The passageway in the human birth canal  is where the fluid passes during menstrual period and where the baby passes during birth.

The human birth canal is the passageway through which the fluid passes out of the body during menstrual periods and also where the baby passes during birth. 

There is no way to say exactly how long the vaginal canal is during pregnancy without understanding the way it works.  The average vagina canal could be anywhere between 3 to 7 inches deep depending on if the vagina canal is relaxed or aroused.

The average depth when relaxed is around 3 to 4 inches deep.  When aroused it could go as deep as 7 inches and in some cases more. 

When relaxed the vaginal walls are tighter bringing the back of the vagina and cervix closer to the opening.  The vagina also has the capacity to expand letting the baby pass through the canal during childbirth.  Just as it does during intercourse it accommodates the penis expanding the upper two thirds of the vagina to lengthen by forcing the cervix and uterus to ascend.

Because the birth canal will expand only so far, sometimes when you have a very large baby you could end up with a birth injury. A birth injury could occur when the baby passes through the birth canal.

The baby being to large or birth canal being too small could cause a birth injury.  You could have difficulty delivering your baby if the shoulders or to large and could cause the clavicle to break.