Anatomy of the female pelvis and vaginal birth

Take a look at the bony pelvis you have been given. View it from the front. In the following slides, the bony landmarks will be described.

Innominate bone

Sacrum

Coccyx

Sacroiliac joint

Sacrococcygeal joint

Symphysis pubis

Ischial spine

Ileopectineal line

Obturator foramen

Pubic arch

Sacral promontory

Anterior foramina

Now look at the pelvis from one side. In the following slides, more landmarks will be shown. You will also see how the pelvis is orientated when a woman is standing up straight.

Anterior superior iliac spine Symphysis pubis Vertical plane

Ileum Pubis Ischium

Acetabulum Obturator foramen

Look at the pelvis from the front again. In the following slides, you will be shown a little more anatomy. Look at the position of the sacrotuberous and sacrospinous ligaments.

Sacrotuberous ligament

Sacrospinous ligament

Look at the pelvis from behind. Look at the position of the sacrotuberous and sacrospinous ligaments. These delineate the greater and lesser sciatic foraminae.

Sacrospinous ligament

Sacrotuberous ligament

Greater sciatic foramen Lesser sciatic foramen

We are now going to add in some muscles. You will see piriformis from front and back. You will see obturator internus from the back.

Piriformis

Piriformis

Obturator internus

We are now going to add in blood vessels and nerves. Look at the pelvis from the front again.

Common Iliac A Internal Iliac A

External Iliac A

Common Iliac V Internal Iliac V 37

The Lumbosacral Plexus

Sciatic nerve

Pudendal nerve

Obturator nerve

Look at the pelvis from the side. We will look at the muscles and ligaments on the side wall of the pelvis. You will see where the levator ani muscles originate. You will also see the critical dimensions of the pelvis.

Sacrotuberous ligament

Sacrospinous ligament

Obturator canal

Obturator internus Muscle Covered by Fascia

Pudendal canal

Line of attachment of levator ani

Critical pelvic dimensions Pelvic inlet

Critical pelvic dimensions Pelvic midplane

Critical pelvic dimensions Pelvic outlet

Female Male Pelvic inlet Pelvic outlet Pelvic cavity Pubic arch

Look at the pelvis from the front again. Imagine a ‘coronal’ plane through the middle of the pelvis. You will see the rectum coming through the pelvis. You will see where the levator ani muscles originate. You will see which structures form the pelvic diaphragm.

Iliac crest Pelvic brim Ischial tuberosity

Rectum

Obturator Internus With Fascia

Levator ani Plus coccygeus Makes Pelvic diaphragm

There are some structures above the pelvic diaphragm. There are some structures below the pelvic diaphragm.

Peritoneum

Subperitoneal space

Contains: Pubocervical Trans cervical Sacrocervical Ligaments

everything under pelvic diaphragm Perineum everything under pelvic diaphragm

Ischiorectal fossae

Now look at the pelvis from below. Look at the layout of the bones and the ligaments. They define the pelvic outlet.

Obturator membrane Obturator canal

Pubic arch Symphysis pubis Inferior pubic ramus Ischial ramus

Ischial tuberosity Sacrotuberous ligament Sacrum / coccyx

Pelvic outlet

Urogenital triangle Anal triangle

Keep looking at the pelvis from below. Imagine the anatomy above the pelvic diaphragm. The following slides show the structures encountered as you descend through the pelvis.

Bladder Above the Pelvic diaphragm Cervix Rectum

Pubocervical ligament Above the Pelvic diaphragm

Above the Pelvic diaphragm Transverse cervical ligament

Above the Pelvic diaphragm Sacrocervical ligament

Pelvic diaphragm Levator ani: Pubococcygeus Iliococcygeus Ischiococcygeus Coccygeus

Keep looking at the pelvis from below. Imagine the anatomy as you descend below the pelvic diaphragm. The following slides show the structures encountered as you continue to descend through the pelvis.

Urogenital diaphragm Superior layer of fascia

Urogenital diaphragm Sphincter urethrae Deep transverse peroneal muscles

Perineal membrane

Structures in Superficial pouch Clitoris & crus Bulb of vestibule Vestibular glands

Muscles in Superficial pouch Ischiocavernosus Bulbospongiosus Supl transverse peroneal muscles

Perineal body

Keep looking at the pelvis from below. You have now reached the most superficial level.

Labium majus Labium minus

Mons pubis Prepuce of clitoris Vestibule vagina Fourchette

Here is the female abdomen and pelvis viewed from one side. The structures shown should now be familiar to you.

Peritoneum

Sacrocervical ligament Pubocervical ligament

Pelvic diaphragm

Urogenital diaphragm

Look at its anatomical relations. Here is the rectum. Look at its anatomical relations.

Rectum Sacrum Perineal body Anococcygeal body

Puborectalis Deep Superficial Subcutaneous

Take your fetal skull and view it from above. Note the near central position of the anterior fontanelle.

parietal eminence coronal sutures frontal bones lambdoid sutures occiput anterior fontanelle posterior fontanelle saggital suture

The following slides will demonstrate the orientation of the fetal skull as it passes through the pelvis in normal labour.

the head flexes as the uterus contracts passenger the head flexes as the uterus contracts the head descends and engages in the pelvis the leading part approaches the ischial spines

passenger the occiput starts to rotate anteriorly the occiput reaches the pelvic floor (levator ani) internal rotation continues to achieve an occipito-anterior position

the occiput clears the symphysis pubis the head extends to deliver passenger the occiput clears the symphysis pubis the head extends to deliver

the head sits on the maternal perineum passenger the head sits on the maternal perineum

the fetal head realigns itself with the fetal shoulders - restitution passenger the fetal head realigns itself with the fetal shoulders - restitution

the head therefore continues to rotate - external rotation passenger the shoulders contact the pelvic floor and rotate so that the bisacromial diameter lies in an anteroposterior orientation the head therefore continues to rotate - external rotation