Dystocia due to abnormal birth canal

　　1. Developmental pelvic abnormalities:The pelvis is influenced by factors such as race, genetics, and nutrition during the development process, and its shape and size vary from person to person. Shapiro classified the different shapes of the pelvis into four types: female, male, flat, and ape-like. In reality, it is not common to find pelvises that perfectly match any of these four shapes, and most are mixed types.

　　Two. Infantile pelvis:Due to the lack of mechanical action factors during the development of the pelvis, long-term bed rest due to illness, resulting in the pelvis still maintaining an infantile state in adulthood.

　　Three. Dwarf pelvis:According to Breus and Kolisko, the dwarf pelvis can be divided into five types, with the cartilage dysplasia dwarf pelvis being the most common.

　　1. True dwarf pelvis:Due to diseases of the anterior lobe of the pituitary gland, the growth and development are delayed, leading to the pelvis not developing proportionally.

　　2. Incomplete dwarf pelvis:Due to incomplete overall development, the growth and development of the pelvis is delayed, and the ossification centers can eventually complete ossification. Although the development of each pelvic bone is normal, the pelvis is narrow, becoming a narrow pelvis.

　　3. Cartilage dysplasia dwarf pelvis:It is a congenital dysplasia of cartilage, which is often considered to be caused by endocrine disorders, but it has a genetic hereditary nature.

　　4. Kretschmer dwarf pelvis:It is a local disease in some mountainous areas, caused by iodine deficiency leading to thyroid dysfunction, which can affect the development of the central nervous system in severe cases. Patients have low intelligence, dwarfism, with the lower limbs shorter than the upper limbs, maintaining the proportion of an infant's body type, with a pelvic inlet that is uniformly small, but it is different from true dwarfism and does not present an infantile type.

　　5. Rickets dwarf pelvis:Due to a lack of vitamin D during physical development, calcium and phosphorus metabolism disorders occur, which hinders bone development, leading to dwarfism, with all pelvic diameters shortened. This is a cause of dystocia.

　　1. Premature rupture of membranes:Cephalopelvic disproportion and abnormal fetal head position may occur because the fetal head cannot adapt to the pelvic inlet plane, causing obstruction of the fetal head entry. Sometimes, the fetal head is even in a floating state, with a large gap between the fetal head and the pelvic inlet, allowing amniotic fluid to enter the anterior amniotic sac through this gap. When the uterine contraction reaches its peak, the amniotic membrane naturally ruptures due to its inability to withstand strong pressure, so premature rupture of membranes is often a herald sign of dystocia. It must be pointed out that premature rupture of membranes can also be caused by chorioamnionitis, so not all cases of premature rupture of membranes will result in dystocia, but the incidence of premature rupture of membranes in dystocia is higher than that in normal delivery.

　　2. Primary uterine contraction weakness:It refers to the onset of weak or uncoordinated uterine contractions at the beginning of labor, which is sometimes difficult to differentiate from false labor. If the uterine contractions become regular and strong after the administration of strong sedatives, and the labor progresses quickly, it indicates that uterine contraction weakness has transformed into coordinated uterine contraction; if the uterine contractions completely stop after the administration of strong sedatives, and the pregnant woman can return to normal life, it is considered false labor; if the uterine contractions do not stop or do not become normal after medication, it should be considered as an early manifestation of obstructive labor caused by organic factors (such as cephalopelvic disproportion, abnormal fetal head position).

　　3. Extended latent phase:The consequences of primary uterine contraction weakness often include an extended latent phase. The normal latent phase generally has an average of 6 to 8 hours, with an upper limit of 16 to 20 hours, and is confirmed as 16 hours in Chinese textbooks. A latent phase exceeding 16 hours is considered extended. It must be pointed out that if the latent phase exceeds 8 hours, it should be considered as having an extended tendency, and appropriate measures should be taken. The incidence of cephalopelvic disproportion and abnormal fetal head position in pregnant women with an extended latent phase is higher than that in pregnant women with a normal latent phase, so the extension of the latent phase is often caused by organic factors.

