Pelvic presentation

In the longitudinal position, in which the long axis of the fetus and the uterus are parallel, the fetus's descending part is the lower half of the body, and is commonly called "breech position." It is divided into four types depending on the part of the fetus that has descended the most.

[1] Single hip position (pure hip position): With the lower limbs extended, the hip joints are flexed so that only the buttocks move forward.

[2] Double Hip Position: This is a position in which the buttocks and both feet move forward at the same time, as if the body is sitting cross-legged.

[3] Knee position: The hip joint is extended and the knee joint is flexed to allow the knee to advance. When both knees advance at the same time, it is called full knee position, and when only one knee advances, it is called incomplete knee position.

[4] Foot position: When the hip and knee joints are extended, the foot moves forward. When both feet move forward at the same time, it is called full foot position, and when only one foot moves forward, it is called incomplete foot position.

Breech presentation is quite common when the fetus is small, but in fetuses weighing 2,500 grams or more, its frequency is around 4% of all births. There are four possible causes: (1) fetal abnormalities such as prematurity, malformations, and twins, (2) uterine abnormalities such as a bicornuate uterus, a septate uterus, or uterine fibroids, (3) polyhydramnios, which increases fetal mobility, or relaxation of the uterus or abdominal wall, and (4) placenta previa or narrow pelvis, which restricts fetal descent.

The prognosis for the fetus is generally poorer than in normal cephalic births, but this is often due to premature birth and the immaturity of the fetus itself, which is the cause of death. In addition, in cephalic births, the head, which is the largest, slowly descends into the bony birth canal while gradually deforming, but in breech presentation, the head passes through the bony birth canal in a few minutes, so this adaptive function is not fully exercised, which can result in intracranial damage, or birth damage such as cervical spinal cord damage due to traction during delivery. When the trunk is delivered up to the navel, the umbilical cord is compressed between the bony birth canal and the descending fetal head, which is also a factor in worsening the prognosis. In foot or knee presentations, if premature or early rupture of membranes occurs, the umbilical cord may escape into the cervix or vagina, worsening the prognosis. Among breech presentations, single hip presentation accounts for 70-80% of cases, and has the best prognosis. The risk of umbilical cord prolapse is the same as in cephalic presentations. The prognosis of double hip presentation is almost the same as in single hip presentation.

If breech presentation is found during pregnancy, the baby can be left alone and allowed to naturally change to cephalic presentation, or from around 30-32 weeks the mother can place herself in a position with her knees pressed together against the chest wall for 10-15 minutes and then lie on her side, or an assistant can use external version to turn the baby from above the abdominal wall to cephalic presentation. If cephalic presentation can be achieved by any of these methods, a cephalic delivery is performed. Breech delivery can be performed as a natural birth using contractions and pushing, as a partial evacuation in which an assistant evacuees the fetus's scapular limbs and head, or as a total evacuation in which an assistant evacuees the fetus from the beginning, but partial evacuation is more common. Trailing fetal head forceps may be used to deliver the head. In a first-time mother with a poor prognosis, such as feet presentation, a Caesarean section may be performed from the start.

[Masao Arai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

胎児と子宮の長軸が平行になっている縦位のうち、胎児の下向部が下半身であるものをいい、俗に「さかご」とよばれる。もっとも下降してきている先進部によって4種に分けられる。

〔1〕単臀位(でんい)（純臀位）　下肢を伸展したまま股関節(こかんせつ)を屈曲して、臀部のみが先進してくるもの。

〔2〕複臀位　あたかも「あぐら」をかいているような姿勢で、臀部と両足が同時に先進してくるもの。

〔3〕膝位(しつい)　股関節を伸展させ、膝関節を屈曲して膝部が先進してくるもので、両膝(ひざ)同時に先進してくる場合を全膝位、片膝のみ先進してくる場合を不全膝位という。

〔4〕足位　股関節も膝関節も伸展したまま足が先進してくるもので、両足同時に先進してくる場合を全足位、片足のみ先進してくる場合を不全足位という。

　骨盤位は胎児が小さいときにはかなり高率でみられるが、2500グラム以上の胎児に限ってみると、その頻度は全分娩(ぶんべん)例の4％前後である。誘因としては次の四つが考えられる。(1)未熟児、形態異常、双胎など胎児の異常、(2)双角子宮、中隔子宮、子宮筋腫(きんしゅ)の合併など子宮の異常、(3)胎児の可動性が大きくなる羊水過多、子宮や腹壁の弛緩(しかん)がある場合、(4)胎児の下降が制限されるような前置胎盤や狭骨盤のある場合。

　胎児の予後は一般に正常の頭位分娩に比べて不良であるが、その原因は早産で胎児の未熟性そのものが死因となることが多い。また、頭位分娩では最大の頭部が少しずつ変形しながらゆっくり骨産道内を下降する応形機能を発揮するが、骨盤位では頭部が数分で骨産道内を通過してしまうためこの応形機能が十分に発揮されず、頭蓋(とうがい)内損傷をきたしたり、娩出時の牽引(けんいん)によって頸髄(けいずい)損傷などの分娩損傷をきたすこともある。臍部(さいぶ)（へそ）まで躯幹(くかん)が娩出されてくると、臍帯（へその緒(お)）が骨産道と下降してきた児頭の間で圧迫されることも、予後を悪くさせる一因となる。足位や膝位の場合は前期破水や早期破水をきたすと、臍帯が頸管や腟内に脱出して予後を悪くする。骨盤位のうちでは単臀位の場合が70～80％を占め、しかもこの予後がもっともよく、臍帯脱出の危険性も頭位分娩と変わらない。複臀位の場合も単臀位と予後はほとんど変わらない。

　妊娠中に骨盤位が判明した場合は、放置して自然に頭位になるのを待つか、30～32週ころから妊婦が自分で胸壁と膝をくっつけるような体位を10～15分間とったのち横向きで寝るとか、介助者の手で腹壁上から頭位に直す外回転術を行うか、いずれかの方法で頭位になるものは頭位分娩を行う。骨盤位分娩は、陣痛といきみで娩出させる自然分娩か、胎児の肩甲上肢と児頭を介助者が牽出する部分牽出術か、最初から介助者が牽出する全牽出術が行われるが、一般には部分牽出術が多い。頭部の娩出には後続児頭鉗子(かんし)が用いられることもある。初産婦で、足位など予後の悪い胎位の場合には、最初から帝王切開が行われることもある。

［新井正夫］

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