indicator of complete or near complete dilation and descent of baby..
This is a quote from the Gentle birth archives:
"You can assess descent fairly well by watching the crease that forms
parallel to the symphysis. In a lean mom, you might not have
any crease at all at the beginning of labor, but the line gets wider as
labor progresses and it should be nearly fully across at full dilation.
Can't always see it if mom is real heavy, but it is pretty dang reliable."
I think it's caused as the bulk of the baby's shoulders come closer
to the symphysis.
There is a voice inside of you
That whispers all day long,
"I feel this is right for me,
I know that this is wrong."
No teacher, preacher, parent, friend
Or wise man can decide
What's right for you--just listen to
The voice that speaks inside."
— Shel Silverstein