Ministry of Health of Ukraine
Vinnitsya National Pirogov Memorial Medical University
METHODOLOGICAL INSTRUCTION
for practical lesson
“Bony pelvis. Fetus as an object of labor”
MODULE 1: Physiology of pregnancy, labor and puerperium
CONTEXT MODULE 1: Physiology of pregnancy, labor and puerperium
Aim; to learn measuring the pelvis sizes, to estimate pelvis from the obstetrics'
point of view, to learn the structure of pelvic floor, sizes of fetus head.
Basic level:
1. Structure of pelvis.
2, Main anatomic landmarks for measuring pelvis.
3. Shape and sizes of pelvic planes.
4. External sizes of pelvis.
5. Structure of pelvic floor.
6. Structure and sizes of in-term fetus head.
Inter-subject integration
Previous
Requested skills
disciplines
Anatomy
To describe anatomy of female pelvis, external and
internal female genitalia, pelvic floor, to define parts
pf pelvis and their dimensions. To know the
dimensions of fetal head
STUDENTS, INDEPENDENT STUDY PROGRAM
I. Objectives for Students' Independent Studies
You should prepare for the practical class using the existing textbooks and lectures.
Special attention should be paid to the following:
1. What bones is the pelvis consist of?
2. Differences between female and male pelvis.
3. The main landmarks of the female pelvis.
4. Planes of pelvis and their sizes.
5. Main and additional external pelvis sizes.
6. Methods of estimation of true conjugate.
7. Solovjov’ index. It’s significance for estimation of internal pelvic sizes.
8. Mikhaelis’ rhomb. Its significance for estimation of result of labor.
9. Structure of pelvic floor.
10. Structure and sizes of fetal head.
11. Body sizes of in-term fetus,
12. Sutures and fontanels and their significance for diagnosing the situation of
fetal head.
13. Definition of «large segment» of fetal head.
14. Relationship of fetal head to pelvic planes.
Key words and phrases: Bony pelvis, pelvic inlet, greatest diameter, midplane,
pelvic outlet, anterior and posterior fontanels; occipitofrontal dimension,
subocccipitobregmatic diameter, occipitomental dimention; biparietal, bitemporal
distance; sagittal, frontal, lambdoid suture.
Summary
Planes and Diameters of the Pelvis.
The pelvis has four imaginary planes:
1 - plane of the pelvic inlet,
2 - plane of greatest pelvic dimensions,
3 - the plane of the midpelvis (least pelvic dimensions),
4 - the plane of the pelvic outlet.
Pelvic inlet is bounded posteriorly by the promontory, laterally by the linea
terminaiis, and anteriorly by the horizontal rami of the pubic bones and symphysis
pubis. Four diameters of pelvic inlet are described: the anterposterior (l 1cm), the
transverse (13cm), and two obliques (12 cm from left or right sacroiliac
synchondroses to the iliopectineai eminence on the opposite side of the pelvis).
The plane of greatest pelvic dimension extends from the middle of the posterior
surface of the symphysis pubis through the ischial bones over the middle of the
acetabulum to the junction of second and third sacral vertebrae. Its anteroposterior
and transverse diameters are 12,5 cm.
The midpelvis at the level of the ischial spines is particular importance following
engagement of the fetus head in obstructed labor. The transverse diameter
(interspinous) is 10,5 cm and anteroposterior is 11cm.
Pelvic outlet has two diameters: anteroposterior extends from the lower margin of
the symphysis pubis to the tip of the coccys (9,5cm) and transverse diameter
between the inner edges of the ischial tuberosities 11,5 cm.
The main external pelvic sizes:
D. Spinarun - distance between anterior superior iliac spines from both sides. It has
25-26 cm.
D. Crlstarum ~ distance between iliac crista from both sides. It is 28-29 cm.
D. Trochanterica - distance between trochanter majors from both sides. It has 3132 cm.
C. Externa - distance between midpoint of superior surface of the symphysis pubis
and suprasacralis fossa.
Michael's' rhomb. It has 4 angles. The upper angle is located in the suprasacralis
fossa. The lower angle is situated in the apex of coccyx, and laterally angles are
situated in the posterior superior iliac spines. In the women with normal pelvis
rhomb has regular form. It vertical size has 11 cm, and horizontal size is 10 cm.
Solovjov' index. It is estimated by the circumference of radiocarpal joint. It has 1416 cm and indicates into bones' pelvic thickness.
