The bone pelvis consists of several bones connected by cartilage. In women, it is conventionally divided into two sections: the upper – large and lower – small.
The first is laterally limited by the upper parts of the iliac bones (they are clearly visible when we lie on the back), behind – the last lumbar vertebrae, in front – the abdominal muscles. The second section is formed by the frontal bones, from the back – the sacrum and tailbone, from the sides – the ischial bones (we lean on them when we sit).
The sizes of both structures influence the outcome of labor, but the dimensions of the pelvis are especially important, because the child must pass through this tight tunnel at the last stage of labor.
Inside the pelvis there are four planes: the entrance plane, the exit plane, the plane of the wide and the plane of the narrow part. The first has the shape of an ellipse.
If inside it to hold an imaginary line from left to right, you get a “transverse dimension of the entrance plane”, from top to bottom – “straight”, diagonally – “oblique”. The head of the baby is inserted into one of these “sizes”. If it corresponds to the norm, there will be no problems with the appearance of a child in this world.
Otherwise, there will certainly be difficulties.
To predict the course of labor and possible complications, it is necessary to determine in advance the dimensions of both basins. However, the results of measurements will be only preliminary and will not give an accurate forecast. That’s because in obstetric practice there is a huge difference between the anatomically and clinically narrow pelvis.
The first diagnosis can be identified even outside of pregnancy. About anatomically narrow pelvis say, if at least one of its linear dimensions is less than the norm by 1.5-2 cm and more.
The diagnosis of a “clinically narrow pelvis” can only be made during childbirth, when the discrepancy between the size of the head of the baby and the small pelvis becomes apparent. Moreover, no woman is insured against such a scenario, even if her anatomical parameters correspond to the average.
For example, if the head of the baby is too large (usually it happens if the weight of the child with headache more than 4 kg), the most common pelvis will be declared clinically narrow. It is almost impossible to predict such scenarios before delivery, so doctors choose the waiting tactics.
Since the pelvis is not available for direct measurements, its dimensions are judged by the big pelvis, and the parameters of the latter are determined quite accurately. External measurements are made using a tazomer (a special compass with rounded ends) and obstetric measuring tape at the first examination at the antenatal clinic. The distances between different points of the iliac bones and large trochantes of the femur bones are calculated and the parameters of the lumbosacral rhombus are determined.
Then, the outer conjugate is measured – the distance between the supracarpal fossa and the upper edge of the pubic symphysis. The capacity of the pelvis is assessed by vaginal examination.
The same parameter can be determined by the circumference of the wrist – the Solovyov index. If all these measurements reveal an anatomically narrow pelvis, doctors must specify the degree of constriction.
For this, the size of the true conjugates is calculated. You can recognize it if you subtract 9 cm from the value of the external conjugates.
About I degree narrowing say 9–11 cm, II – 7.5–9 cm, III –– 6–7.5 cm, IV –– figure less than 6. Most women diagnosed with “anatomically narrow pelvis” show first degree narrowing. The second is extremely rare.
The third and fourth – and at all an exception to the rules. In addition to the degree of pelvic narrowing, doctors necessarily determine its shape (transversely narrowed, evenly narrowed, flat, etc.).
The tactics of childbirth depend directly on these two indicators.
A couple of weeks before giving birth, doctors need to repeat the measurements, because the baby is constantly growing. The dimensions of a child (measurements of the circumference of his head, the diameter of the abdomen and thigh bones during an ultrasound scan), as well as the abdominal circumference of the future mother make it possible to judge the dimensions of the child.
The pelvic dimensions at this stage specify the distance from the upper pubic peak to the highest point of the uterus, which in late pregnancy can be felt through the abdominal wall (in obstetrics, this value is called the height of the bottom of the uterus).
Until the birth began, doctors will not talk about a clinically narrow pelvis, but about a clinical discrepancy between its parameters and the size of the baby’s head (by the way, with an anatomically narrow pelvis, this probability is only 25-30%). In this case X-ray heliometry may be required.
The method allows you to accurately compare the size of the internal cavities of the pelvis with the volume of the baby’s head and make a fairly accurate prediction of childbirth. The survey is carried out only after 38 weeks of pregnancy, when the baby is already older.
But the final diagnosis is still made only in the maternity hospital. In this case, the parameters of the pelvis and baby tops are measured again in the receiving room.
The absolute indications for cesarean section are: diagnosis “clinically narrow pelvis”, III and IV degree of pelvic contraction, bone tumors in the small pelvis or its deformation due to traumas and diseases. If the expectant mother is more than 30 years old, she has serious systemic diseases or the presentation is not a headache, the baby will also be born operatively, even if the degree of contraction is I or II.
Natural childbirth is possible only if the anatomically narrow pelvis of the first or second degree of constriction during labor has not become clinically narrow. But in this scenario, complications are very likely.
The most frequent of these are prolonged labor, in which the pathogenic flora penetrates the uterus from the vagina, so both the mother and the baby at the same time increase the likelihood of contracting a bacterial infection; premature rupture of amniotic fluid and weak labor activity. In addition, the risk of birth injuries for both women and children increases markedly. For example, if the head does not move forward for 1.5-2 hours, then in this area it presses the soft tissue of the mother to the bones of the pelvis.
In such a “vice” may be the rectum or bladder. If this happens, blood circulation is disturbed and tissue necrosis begins, which will lead to the formation of fistulas – channels connecting organs that should not be reported.
The kid is also in danger. Squeezing through the bones of the narrow pelvis, the head of the child experiences mechanical loads, as a result of which hematomas can appear, small vessels rupture and even the cervical spine can be damaged.
Knowing about all these subtleties, doctors always warn the future mother about possible complications and suggest carefully weighing the pros and cons before venturing into natural childbirth.