The birth canal. It has to be removed from common vernacular pertaining to birth.
Here's the heads up... or down... follow me for a second.
Lie down, on your back, knees bent up.
Take a deep breath. Inhale. Exhale. Repeat a few times.
Now feel with your right hand, the front of your pubic bone.
With your left hand, feel your perineum.
Now close your eyes, and imagine the 'tunnel' that exists between them. It's not made of steel and concrete. It's not kilometres long.
It's a vagina.
Purpose built for birthing babies.
And it's about the length of you middle finger.
Your baby's head will be behind that same pubic bone you are now touching with your right hand when you are at term, and often it is LOWER than that.
Is it really a birth canal in your mind? A passage between the womb and the world, yes... but not long enough to house a ship or 20.
Myth of tunnel of doom = busted.