Anatomical norms and criteria

47. An anatomically narrow pelvis is a pelvis whose size can interfere with the delivery process. One or more sizes of such a pelvis is 1.5-2 cm less than normal. Pathology occurs due to congenital anomalies, rickets, polio, poor living conditions in childhood, stress, intensive sports, traumatic injuries and bone tumors

Causes of development

A narrow pelvis occurs under the influence of many different factors, including developmental anomalies, cerebral palsy, malignant and benign neoplasms, osteomalacia, scoliosis, kyphosis, spondylolisthesis, and lameness of various origins (for example, as a result of a mismatch in the length of limbs or innervation disorders). In classical studies dealing with the problem of anatomically narrow pelvis , rickets, bone tuberculosis , polio, poor nutrition and hard work in childhood were usually listed as the main causes, but these factors are rarely identified today. 

An anatomically narrow pelvis can develop under the influence of changes in the hormonal background during puberty. Experts note that the increase in the number of patients with a transversely constricted pelvis is associated with acceleration, because with intensive growth, the pelvis increases faster in length than in width

Anatomical norms and criteria

To determine the size of the pelvis, the gynecologist makes standard measurements between certain bone protrusions, using a pelvis meter or a centimeter tape. The minimum distance between the anterior spines of the iliac bones is 25 cm, between the iliac ridges – 28 cm, between the large trochanters-30 cm, between the upper corner of the Michaelis rhombus and the upper edge of the pubic joint (external conjugate) - 20 cm. If one or more of these dimensions are smaller than normal, there is reason to suspect an anatomically narrow pelvis. 

The fifth size (the distance between the most prominent part of the base of the sacrum and the pubic joint, the true conjugate) is determined during a gynecological examination or by calculations. Normally, the specialist cannot reach the protruding part of the sacrum during the examination on the chair, so the calculation of the true conjugate is carried out taking into account the size of the external conjugate and the circumference of the wrist, or taking into account the size of the diagonal conjugate and the circumference of the wrist. It is also believed that the size of the true conjugate corresponds to the height of the Michaelis rhombus and the Frank index (the distance between the jugular notch and the spinous process of the seventh cervical vertebra). Normally, the true conjugate value is 11 cm. 

Classification

Common variants of an anatomically narrow pelvis are:

• Flat – reduced sagittal size
• Cross-narrowed – reduced front size
• Generally uniform narrowed – all dimensions are reduced while maintaining the normal shape. 

Rare variants of an anatomically narrow pelvis include:

• Oblique and oblique-with asymmetrical halves
• Narrowed due to abnormal bone protrusions (exostoses, neoplasms)
• Common constricted flat pelvis
• Other forms of narrowed pelvis. 

The degree of pelvic narrowing is determined taking into account the true conjugate value: 1 degree-9-11 cm; 2 degree-7.5-9 cm; 3 degree-5-7. 5 cm; 4 degree-less than 5 cm. Anatomically narrow pelvis of the first degree occurs in more than 90% of cases, the second degree – in 8-9% of cases, the third-in 0.2-0.3% of cases. The fourth degree is extremely rarely diagnosed. 

With a transversely narrowed pelvis, the degree of narrowing is estimated using the size of the entrance to the small pelvis: 1 degree-11.5-12.5 cm; 2 degree-10.5-11.5 cm; 3 degree-less than 10.5 cm. 

Diagnostics

The diagnosis is made taking into account the history of life, the results of general and gynecological examination, and ultrasound data. The doctor finds out if there have been illnesses, surgeries, and injuries that increase the likelihood of developing an anatomically narrow pelvis. During an external examination, the specialist takes all the necessary measurements, determines the patient's height and weight, reveals lameness, limited joint mobility, and curvature of the spinal column. When performing a gynecological examination, it determines the size of the true conjugate. 

To determine the treatment plan, the obstetrician-gynecologist collects information about previous births and the number of children born. Then the patient with a suspected anatomically narrow pelvis is referred for ultrasound. In difficult cases, X-ray pelviometry is used. The study is prescribed only if there are sufficient indications and is carried out either before the start of gestation, or for a period of more than 38 weeks. The decision on the need for X-ray examination is made if the risk of birth trauma exceeds the risk of minimal radiation exposure to the fetus. 

Indications for X-ray pelviometry with an anatomically narrow pelvis are the first pregnancy in a patient over 30 years of age, complicated childbirth , infertility, miscarriage, increased risk of intrauterine pathology, endocrine disorders, somatic diseases, developmental abnormalities and previous skeletal injuries of the mother, suspicion of a discrepancy between the size of the fetal head and the size of the patient's pelvis. A promising diagnostic method isHowever , the widespread use of this instrumental technique is difficult due to its high cost. 

Childbirth with an anatomically narrow pelvis

With a narrow pelvis, incorrect fetal positions are often detected. Usually, pelvic presentation is diagnosed, less often-transverse and oblique presentation. Due to the insufficient width of the pelvis in late gestation, the fetus is located too high and presses on the diaphragm, causing shortness of breath and heart rhythm disorders. There is a tendency to overstrain, which, due to the loss of mobility of the skull bones, further increases the risk of complications in childbirth. 

Absolute indications for caesarean section are anatomically narrow pelvis of 3 and 4 degrees, deformities and bone neoplasms, pelvic injuries during previous labor. In all other cases, the operation is decided individually. In the process of natural labor, a number of complications are possible, including premature discharge of water; slowing the opening of the cervix and lengthening the first period of labor; weak, too strong or discoordinated labor.