Narrow pelvis

This refers to a condition in which the bony birth canal through which the fetus passes during childbirth is narrow, and a normal-sized fetus often cannot pass through. The three elements of childbirth, the birth canal, the fetus, and labor pains, are all related to each other, and if the bony birth canal is narrow, the fetal head does not rotate properly, and labor pains are too strong or too weak. Except for cases in which the pelvic cavity is partially narrowed due to deformation caused by pelvic fractures, the pelvic cavity is generally small in most cases. An absolutely narrow pelvis is when the shortest distance (obstetric symphysis) between the posterior surface of the pubic symphysis and the upper edge of the sacrum (promontory), which is the vertical diameter of the pelvic inlet equivalent to the upper edge of the bony birth canal, is less than 9.5 cm, or the maximum horizontal distance is less than 10.5 cm. In this case, 40 to 73% of cases are performed by Caesarean section. In cases where the vertical diameter is 9.5 to 10.5 cm or the horizontal diameter is 10.5 to 11.5 cm, the pelvis is said to be relatively narrow, and about 24% of cases are performed by Caesarean section. In general, short women have smaller pelvic cavities. To determine the size of the pelvic cavity, pelvic measurements using X-rays are often used. Recently, it has become possible to measure not only the size of the pelvis but also the size of the fetal head using ultrasound tomography, and so the concept of fetal head-pelvic disproportion, which is an issue of the relative sizes of the fetal head and the pelvic cavity, is often considered. The presence or absence of fetal head-pelvic disproportion is often determined by pelvic and head measurements, as well as by test deliveries to observe the progress of labor.

[Masao Arai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

分娩(ぶんべん)時に胎児が通過する骨産道が狭く、通常の大きさの胎児が通過できないことが多いものをいう。分娩の3要素である産道、胎児、陣痛は互いに関連があり、骨産道が狭いと児頭の回旋がうまく行われなかったり、陣痛も強くなりすぎたり微弱になったりする。骨盤骨折などが原因で、変形のために骨盤腔(くう)が一部狭くなっているものを除けば、骨盤腔が全体的に小さいものが多い。骨産道の上縁に相当する骨盤入口部の縦径である恥骨結合後面と仙骨上縁（岬角(こうかく)）を結ぶ最短距離（産科的結合線）が9.5センチメートル未満か、横径の最大距離が10.5センチメートル未満の場合を絶対的狭骨盤といい、40～73％が帝王切開となる。この縦径が9.5～10.5センチメートルあるいは横径が10.5～11.5センチメートルの場合は比較的狭骨盤といい、約24％が帝王切開となる。一般に、身長の低い女性は骨盤腔も小さい。骨盤腔の広さを知るためには、X線撮影による骨盤計測が用いられることが多い。最近では、骨盤の大きさだけでなく、児頭の大きさも超音波断層法によって計測できるようになったので、児頭と骨盤腔の相対的な大きさを問題とする児頭骨盤不均衡という考え方がなされることが多い。児頭骨盤不均衡の有無は、骨盤計測と児頭計測のほか、分娩の進行ぐあいを観察する試験分娩によって判定されることが多い。

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