Obstetrical anatomy


Clinical examination of pregnant woman

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Obstetricians use the knowledge of anatomy in their day to day practices.

General

The height of a pregnant woman is very vital in the elucidation of the outcome of labor. Generally, a woman below the height of 5ft is considered to have a small pelvis and will have difficulty in the delivery of baby.

In the condition recognised as dystocia dystrophia syndrome, the patient is short, very sturky and has an android (male) type of pelvis. The patient is very likely to have difficulty in delivery.

 

Fundal height measurement is utilised in the determination of the age of gestation by the method of palpation in routine obstetrical care. The fundus rises from the false pelvis and is palpable through the anterior wall of the abdomen at the hypogastrium at 8-12 weeks of gestation.

 

At the week 16 weeks of gestation the uterine fundus has reached midway between the umbilicus and the pubic symphysis. At 20 weeks it has reached the umbilicus. Thereafter it increase by about 2.5cm for every 2 weeks. It reaches midway between the umbilicus and the xiphoid process at 28 weeks of gestation.

 

 

Clinical examinations

Pelvimetry

There are three anatomical cavities of the lesser pelvis

  • Inlet

  • Midcavity

  • Outlet

 

 

The inlet is placed at the level of the pelvic brim which extends from the superior surface of the symphysis pubis, the pubic crest, the pectineal line and the pectin pubis to the iliopubic eminence and finally ends posteriorly at the sacral promontory.

 

The midcavity is placed at the level of ischial spines.

The outlet is placed at the inferior aperture of the lesser pelvis and its boundaries are osteofascial. It runs anteriorly from the inferior surface of the pubic symphysis to the inferior surfaces of the ischiopubic rami and then to the margin of the sacrotuberous ligaments and finally to the posterior aspect at the coccyx.

The inlet cavity can be measured by both clinical assessment and X ray pelvimetry.

The dimensions under the Xray pelvimetry are as follows

Anteroposterior diameter of the inlet – this is measured from the superior surface of the pubic symphysis to the sacral promontory.

Transverse diameter measures the widest portion between the lateral walls of the inlet.

Oblique diameter measures the diameter from the iliopubic eminence to the sacroiliac joint of the other side.

 

In clinical assessment of the inlet dimensions, the diagonal conjugate diameter is the single most important measurement to be taken. It measures the diameter from the inferior surface of the pubic symphysis to the sacral promontory. This is best assessed by vaginal (pelvic) examination with the use of the index finger. The examine first of all measures his own index finger length and determines during examination whether he can feel for the sacral promontory or not.

  1. If the examining finger just touches the sacral promontory then the diagonal conjugate diameter is equal to the length of he examining finger.

  2. If the examining finger does not reach the sacral promontory then the diagonal conjugate diameter is less then the length of the examining finger.

 

The midcavity can also be assessed by Xray and clinical pelvimetry. The dimensions are as follows

 

In cm

                                   Trans          Oblique          *AP

Inlet                              12.5             11.5           10

Mid-pelvis                     11.5            11.5           11.5

Outlet                            10               11.5           12.5

 

*AP- anteroposterior

Midcavity
The most used methods of clinical assessment of the dimensions of the midcavity are

1.      Morphology of ischial spines. The spines may be heavy and long or small. If heavy then they should decrease the transverse diameter of the midcavity and probably cause transverse arrest of the head of the baby during labour.

2.      Sacropspinous diameter is measure by insinuating two fingers into the interval between the ischial spine and the lateral border of the sacrum during pelvic examination. The width of the two examining fingers are initially noted to determine the approximate diameter of the sacrospinous interval.

Outlet cavity

Outlet dimensions are assessed by both Xray and clinical pelvimetry – as follows

·        Anteroposterior diameter

·        Oblique diameter

·        Transverse diameter

Clinically the following are important measurements of the outlet cavity.

1.      Intertubersichial diameter which measures the diameter between the ischial tuberosities. It is best assessed with the knuckle of the examining hand pressed firmly in the interval between the ischial tuberosities. It should be about 8cm or above.

2.      Subpubic angle is assessed by the use of the angle between the index finger and the thumb in determining the angle between the two inferior rami of the pubis.

The following problems may be identified by the use of clinical assessment in labor

  • 1.      General contraction of all 3 types of cavities- possible justominor gynecoid pelvis

  • 2.      Contraction only at the outlet which may point to an android pelvis

  • 3.      Contraction at the midcavity with heavy spines which may be indicative of anthropoid pelvis.

