What is the birth canal?

The birth canalalso called pelvic excavationis the tunnel or channel through which passes the baby during childbirth to go outside and be born.

It is made up of the bony structures of the pelvis and the muscular walls of the uterus, cervix, vagina, and vulva.

Anatomy and characteristics 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 tilted and curved position with three bony constrictionscolloquially called upper, middle and lower straitswhich represent the key points for the passage of the baby during childbirth.

The baby is forced to flex and twist like a screw in order to get through the birth canal and past the three constrictions.

upper narrow

The upper strait forms the entrance to the birth canal and it is the first difficulty that the baby faces to be born. It is the place where the baby enters the birth canal and it has to start to twist in order to move forward.

The bony structures that delimit the superior strait are the anterior part of the sacrum bone and the inner borders of pelvic bones.

The diameter of the superior strait and the diameter of the baby's head determine the possibility of a vaginal delivery, always bearing in mind that the baby's skull bones are relatively flexible.

In fact, the diameter of the superior strait and the diameter of the baby's head have co-evolved in hominids. The Homo sapiensthe current human species, has a wider birth canal than its ancestors.

medium narrow

Following the upper strait is the middle strait, formed by the ischial spines. The middle strait is oval in shape with the largest diameter at anteroposterior directionforcing the baby to turn to orient the head in the same direction.

In the middle strait is also located the curved part of the sacrum and pelvic floorwhich also makes the baby have to flex the head and neck.

narrow bottom

The inferior birth canal is the last bony narrowing of the birth canal before the baby can come out. It is formed by the coccyx on the back and on the sit bones.

Baby's passage through the birth canal

The force and push for the baby to move through the birth canal comes from the uterine contractions and, in the last stages of labor, also of the voluntary contraction of abdominal press.

The baby's passage through the birth canal begins with the dilation of the cervix due to hormonal effect. Dilation and contractions push the baby until she 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 bony constriction. The baby's body takes on an almost cylindrical shape that simply follows the head.

For a vaginal delivery to be possible, the measurements of the pelvic inlet and the diameter of the baby's head must be adequate, always bearing in mind that the baby's skull bones are not yet fused, and also that the pelvic ring bones of the mother also show some flexibility due to the action of birth hormones, so that the baby and the birth canal have a certain ability to adapt to each other.

Alterations of the birth canal

The dystocia is the medical term used to describe a delivery with difficult or abnormal expulsion. Among the many causes of dystocia, anatomical abnormalities of the birth canal are the most frequent and common, especially bone alterations.

Dystocia due to bone disorders

  • Reduced diameter of some of the narrowing of the birth canal in the pelvis of normal morphology.
  • Bone alterations in the pelvic bones, in the spine or in the lower extremities.

Dystocia due to soft tissue disorders:

  • Uterus: malformations, tumors, cervical dystocia.
  • Vagina: tumors, edema, vaginal surgeries
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