The maternal pelvis, the fetal skull, mechanisms of normal labour and malpositions

Question 1

What part of the fetal skull is the sinciput?

Answer 1

• extends from the orbital ridges (bridge of the nose) to the anterior fontanelle and the coronal suture
• the sinciput is also called the brow.

Question 2

What part of the fetal skull is the vertex?

Answer 2

Extends from the anterior fontanelle and the coronal suture to the posterior fontanelle and the lambdoidal suture

Question 3

What part of the fetal skull is the occiput?

Answer 3

Extends from the posterior fontanell and the lambdoidal suture to the base of the skull

Question 4

What are the bones of the fetal skull?

Answer 4

2 frontal bones
2 parietal bones
2 temporal bones
1 occipital bone

Question 5

What is the suture between the two frontal bones?

Answer 5

the frontal suture

Question 6

What is the suture between the frontal bones and the parietal bones?

Answer 6

the coronal suture

Question 7

What is the suture that runs between the anterior fontanelle and the posterior fontanelle?

Answer 7

the sagittal suture

Question 8

What is the suture that runs between the parietal bones and the occiput?

Answer 8

the lambdoidal suture

Question 9

What is the shape, alternative name and position of the anterior fontanelle?

Answer 9

• diamond shaped
• also called bregma
• at the junction of the frontal, coronal and sagittal sutures

Question 10

What is the shape, alternative name and position of the posterior fontanelle?

Answer 10

• triangular
• also called lambda
• bound by the sagittal and lambdoidal sutures

Question 11

What are the 3 sections of the true pelvis?

Answer 11

• brim
• cavity
• outlet

Question 12

What are the landmarks of the pelvic brim from back to front? Think about how these landmarks relate to position

Answer 12

• sacral promontory (Direct OP)
• sacroiliac joint (LOP/ROP)
• iliopectineal line (LOT/ROT)
• iliopectineal eminence (LOA/ROA)
• upper border of the symphysis pubis (Direct OA)

Question 13

What are the 4 types of pelvis?

Answer 13

• Gynecoid
  oval shaped, parallel sides, dull ischial spines and wide pubic arch (3 fingers)
• Android
  heart-shaped, prominent ischial spines and narrow pelvic arch
• Anthropoid
  longer front-to-back compared to gynecoid
• Platypelloid
  longer side-to-side compared to gynecoid

Question 14

What is the shape of the pelvic brim, and what are its anteroposterior, oblique and transverse diameters?

Answer 14

Oval or heart-shaped
AP 11cm
O  12cm
T   13cm
wider in the transverse diameter

Question 15

What is the shape of the pelvic cavity, and what are its anteroposterior, oblique and transverse diameters?

Answer 15

Circular
AP 12cm
O   12cm
T    12cm
same width in AP and T diameters, allows space for rotation

Question 16

What is the shape of the pelvic outlet, and what are its anteroposterior, oblique and transverse diameters?

Answer 16

Diamond
AP 13cm
O   12cm
T    11cm
wider in the AP diameter

Question 17

What is the anatomical name for the top of your hip bones?

Answer 17

Iliac crests

Question 18

What is the anatomical name for your sit bones?

Answer 18

Ischial tuberosities

Question 19

What are the bones of the pelvis?

Answer 19

• 2 innominate bones each with an ilium, ischium and a pubic bone
• sacrum
• coccyx

Question 20

What is the meeting point of your pubic bones called?

Answer 20

the pubic symphysis

Question 21

What is the name for the underside of the pubic symphysis?

Answer 21

the pubic arch

Question 22

What are the 4 pelvic joints?

Answer 22

- the pubic symphysis
between two pubic bones
- two sacroiliac joints 
between sacrum and ilium
- the sacrococcygeal joint
between sacrum and coccyx

Question 23

What is the curve of carus?

Answer 23

• the imaginary line which passes through the centre of the theoretical plane at the pelvic brim, cavity and outlet
• follows the curve of the sacrum
• the anterior wall of the pelvis (the pubic symphysis) is shorter than the posterior wall (the sacrum), which creates a curved canal

Question 24

what are the landmarks of the pelvic outlet?

Answer 24

• sacrococcygeal joint
• ischial spines
• symphysis pubis

Question 25

What is moulding (of the fetal skull)?

Answer 25

the change in the shape of the fetal head as it passes through the birth canal, possible due to slight bending and overlapping of the bones of the fetal skull

Question 26

What is attitude of the fetal skull?