 

  • Android pelvis is associated with outlet dystocias (difficulty labour caused by outlet problem) and engagement of the fetal head may be delayed by encroachment of  the sacral promontory. Occipitolateral and occipitoposterior positions are common (see below).

  • Gynecoid pelvis is the normal female pelvis but it may be generally contracted in the gynaecoid pelvis of a small woman which is called the justominor variety.

  • Anthropoid pelvis is associated with transverse arrest of the head of the fetus in labour because of the usual prominence of the ischial spines.

  • Flat pelvis is always associated with severe with severe labour difficulty and its dimensions of the inlet are heavily contracted.

 

Pelvic types (After Caldwell and Molloy)

Gynecoid

Android

Anthropoid

 

Gynaecoid

  1. Inlet: Rounded or slightly heart shaped. Ample anterior and posterior segments.

  2. Sacrum: Curved, average length

  3. Sacrosciatic notch: Medium width

  4. Side walls (lateral view): Straight, divergent or convergent.

  5. Side walls (lateral view): Straight, divergent or convergent

  6. Interspinous diameter: Wide

  7. Pubic arch: Curved

  8. Subpubic angle: Wide.

  9. Intertuberous diameter: Wide.

 

 

Android

  1. Inlet: Wedge shaped or rounded triangle. Posterior segment usually flat, and narrow and pointed.

  2. Sacrum: Straight with forward inclination

  3. Sacrosciatic notch: Narrow

  4. Side wall: Usually convergent

  5. Side walls (lateral view): Usually convergent

  6. Interspinous diameter: Shortened

  7. Pubic arch: Straight

  8. Subpubic angle: Very narrow.

 

 

Anthropoid

  1. Inlet: Anteroposterior ovoid with length of anterior and posterior segment increased: Transverse diameter reduced.

  2. Sacrum: Normally curves, but long and narrow.

  3. Sacrosciatic notch: Wide, shallow

  4. Side walls: Straight

  5. Side walls (lateral view): Often straight

  6. Interspinous diameter: Shortened.

  7. Pubic arch: Slightly curved

  8. Subpubic angle: Narrow

  9. Intertuberous diameter: Often shortened.

 

 

Platypelloid

  1. Inlet: Transverse ovoid; increased transverse and anteroposterior diameter of both segments.

  2. Sacrum: Curved short.

  3. Sacrosciatic notch: Slightly narrowed.

  4. Side walls: Straight or slightly divergent

  5. Side walls: (lateral view): straight or divergent

  6. Interspinous diameter: Increased

  7. Pubic arch: Increased

  8. Subpubic angle: Wide.

  9. Intertuberous diameter: Wide.

 

 

Male and female pelves

The pelvis of male and female are configured differently in order to give the advantage of child birth to the female.

  • The male pelvis has a heart shaped pelvic brim (due to protrusion of sacral promontory) while the female has an oval or rounded one.

  • The female pelvis has an obtuse pubic angle while the male has an acute one.

  • The female osteological features are non prominent (e.g. sacral promontory, ischial spine, osteological lines). In the male these are prominent.

  • Obturator foramen is round in the male but oval in female

  • False pelvis is deep in male but shallow in female

  • Pelvic canal is convergent in male but has parallel walls in female.

Anatomy of labour

During labour a curve is formed by the birth canal called the curve of Carus. This curve is formed by the prolongation of  perineum below its normal position inferiorly and by the vagina superiorly.

 

 

Curve of Carus

Pelvic floor 1. pubococcygeus 2 iliococygeus 3 arcus tendineus 4 rectum 5 puborectalis 6 levator prostatae

 

The curve of Carus has to be negotiated by the fetus during labour. The bend of the curve is at the level of the ischial spines and the fetus hits this with its head and the head is made to rotate as it hits the pelvic floor at this bend. The pelvic floor is made up of the levator ani and its constituent muscles are as follows

  • Pubococcygeus

  • Iliococcygeus

  • Ischiococcygeus

The shape of the pelvic floor forms a gutter which is directed downwards and anteriorly. It is the bend of this gutter tht is accentuated with the formation of the Carus curve.

 

Once the head of the baby hits the pelvic floor the pelvic gutter directs the head along its own configuration, hence the rotation of head.