　　4, Non-engagement or delayed engagement of the fetal head:The signs of the fetal head not being engaged at the time of labor may be related to the shape of the pelvis entrance, there is no need to worry too much about this, but the progress of labor should be closely observed. However, it is necessary to be vigilant about the fetal head floating high, high position -3cm or above. Normally, when the cervix is dilated 5cm, the fetal head should have engaged. If the fetal head engages only after the cervix is dilated 5cm, it is called delayed engagement, indicating that the fetal head encountered difficulties when passing through the pelvis entrance plane. If the fetal head does not engage until the cervix is fully dilated, it is called non-engagement of the fetal head, indicating that there is a serious malposition of the head and pelvis or abnormal fetal head position at the pelvis entrance plane.

　　5, Delayed cervical dilation:The cervix of the primipara expands and accelerates after entering the active phase, it is generally believed that the acceleration stage is when the cervix is dilated 3-4cm, and the maximum acceleration stage is 4-9cm. 9-10cm (cervical dilatation is complete) is the deceleration stage.

　　First, classified according to the narrow plane of the pelvis

　　1, Entrance stenosis:Mostly manifested as the anteroposterior diameter of the entrance plane is narrow, that is, flat stenosis.

　　2, Middle pelvis-outlet stenosis:The outlet stenosis referred to here refers to the stenosis of the outlet surface surrounded by bone, due to its close proximity to the middle pelvis, similar in size and shape, even slightly smaller than the middle pelvis, it is the last stage of vaginal delivery, therefore, in fact, outlet stenosis also indicates middle pelvis stenosis. Therefore, Benson believes that the middle pelvis and outlet are the same thing, and put forward the concept of middle pelvis-outlet dystocia.

　　The middle pelvis-outlet stenosis is also known as the funnel-type stenosis, which is divided into 3 types:

　　(1) Narrowness of the transverse diameter of the middle pelvis and outlet: The pelvic walls on both sides are convergent, commonly seen in the flat type of anthropoid pelvis.

　　(2) Narrowness of the anteroposterior diameter of the middle pelvis and outlet: The pelvic anterior and posterior walls are convergent, mostly due to the straight type of sacrum being simple.

　　(3) Mixed type: The transverse diameter and anteroposterior diameter of the middle pelvis and outlet are both narrow, the pelvic walls on both sides and the anteroposterior walls are all convergent, commonly seen in male-type pelvis. The middle pelvis and outlet transverse diameter narrowness and mixed type of pelvis are prone to persistent occiput posterior position, because the anterior half of the anthropoid and male pelvis is narrow, and the posterior half is wide, the fetal head often enters the pelvis in the occiput posterior position, but it is difficult for the fetal head to rotate 135 degrees forward to the occiput anterior position in the transverse diameter narrow middle pelvis plane. The anteroposterior diameter of the middle pelvis and outlet is narrow, and the pelvis entrance is often flat. The fetal head enters the pelvis in the occiput transverse position. Due to the narrow anteroposterior diameter of the middle pelvis and the normal transverse diameter, the fetal head remains in the occiput transverse position, even reaching the pelvic floor. If the fetus is not too large, the fetal head can be manually rotated to the occiput anterior position for delivery; if the fetus is slightly larger, it is easy to occur obstructive dystocia, and cesarean section is required to end the delivery.

　　The pelvis with a normal inlet but narrow middle and outlet, presenting a funnel shape. The fetal head can usually engage the pelvis, but after reaching the middle pelvis, the descent of the fetal head is slow or even stops. Clinical manifestations include normal first stage of labor in the first half, delayed or stopped cervical dilation in the late first stage, and prolonged second stage of labor. Therefore, when the cervix is fully dilated and the presenting part of the fetus descends to below the ischial spines and stops, attention should be paid to whether there is a funnel-shaped narrow pelvis, and whether the fetal head is in a persistent occiput transverse or posterior position. At this time, one should not be deceived by the false appearance of the fetal head having entered the pelvic floor caused by severe deformation and edema of the fetal head and blindly decide to assist with vaginal delivery. Otherwise, it will bring great harm to the mother and child. Therefore, if it is a funnel-shaped narrow pelvis, it is not advisable to try labor for too long, and the indication for cesarean section should be relaxed. Severe narrowness should undergo selective cesarean section.

