The maternal pelvis, the fetal skull, mechanisms of normal labour and malpositions Flashcards
What part of the fetal skull is the sinciput?
- extends from the orbital ridges (bridge of the nose) to the anterior fontanelle and the coronal suture
- the sinciput is also called the brow.
What part of the fetal skull is the vertex?
Extends from the anterior fontanelle and the coronal suture to the posterior fontanelle and the lambdoidal suture
What part of the fetal skull is the occiput?
Extends from the posterior fontanell and the lambdoidal suture to the base of the skull
What are the bones of the fetal skull?
2 frontal bones
2 parietal bones
2 temporal bones
1 occipital bone
What is the suture between the two frontal bones?
the frontal suture
What is the suture between the frontal bones and the parietal bones?
the coronal suture
What is the suture that runs between the anterior fontanelle and the posterior fontanelle?
the sagittal suture
What is the suture that runs between the parietal bones and the occiput?
the lambdoidal suture
What is the shape, alternative name and position of the anterior fontanelle?
- diamond shaped
- also called bregma
- at the junction of the frontal, coronal and sagittal sutures
What is the shape, alternative name and position of the posterior fontanelle?
- triangular
- also called lambda
- bound by the sagittal and lambdoidal sutures
What are the 3 sections of the true pelvis?
- brim
- cavity
- outlet
What are the landmarks of the pelvic brim from back to front? Think about how these landmarks relate to position
- sacral promontory (Direct OP)
- sacroiliac joint (LOP/ROP)
- iliopectineal line (LOT/ROT)
- iliopectineal eminence (LOA/ROA)
- upper border of the symphysis pubis (Direct OA)
What are the 4 types of pelvis?
- 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
What is the shape of the pelvic brim, and what are its anteroposterior, oblique and transverse diameters?
Oval or heart-shaped AP 11cm O 12cm T 13cm wider in the transverse diameter
What is the shape of the pelvic cavity, and what are its anteroposterior, oblique and transverse diameters?
Circular AP 12cm O 12cm T 12cm same width in AP and T diameters, allows space for rotation
What is the shape of the pelvic outlet, and what are its anteroposterior, oblique and transverse diameters?
Diamond AP 13cm O 12cm T 11cm wider in the AP diameter
What is the anatomical name for the top of your hip bones?
Iliac crests
What is the anatomical name for your sit bones?
Ischial tuberosities
What are the bones of the pelvis?
- 2 innominate bones each with an ilium, ischium and a pubic bone
- sacrum
- coccyx
What is the meeting point of your pubic bones called?
the pubic symphysis
What is the name for the underside of the pubic symphysis?
the pubic arch
What are the 4 pelvic joints?
- the pubic symphysis between two pubic bones - two sacroiliac joints between sacrum and ilium - the sacrococcygeal joint between sacrum and coccyx
What is the curve of carus?
- 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
what are the landmarks of the pelvic outlet?
- sacrococcygeal joint
- ischial spines
- symphysis pubis
What is moulding (of the fetal skull)?
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
What is attitude of the fetal skull?
the degree of flexion or extension of the of the fetal head relative to the neck
What part of the fetal skull is the face?
extends from the orbital ridges to the junction of the chin and neck. the chin is called the mentum
What are the names and measurements of the two transverse diameters of the fetal skull?
The bitemporal diameter is 8.2cm and the biparietal diameter is 9.5cm
What are the presenting diameters in a well flexed vertex presentation?
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.
What are the presenting diameters in a deflexed vertex presentation?
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.
What is the presenting diameter in an extended or brow presentation?
the mentovertical (MV) diameter of 13.5cm, if this persists it makes vaginal birth less likely
What is the presenting diameter in a face presentation?
the submentobregmatic (SMB) diameter of 9.5cm
What are the mechanisms of normal labour?
- 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)
What are the common principles to all mechanisms of labour?
- 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
What are 13 signs of occipitoposterior position?
- 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
What is the mechanism of the long rotation in posterior labour?
- 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
What is the mechanism of the short rotation in posterior labour?
- 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
What are the three possible courses of labour where there is a posterior position?
- long rotation to OA
- short rotation to OP
- deep transverse arrest
According to spinningbabies, in general which of the ROP and LOP positions are more likely to undergo the short rotation or the long rotation?
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.
What is deep transverse arrest?
- 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.