 

 

Mechanism of labor

Fetal skull

Fetus

1 anterior fontanelle 2 frontal eminence 3 sinciput 4 orbital ridge 5 glabella 6 femporal bone 7 chin 8 submentobregmatic diameter 9 temporal suture 10 suboccipitobregmatic diameter 11 occipital bone 12 poterolateal fontanelle 13 lambdoidal suture 14 posterior fontanelle 15 parieral eminence 16 coronal suture 17 sagittal suture 18 metopic suture19 biparietal diameter 20 bitemporal diameter

 

Descent of head

 

The head of the fetus at the beginning of labor is usually in an occipitotransverse (or occipitolateral) position with a posterior or anterior asynclitism- i.e. the parietal bone presenting. 

As its descent continues to the level of the ischial spines, the occiput rotates at the false pelvis into the left position in most cases. The head is now in a left occipitoanterior position. This initial deflection of the head to the occipitoanterior position is due to the fullness of the sigmoid colon which pushes the head slightly upwards. The pelvic corridor which is a wide opening connecting the abdominal cavity with the false pelvis prevents a similar condition occurring in the other side.

 

As the head hits the pelvic floor, internal orientation occurs and the head rotates through an angle of 45o. This rotation brings the head into a direct occipitoposterior position, a position it keeps till it crowns.

Further descent will cause the head to be delivered through a movement of extension so tht the occiput is delivered first.

1 fetal head 2 anterior fontanelle 3 posterior fontanelle 4 occiput 5 path of descent of head in labour

 

As the shoulder enters the pelvic floor and proceeds to pass through the same process as the head, the bisacromial diameter passes through first and becomes the left acromioanterior presentation as a result of the sigmoid (pelvic) colon. It then turns to a direct acromioanterior position at the pelvic floor. At the stage in which the shoulder is at oblique position (i.e. left acromioanterior position) the head is already delivered and it proceeds to undergo restitution by which it rotates through an angle of 45o to the left in order to be in alignment with the orientation of the shoulder at the pelvis. The head then undergoes external rotation to an occipitolateral postion so that the occiput lies next to the left thigh. At this stage the shoulder has hit the pelvic floor and it has orientated itself into direct anteroposterior position. This is the main reason for the external rotation of the head to the occipitolateral position in order to bring the head in normal alignment with the shoulder.

 

The anterior shoulder then slips under the symphysis pubis and is delivered. This is followed by the posterior shoulder. The rest of the baby’s body allows the delivery of the shoulders. This normal process of labour may develop complication sat any stage during labour.

 

 

 

 

 

Caesarean section

 37 steps in Caesaraean section

Linea alba incision

Pfannenstiel incision

1 Wash your hands with soap and then scrub the hands thoroughly with savlon (or other scrubbing agent)

 

 

2 Put on sterile gloves and scrub the patient’s anterior abdominal wall skin thoroughly

 

 

3 Scrub the skin with savlon, spirit, then TBC or iodine in that order.

 

 

4 Rescrub your hands and ten gown and glove.

 

 

5 Proceed to place the simplest of the drapes on the anterior abdominal wall, allowing a small hole for incision.

 

 

6 A single sheet with a middle hole with suffice in most cases.

 

7 When everything is set (including trolley) make your first incision into the skin and clap major vessels in the subcutaneous tissue. An incision may be on the linea alba running form the umbilicus to the symphysis pubis or Pfannenstiel incision which is a curved transverse skin crease incision at the hypogastrium.

 

8  Identify the Scarpa’s and Camper’s fasciae and incise them (transversely in Pfannenstiel and longitudinally in linea alba incisions) together with linea alba.

9 Incise the anterior wall of rectus sheath

10.Separate the rectus muscle

 

 11 Incise the posterior wall of rectus sheath only in linea laba incision. Therw is no posterior wall of rectus from midway between the umblilicus and the symphysis pubis (arcuate line of Douglas) and the symphysis pubis.

 

 

 12 Incise the parietal peritoneum longitudinally in both Pfannenstiel and linea alba incisions and enter the abdominal cavity.

 

 

 13 Put in two large abdominal packs with long tails held out with artery forceps into the two paracolic (paravertebral) gutters of the abdominal cavity. After this remove all swabs and use only large abdominal packs.

 

 

14 Incise the uterovesical peritoneum at the level of the lower segment of the uterus after clearing the bladder out of the way with the user of Doyen’s retractor.