　　3. Inlet, middle, and outlet all narrow (uniformly small narrow pelvis):When the inlet, middle, and outlet planes of the pelvis are all narrow, it is called uniformly small narrow pelvis, which can be divided into three types:

　　(1) The pelvic shape still maintains the shape of the female pelvis, but the diameters of each plane are 1 to 3 cm smaller than the normal value. Uniformly small pelvises are more common in women with poor development and short stature;

　　(2) Simple flat pelvis, but the anteroposterior diameters of all three planes are shortened;

　　(3) Simian pelvis, with the transverse diameters of all three planes being small, among which the type ① is the most common. Although the transverse diameters of this type of pelvis are slightly smaller, if the fetus is not large, the fetal position is normal, and the uterine contraction is strong, it is sometimes possible to deliver vaginally. However, most often due to poor overall physical development, uterine contraction insufficiency often occurs, and cesarean section assistance is required. If the fetus is large, or the fetal head is in a persistent occiput posterior or occiput transverse position, the chance of dystocia is greater. Therefore, the indication for cesarean section in uniformly small pelvises should not be too strict.

　　Secondly, classification of pelvic shape abnormalities Pelvic shape abnormalities are divided into three categories:

　　1. Developmental pelvic abnormalities:The pelvis is influenced by factors such as race, genetics, and nutrition during the development process, and its shape and size vary from person to person. Shapiro classified the different pelvic shapes into four types: female, male, flat, and simian. In fact, pelvises that completely conform to these four shapes are not common, and most are mixed types. The characteristics of the four basic pelvic shapes.

　　(1) Female pelvis: The most common type, also known as the normal type of pelvis, with the transverse diameter of the pelvic inlet slightly longer than the anteroposterior diameter, presenting an oval shape, which is conducive to delivery. The fetal head mostly enters the pelvis in the occiput anterior or occiput transverse position. However, if the pelvic cavity is uniformly narrow, it is a uniformly small pelvis, which is not conducive to delivery.

　　(2) Male pelvis: The inlet of the male pelvis is heart-shaped or wedge-shaped, with the two walls converging, the pubic arch is small, the ischial spines are prominent, the sacral sciatic notch is narrow, and the interspinous diameter is not specified.

　　(3) Flat pelvis: The anteroposterior diameter of the inlet of the flat pelvis is short, and the transverse diameter is relatively longer, forming a flat circular shape horizontally. The pelvis is shallow, the lateral walls are upright, the posterior angle of the pubic symphysis and the pubic arch are both wide, the ischial spines are slightly prominent, the interspinous diameter is large, the sacral sciatic notch is narrow, the sacrum is wide and short, and the fetal head often enters the pelvis in the occiput transverse position. Once it passes through the inlet, delivery can proceed smoothly.

　　(4) Ape-like pelvis: The anteroposterior diameter of each plane of the ape-like pelvis is long, and the transverse diameter is short, forming a longitudinal elliptical shape. The pelvis is deep, the lateral walls are upright, slightly convergent, the ischial spines are slightly prominent, the interspinous diameter is short, the sacral sciatic notch is wide, the sacrum is narrow and long, and the fetal head often enters the pelvis in the occiput posterior position and remains in this position. If the uterine contraction is good, the fetal head can descend to the pelvic floor and turn to the occiput posterior position for delivery.

