The birth canalAlso called pelvic excavation is the tunnel or channel through which the baby passes during childbirth to come out and be born.
It is formed by the bony structures of the pelvis and the muscular walls of the uterus, cervix, vagina and vulva.
Anatomy and features of the birth canal
The birth canal can be described as a tube in the female reproductive tract that runs from the top of the uterus to the outside of the vagina. This soft tissue tube is surrounded by the pelvic bones and together they form the birth canal.
The birth canal assumes a slanted and curved position with three bony constrictions colloquially called superior, middle and inferior straits which represent the key points for the baby’s passage during delivery.
The baby is forced to flex and twist like a screw to pass through the birth canal and through the three constrictions.
The upper strait forms the entrance to the birth canal and is the first difficulty that the baby faces to be born. It’s the place where the baby enters the birth canal and has to start rooting to move forward.
The bony structures that delimit the superior strait are the anterior part of the sacrum bone and the inner edges of the pelvic bones.
The diameter of the upper neck and the diameter of the baby’s head determine the possibility of vaginal delivery, always remembering that the bones of the baby’s skull are relatively flexible.
In fact, the diameter of the upper strait and the diameter of the baby’s head co-evolved in hominids. Homo sapiensa, the current human species, has a wider birth canal than its ancestors.
Following the upper strait is the middle strait, formed by the ischial spines. The middle strait is oval in shape with the largest diameter in the anteroposterior direction forcing the baby to rotate to orient the head in the same direction.
In the middle strait is also located the curved part of the sacrum and pelvic floor which also makes the baby have to flex the head and neck.
The lower birth canal is the last bony narrowing of the birth canal before the baby can come out. It is formed by the coccyx in the back and in the sitting bones.
The baby’s passage through the birth canal
The force and impulse for the baby to move through the birth canal comes from the uterine contractions and, in the later stages of labor, also from the voluntary contraction of the abdominal press.
The passage of the baby through the birth canal begins with the dilation of the cervix due to the hormonal effect. Dilation and contractions push the baby forward until it enters the birth canal and fits into the upper strait.
To pass through the middle and lower straits, the baby performs twisting and turning movements to adapt its head and diameter to the shape and orientation of each bone constriction. The baby’s body takes on an almost cylindrical shape that simply follows the head.
For vaginal delivery to be possible, the measurements of the pelvic inlet and the diameter of the baby’s head must be adequate, always remembering that the bones of the baby’s skull are not yet fused, and also that the bones of the mother’s pelvic ring also show some flexibility due to the action of the birth hormones, so that the baby and the birth canal have some adaptability to each other.
Birth canal changes
dystocia is the medical term used to describe a delivery with difficult or abnormal delivery. Among the many causes of dystocia, anatomical abnormalities of the birth canal are the most frequent and common, especially bone changes.
Dystocia due to bone disordersReduced diameter of some of the birth canal narrowings in the pelvis of normal morphology. Bone changes in the pelvic bones, spine, or lower extremities.
Dystocia due to soft tissue disorders:Uterus: malformations, tumors, cervical dystocia. Vagina: tumors, edema, vaginal surgeries