 

15 Before incising the uterus, put a stay stitch on its lower portion to help identify this portion (ie the portion inferior to the incision) which usually slips under the bladder (at its posterior aspect) after the delivery of the baby.

16 Incise the uterus with a knife above your stay stitch and then widen your incision carefully with scissor. Avoid the extreme angles of the uterus and the broad ligament in order not to enter the uterine vessels or the ureters. Avoid widening the incision with your fingers; use knife carefully.

 

 

17 Suck up the amniotic fluid from the uterus vigorously with a suction machine- a simple foot operated machine will suffice

 

 

 18 Deliver the baby’s head into your uterine wound and proceed to give fundal pressure in order to push the baby out of the wound.

 

 

 19 Deliver the remains of the baby; clamp the cord in any two places separated by a small interval and divided between the your clamps.

 

 

 20 Give the baby to the midwife or if present, the paediatrician.

 

 

 21 Ask the circulation nurse or the anesthetist to administer IV ergometrine 0.5mg after the delivery of he baby.

 

 

 22 Deliver the placenta by manual scooping from its uterine position. Proceed with vigorous suction of blood which will now make the entire field of operation very messy indeed.

 

 

 23 Apply Green-Amytage clamps to the edges of the uterine wound to control bleeding from the cut edges and suck the uterus

 

 

 24 After sucking the uterus dry, proceed to suture the two layers of he wound together. Start at one angle and proceed to the other.

 

 

 25 Use size 1 or 2 chromic catguts on round bodied stout needle.

 

 

 26 Hold the knotted sutures at the uterine angles of the wound on arterial clamps. Place two rows of stitches without locking. Always avoid the uterine mucosa as much as possible.

 

 

 27 Lock the first row of stitches and run the 2nd row of stitches without locking. Always avoid the uterine mucosa as much as possible.

 

 

 28 After the successful stitching of uterine wound, knot the two end stitches at the uterine angles.

 

 

 29 Mop the wound to see if there is further bleeding. If this occurs, proceed to give the Z-type of lock stitch in order to occlude the open vessel; first pass the needle in a longitudinal fashion and then transverse, then the knot. Make sure you take a god muscular bite.

 

 

 30 Reconstruct the utero-vesical peritoneum using the Doyen’s retractor for proper visualisation.

 

 

 31 Remove the abdominal packs. Do a general toileting with warm saline and suction.

 

 

 32 Count all swabs and make sure no swab is unaccounted for (this is for the circulation nurse)

 

 

 33 Close the peritoneum with chromic 2/0 catgut using continuous stitches

 

 

34 Close up the anterior wall of rectus sheath with 1 chromic catgut using continuous stitches.

 

35 Approximate the Scarpa’s and Camper’s fasciae together using plain catgut 2/0 with interrupted stitches (this step may be avoided).

36 Stitch up the skin using silk with interrupted stitches. Use subcuticular stitches preferably with monofilament nylon for Pfannestiel incision. Dress your wound with sterile gauze and tapes after mopping and cleaning with hibitane.

37 Perform a vaginal toileting with swab on sticks (swab on sponge holding forceps) three times.

 

Paracervical block

Anesthetic solution is delivered into the paracervical tissue (around the cervix) in two sites -at 3 and 4 o'clock and 8 and  o'clock. It blocks the nerves which supply the uterus and cervix, which may include uterine nerves and branches of the pudendal nerve.

 

Pudendal block

There are two methods of pudendal block

  • Vaginal

  • Extravaginal

The vaginal method is performed directly inside the vagina. The ischial spine is sought for and the anesthetic solution is delivered medial t it to reach the pudendal nerve which hooks around it. In extravaginal method, the ischial tuberosity is sought for and anesthetic solution is delivered posterior and medial to it it.1 ischial spine 2 ischial tuberosity

 

Episiotomy

Episiotomy is a procedure used to widen the vagina in order to have an easy vaginal delivery. It is recommended in all first deliveries (primigravids) and all cases of soft tissue dystocias (difficult labor caused by soft tissue rigidity). A Mediolateral, B J shaped mediolateral C Median D Schuchardt. Episitomy incision passes through mucous membrane of vagina, superficial transverse perineal muscle and perianal fat. The median incision passes through the central perineal tndon. A lateral incision is avoided because it destroys the greater vestibular gland.

1  ischiocavernosus 2 bulbospongiosus 3 superficial transverse perineal muscle 5 central perineal tendon 6 anus

Repair of episiotomy

 

 

 



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