　　2. Pelvic diseases or injuries

　　(1) Vitamin D deficiency pelvis: Caused by insufficient vitamin D supply during childhood or long-term lack of sun exposure. The formation of vitamin D deficiency pelvis is mainly due to the mechanical action of the patient's body weight pressure and the traction of the muscles and tendons on the pelvis, followed by pathological changes in the development of the pelvic bones. It is now very rare. The main characteristics of the pelvis are: the sacrum is wide and short, tilting forward due to the concentrated pressure of its own trunk weight. The sacral promontory protrudes into the pelvic cavity, making the pelvic inlet face appear kidney-shaped horizontally. The anteroposterior diameter is significantly shortened. If the sacrotuberous ligament is relaxed, the distal end of the sacrum will be upturned, and only the anteroposterior diameter of the inlet will be shortened. If the sacrotuberous ligament is firm, the sacrum will be deep-arc or hook-shaped, shortening the anteroposterior diameter of both the inlet and outlet. The lateral walls of the pelvis are upright or even everted, and the transverse diameter of the outlet increases. The deformation of the vitamin D deficiency pelvis is severe, which is extremely unfavorable for delivery, so it is not suitable to try labor.

　　(2) Osteomalacia pelvis: Vitamin D deficiency occurring in adults whose epiphyses have closed is called osteomalacia. The main characteristics of the pelvis in osteomalacia are due to the pressure of the trunk weight and the upward and inward support force of the two femurs, as well as the traction effect of the adjacent muscle groups and tendons, resulting in significant deformation of the pelvis but not in proportion. The anteroposterior diameter and transverse diameter of the pelvic inlet are shortened, forming an 'inverted triangle', and the middle pelvis is significantly reduced. The anteroposterior diameter of the outlet is also severely reduced, making it impossible for the fetus to be delivered vaginally. Even if the fetus is dead, since the fetal head cannot enter the pelvis, it cannot be delivered vaginally by craniotomy, and only cesarean section can be performed. Osteomalacia pelvis is now extremely rare.

　　(3) Pelvic fractures: Commonly occur after traffic accidents or falls, with the fracture site often seen in the bilateral pubic rami, ischial ramus, and iliac wings. Severe pelvic fractures may result in pelvic deformity and significant ossification after healing, which can hinder delivery. It is important to take pelvic X-rays after the fracture has healed to provide a basis for whether future pregnancy can be delivered vaginally. After pregnancy, careful internal examination should be performed to determine if there are any pelvic abnormalities and decide on the caution needed for trial labor.

　　(4) Pelvic tumors: Rare, with reported cases of chondroma, osteoma, and chondrosarcoma. They can be found near the posterior wall of the pelvis adjacent to the sacroiliac joint, with the tumor protruding into the pelvic cavity. During labor, it can obstruct the descent of the fetal head, leading to dystocia.

　　3. Abnormal pelvis caused by spinal, hip, or lower limb diseases

　　(1) Scoliotic deformation pelvis: Most spinal lesions are caused by bone tuberculosis, which can lead to the following two types of deformed pelvises:

　　A. Kyphotic pelvis, mainly caused by tuberculosis and vitamin D deficiency, the effect of kyphosis on the pelvis varies with the location of the lesion. The lower the location of the lesion, the greater the effect on the pelvis. If the kyphosis occurs in the thoracic spine, it has no effect on the pelvis; if the kyphosis occurs in the lower part of the thoracic and lumbar spine, it can cause the anteroposterior diameter and transverse diameter of the middle pelvis to shorten, forming a typical funnel-shaped pelvis, which can lead to obstructive dystocia during delivery. Due to the severe deformation of the spine, it compresses the thoracic cavity, reducing the thoracic cavity capacity, increasing the pressure on the heart and lungs. The vital capacity is only half of that of a normal person, and the right ventricle must increase the pressure to maintain the increasing pulmonary blood flow due to pregnancy, resulting in an increased workload and hypertrophy of the right ventricle. Therefore, kyphosis affects the function of the heart and lungs, and close monitoring should be strengthened in the late pregnancy and during delivery to prevent heart failure.