Answer 26

the degree of flexion or extension of the of the fetal head relative to the neck

Question 27

What part of the fetal skull is the face?

Answer 27

extends from the orbital ridges to the junction of the chin and neck. the chin is called the mentum

Question 28

What are the names and measurements of the two transverse diameters of the fetal skull?

Answer 28

The bitemporal diameter is 8.2cm and the biparietal diameter is 9.5cm

Question 29

What are the presenting diameters in a well flexed vertex presentation?

Answer 29

the suboccipitobegmatic (SOB) diameter (9.5cm) and the biparietal diameter (9.5cm). These diameters form a circle and are most effective at dilating the cervix.

Question 30

What are the presenting diameters in a deflexed vertex presentation?

Answer 30

the occipitofrontal (OF) diameter (11.5cm) and the biparietal diameter (9.5cm). These diameters mean the cervix tends to dilate more readily in the transverse than in the AP diameter.

Question 31

What is the presenting diameter in an extended or brow presentation?

Answer 31

the mentovertical (MV) diameter of 13.5cm, if this persists it makes vaginal birth less likely

Question 32

What is the presenting diameter in a face presentation?

Answer 32

the submentobregmatic (SMB) diameter of 9.5cm

Question 33

What are the mechanisms of normal labour?

Answer 33

- descent (moves down into the pelvis in longer transverse diameter) - flexion (head moves from deflexed to fully flexed and occiput becomes leading part) - internal rotation (pressure on the curved pelvic floor means the occiput rotates anteriorly one eighth of a circle to lie in the longer AP diameter causing a slight twist of babies head relative to shoulders, imagine LOA to direct OA) - crowning (occiput slips under pubic arch, crowning is when the widest biparietal diameter is born) - extension (the fetal head extends, pivoting on the suboccipital region around the pubic bone, the sinciput, face and chin sweep the perineum and are born) - restitution (the slight twist from internal rotation corrects by moving one of a circle to realign with shoulders) - internal rotation of the shoulders (pressure on pelvic floor causes anterior shoulder to rotate anteriorly to sit under the symphysis pubis, in AP diameter), causing external rotation of the head (looking towards the mothers leg) - lateral flexion (the remainder of the baby is born as the spine bends sideways through the curved birth canal)

Question 34

What are the common principles to all mechanisms of labour?

Answer 34

- descent takes place - whatever part hits the pelvic floor will rotate to come under the symphysis pubis - the part the emerges from the pelvic outlet will pivot around the pubic bone

Question 35

What are 13 signs of occipitoposterior position?

Answer 35

- dip at umbilicus - difficult to palpate fetal back - limbs on both sides of midline - high or deflexed head - fetal heart heard in midline or flank - prolonged or start-and-stop labour - backache in labour - early SROM - incoordinate contractions - slow descent - early urge to push - anterior fontanelle in anterior position on vaginal examination (confirmed by location of sagittal suture and post fontanelle) - dilatation of the anus, vaginal gaping while fetal head is barely visible, excessive bulging of the perineum

Question 36

What is the mechanism of the long rotation in posterior labour?

Answer 36

- good contractions cause descent and flexion, the occiput is the first part to hit the pelvic floor - it then rotates 3/8ths of a circle anteriorly to lie in the direct OA position under the symphysis pubis - the shoulders rotate 2/8ths of a circle into the opposite oblique, the neck is slightly twisted. - labour continues as in normal mechanisms of OA labour

Question 37

What is the mechanism of the short rotation in posterior labour?

Answer 37

- the fetal head doesn't flex sufficiently, the sinciput is the first part to hit the pelvic floor - it then rotates 1/8th of a circle to lie in the direct OP position under the symphysis pubis - the baby is born facing the pubic bone

Question 38

What are the three possible courses of labour where there is a posterior position?

Answer 38

- long rotation to OA - short rotation to OP - deep transverse arrest

Question 39

According to spinningbabies, in general which of the ROP and LOP positions are more likely to undergo the short rotation or the long rotation?

Answer 39

ROP tends towards straighter fetal back and deflexed head, so more likely to take short rotation to birth OP. LOP puts babies back against maternal liver, so may encourage curling of the babies back, flexion of the head and long rotation to birth OA.

Question 40

What is deep transverse arrest?

Answer 40

- where an OP baby begins a long rotation but flexion is not maintained. The occipitofrontal diameter (of a deflexed head) may become trapped between the ischial spines. - on VE the sagittal suture may be found in the transverse diameter of the pelvis, with both fontanelles palpable, with no advance at the level of ischial spines.