The additional external pelvic sizes:
Lateral conjugate —is a distance between the anterior superior iliac spine and
posterior superior iliac spine of the same iliac bone. It has 14.5-16 cm
Oblique conjugate -is a distance between the right anterior superior iliac spine to
the left posterior superior iliac spine. It has 14.5-16 cm
Anteroposterior diameter of the pelvic outlet is a distance between the lower part
of symphysis pubis and apex of the coccyx. It has 9.5 a
Transverse diameter of the pelvic outlet is a distance between t posterior portions
of the ishial tuberosities. It has 11.5 cm.
The main internal pelvic sizes:
The widest anteroposterior diameter of the pelvic inlet is called obstetric
conjugate. It runs from the upper midpoint of the symphysis to the promontorium.
It has 11 cm. It is one of the most important pelvic dimension.
Indirect ways of true conjugate estimation:
1. An estimate of the obstetric conjugate is made by determining the diagonal
conjugate. During a vaginal examination, the physician tries to reach the sacral
promontory with the middle finger of the examining, hand. The index finger of the
free hand marks the point where the lowt border of the pubic symphysis impinges
on the examining hand proximal 1 the metacarpophalangeal joint of the index
finger. This measurement, th diagonal conjugate, usually exceeds the obstetric
conjugate by 1.5 to 2 en
2. External conjugate exceeds the obstetric conjugate by 9 cm.
3. Vertical dimension of Michael's, rhomb equal obstetric conjugate
Fetal head. Obstetrically, the head of fetus is most important part, since an
essential feature of labor is an adaptation between the fetal head and the maternal
bone pelvis. Only a comparatively small part of the head of the fetus at term is
represented by the face; the rest is composed of the firm skull, which is made up of
two frontal, two parietal, and two temporal bones, along with the upper portion of
the occipital bone and the wings of the sphenoid. The bones are not united rigidly
but are separated by membranous spaces, the sutures. The most important sutures
are sagittal, between the two pariete bones, frontal between the two frontal bones,
the two coronal, between the frontal and parietal bones; and the two lambdoid,
between the posterior margin of the parietal bones and upper margin of the
occipital bone.
Where several sutures meet an irregular space forms, which is enclosed by a
membrane and designated a fontanel. The greater, or anterior fontanel is a lozengeshaped space situated at the junction of the sagittal and coronal sutures. The lesser,
or posterior fontanel is represented by a small triangular area at the intersection of
the sagittal and lambdoid sutures. Both may be felt readily during labor, and their
recognition gives important information concerning the presentation and position
of the fetus.
The circumference and their average lengths are: suboccipitobregrnatic 32cm (9,5
cm); suboccipitofrontal 33cm (l0cm); occipitomental 38cm (13cm); occipitofrontal
34cm (12cm); sublinguobregmatic 32cm (9,5cm), bi parietal (9,5cm); bitemporal
(8cm). Circumferences of shoulders is 34cm (12cm), circumferences of pelvic part
is 28cm (9,5cm).
II. Tests and Assignments for Self - assessment.
Multiple Choice.
Choose the correct answer / statement:
1. The innominate bones are composed of the:
A - Ilium; B - Tibia; C - Ischium; D - Pubis; E - A, B, and C; F - A, C, and D.
2. The false pelvis and true pelvis are separated by the:
A - Acetabulum; B - Sacrospinous ligament; C - Linea terminalis; D - Obturator
membrane; E - Obturatory foramen.
3. The false pelvis is separated from the true pelvis by the plane of: A - Pelvic
inlet;
B - Greatest diameter;
C - Least diameter;
D - Pelvic outlet.
Real - life situations to be solved:
4. During examination of female pelvis it was reveal that distantia spinarum 26cm, distantia cristarum -29 cm, distantia trochanterica - 30 cm, conjugata
externa -21 cm. Solovjovs' index is 15 cm. Definite the size of true conjugate.
III. Answers to the Self- Assessment.
1. F. 2. C. 3. A. 4. Size of true conjugate is 12 cm. Pelvis has normal sizes.
Students must know:
1 .Structure of female pelvis.
2. Planes of true pelvis and their sizes.
3. Main and additional sizes of pelvis.
4. Structure of interm fetus head.
5. Definition of “large segment” of fetal head.
6.Relationship of fetus head to pelvic planes.
Student should be able to:
1.To measure of pelvic sizes. 2.To measure diagonal conjugate. 3.To measure true
conjugate.
4.To measure Solovjovs' index and to apply it for estimation of internal sizes of
pelvis.
5.To definite the relationship of fetus head to pelvic planes.
References:
1. Danforth's Obstetrics and gynaecology. - Seventh edition- 1994. - P. 108- 112.
2. Basic Gynecology and Obstetrics. - Norman F. Garit F. Gary Cunningham. 1993. - P. 299 - 303