　　B. Scoliotic pelvis, if the scoliosis only affects the thoracic segment above the lumbar spine, the pelvis is not affected; if the scoliosis occurs in the lumbar spine, the sacrum will deviate to the opposite side, causing the pelvis to be skewed and asymmetric, which affects delivery.

　　(2) Hip joint and lower limb deformity pelvis: Hip joint arthritis (usually tuberculous), poliomyelitis lower limb paralysis atrophy, knee or ankle joint lesions, etc. If the disease occurs in childhood, it can cause limping. When walking, the affected limb is too short or painful to touch the ground, and the healthy limb bears all the body weight, resulting in a skewed pelvis. Due to the functional decline on the affected side, the iliac wing and hip bone on the affected side are underdeveloped or atrophic, which further aggravates the degree of pelvic skewness. During pregnancy, a skewed pelvis is unfavorable for delivery.

　　3. The degree of pelvic constriction:Currently, there is no unified standard for the classification of the degree of pelvic constriction, mainly because there is no consensus on the method of measuring the pelvis. The measurement of the pelvis can be done in three ways: clinical measurement, X-ray measurement, and ultrasound measurement. Due to the potential harm of X-rays to the fetus, most people do not advocate the use of X-ray measurement of the pelvis, at least it should not be used routinely. Ultrasound measurement has not been widely used in clinical practice, so clinical measurement is still the main method to measure the size of the pelvis. External measurement is affected by the thickness of the bone, so it is sometimes necessary to make corrections, especially the sacroiliac diameter of the pelvic inlet is most affected by the bone, so wrist circumference measurement should be done to understand the thickness of the bone and make corrections, or the diagonal diameter (not affected by the thickening of the bone) should be checked.

　　The degree of pelvic constriction is generally divided into 3 levels: Level Ⅰ: Critical constriction, that is, the diameter is at the critical value (the intersection of normal and abnormal values), the labor process of such parturients should be observed with caution, but the majority of cases can be delivered naturally; Level Ⅱ: Relative constriction, including a wide range, divided into mild, moderate, and severe constriction, such cases need to be tested for a certain period of time to determine whether it is possible to deliver vaginally, the possibility of vaginal delivery is extremely small in severe constriction; Level Ⅲ: Absolute constriction, there is no possibility of vaginal delivery, cesarean section must be performed to end the delivery.

　　1、入口平面狭窄：The anteroposterior diameter of the inlet plane is more common than the transverse diameter. According to the length of the sacropubic diameter (external conjugate diameter), the anteroposterior diameter of the inlet plane (true conjugate diameter), and the diagonal diameter, the inlet plane stenosis can be divided into three levels.

　　2. Middle pelvis stenosis:The middle pelvis stenosis can be divided into three levels according to the length of the interspinal diameter, the posterior sagittal diameter behind the interspinal diameter, and the anteroposterior diameter of the middle pelvis. The interspinal diameter and the posterior sagittal diameter need to be measured by X-ray film, while the anteroposterior diameter of the middle pelvis can still be measured by vaginal examination (internal measurement).

　　3. Outlet plane stenosis:

　　The outlet plane stenosis can be divided into three levels according to the length of the intertrochanteric diameter, the posterior sagittal diameter behind the intertrochanteric diameter, and the anteroposterior diameter of the outlet. The clinical significance of the intertrochanteric diameter (transverse diameter of the outlet) and the posterior sagittal diameter is the greatest, and the former is more important. If the intertrochanteric diameter is short, the angle of the pubic arch becomes acute, and the area that can be used in front of the outlet face will decrease. If the posterior sagittal diameter is sufficient in length, it can compensate for the lack of the intertrochanteric diameter, and the fetus still has the possibility of being delivered. However, if the intertrochanteric diameter is too short (≤6cm), even if the posterior sagittal diameter is large, it cannot compensate. For the classification of outlet plane stenosis, in addition to measuring the intertrochanteric diameter and the posterior sagittal diameter, the size of the anteroposterior diameter of the outlet face should also be referred to. The anteroposterior diameter of the outlet face is the straight distance from the pubic symphysis to the sacrococcygeal joint, which is also the outlet diameter that the fetal head must pass. If this diameter is short, the fetal head often needs to be in the occipitotransverse position to pass this diameter with the biparietal diameter. The normal value is 11.8cm, and the shortest cannot be less than 10cm.

　　The degree of pelvic stenosis is generally divided into three levels.

　　Ⅰ level, critical stenosis:That is, the diameter is at the critical value (the junction of normal and abnormal values), and it is necessary to carefully observe the labor process of such patients, but the vast majority of cases can be delivered naturally.

　　Ⅱ level, relative stenosis:The scope of application is relatively wide, divided into three levels: mild, moderate, and severe stenosis. Such cases need to undergo a certain period of trial labor to determine whether it is possible to deliver vaginally, and the possibility of vaginal delivery during severe stenosis is extremely small.

　　Ⅲ level, absolute stenosis:There is no possibility of vaginal delivery, and cesarean section must be used to end the delivery.

　　1. X-ray pelvis measurement:X-ray pelvis imaging is more accurate than clinical measurement, and it can directly measure the diameters of all sides of the pelvis and the tilt of the pelvis, and can also understand the shape of the pelvic inlet and the sacrum, the position and flexion of the fetal head, to determine whether there are any abnormalities in these aspects. However, since X-rays may have radioactive damage to pregnant women and fetuses, most obstetricians in China and abroad believe that X-rays should only be used when absolutely necessary.

　　2. Ultrasound pelvic measurement:Pelvic measurement is an important basis for diagnosing cephalopelvic disproportion and determining the mode of delivery. Since X-ray pelvic measurement is harmful to the fetus, it is rarely used in obstetrics at present. Although the clinical external measurement of the pelvis is simple in method, its accuracy is poor. Since 1991, Bian Xuming and others at Peking Union Medical College Hospital have explored the method of vaginal ultrasound pelvic measurement to assist in the diagnosis of cephalopelvic disproportion. The method is as follows:

　　1. Perform vaginal ultrasound measurement of pelvic size at 28-35 weeks of pregnancy:After the pregnant woman empties her bladder, take the lithotomy position, place the vaginal ultrasound probe into the vagina 3-5 cm, and when the pubic bone and sacrum are displayed on the screen at the same time, it is the longitudinal section of pelvic measurement, which can measure the anteroposterior diameter of the pelvic cavity. The anterior reference point is the inner margin of the inferior edge of the pubic symphysis, and the posterior reference point is between the fourth and fifth sacral vertebrae. Then, rotate the vaginal probe 90 degrees, lower the handle to make the boundaries of the pelvis on both sides clearly and symmetrically displayed, which is the transverse section of pelvic measurement. It can measure the transverse diameter of the pelvic cavity. The two ends are the most prominent parts of the ischial spines. According to the anteroposterior diameter and transverse diameter of the pelvic cavity, using the formula of the circumference and area of an ellipse, the circumference and area of the pelvic cavity can be calculated separately.

　　2. One week before labor at the end of pregnancy, use abdominal ultrasound to measure the biparietal diameter and occipitofrontal diameter of the fetal head, and calculate the head circumference.

　　1. Eat foods rich in protein and iron:Such as lean meat, fish and shrimp, animal blood, animal liver and kidney, egg yolk, soy products, and jujube, green leafy vegetables, sesame paste, etc.; choose vegetable oils, and mostly use cooking methods such as boiling, steaming, pickling, roasting, braising, and stewing; it is forbidden to eat fatty meat, internal organs, fish eggs, cream, and other high-cholesterol foods.

　　2. Avoid coarse foods:Three to four days after surgery, after anal exhaust, it indicates that the intestinal function begins to recover. At this time, a small amount of liquid food can be given, and it can be changed to semi-liquid food with less residue after 5 to 6 days. It is forbidden to eat chicken, ham, pigeon meat, and soup of various vegetables. Even if it is cooked very soft, one should not act in haste.

　　Severe pelvic stenosis is rare. Clinically, most abnormalities of the birth canal are mild pelvic stenosis, but it is often one of the important causes of difficult labor and dystocia.

　　The narrowness of a single diameter does not necessarily affect delivery, so a comprehensive assessment of the size and shape of the entire pelvis is necessary to make a more accurate estimate. Whether the fetus can be delivered naturally is closely related to the strength of labor, the position of the fetus, the size and plasticity of the fetal head, the resistance of soft tissues, and whether the diagnosis and treatment are timely and correct.

　　1. Treatment of pelvic inlet stenosis:The narrowness of a single diameter at the pelvic inlet is often flat, and if the ischial pubic diameter is 17-18 cm, and the fetus is of normal size, the fetus should be given a full opportunity for labor. For those with an intact amniotic membrane, artificial rupture of the membrane should be performed first to enhance uterine contractions. Some authors believe that labor without rupture of the amniotic membrane cannot be considered effective labor in the treatment of mild pelvic inlet stenosis.

　　The time for trial labor in pelvic inlet stenosis can be slightly longer, and after the cervix enters the active phase, trial labor can be performed for 6-8 hours. However, if the labor process shows primary uterine contraction weakness or disharmonious uterine contractions at the beginning, and the contractions cannot be interrupted by strong sedatives, it indicates an obvious mismatch between the pelvis and the head. A vaginal examination should be performed, the diagonal diameter measured, and the relationship between the pelvis and the head re-estimated. Trial labor should be performed with caution, and if it is determined that there is a mismatch between the pelvis and the head, cesarean section should be performed as soon as possible.

　　If the uterine contraction is weak during the trial labor process, oxytocin can be used for intravenous infusion to strengthen uterine contractions. When using oxytocin, the condition of the mother and child should be closely monitored. If there is no obvious progress in the labor process after observing effective uterine contractions for 2 hours, it can be considered that the trial labor has failed, and cesarean section should be performed as soon as possible.

　　Indications for selective cesarean section for pelvic inlet stenosis: ① The fetal head is floating and cannot enter the pelvis, or the fetal head is riding over; ② Severe stenosis of the pelvic inlet, the biparietal diameter ≤ 16cm; ③ Significant pelvic deformity or obvious mismatch between the pelvis and the head.

　　Second, treatment for middle pelvis-outlet stenosis

　　1. Treatment for middle pelvis stenosis:During the labor process, the fetal head completes flexion and internal rotation at the middle pelvic plane, and stenosis of the middle pelvis will affect the internal rotation of the fetal head in the pelvic cavity, and thus is the main cause of persistent occipito-transverse or occipito-posterior position. At this time, the fetal head cannot flex well enough to pass through the pelvic diameter. If the cervix is fully dilated, the fetal head can be manually rotated to the anterior position to shorten the diameter of the fetal head passing through the pelvis, which is conducive to natural delivery, but most need the use of forceps or vacuum extraction for delivery. If the labor process does not show obvious progress, and the biparietal diameter of the fetal head still stays above the ischial spines, or fetal distress occurs, cesarean section should be performed immediately.

　　2. Treatment for pelvic outlet stenosis:The pelvic outlet is the lowest part of the birth canal. If there is a suspicion of outlet stenosis, a careful estimate of the size of the fetus, the relationship between the pelvis and the head, should be made before labor to determine whether a vaginal delivery is possible. When the transverse diameter of the outlet is narrow, the triangular space below the pubic arch cannot be utilized, the presenting part can be moved backward, and the posterior triangular space can be used to deliver. Clinically, the sum of the transverse diameter of the outlet and the posterior sagittal diameter is often used to estimate the size of the outlet. If the sum is greater than 15cm, most fetuses can be delivered vaginally; if the sum is 13-15cm, most need the use of vacuum extraction or forceps for delivery, and a larger episiotomy should be performed to avoid severe perineal laceration.