Fetal position - Taii (English spelling) Presentation

This refers to the position of the fetus within the uterine cavity, and is indicated by the directional relationship between the fetal long axis and the vertical axis of the uterus. When the two directions are aligned, it is called the longitudinal position, and when they cross, it is called the transverse position.

Longitudinal presentation is divided into cephalic presentation (head first), which includes normal presentation, and breech presentation (pelvis first), which is abnormal presentation, and cephalic presentation is further divided into cephalic presentation (occipital and parietal), brow, face, etc. Of these, occipital presentation is a normal presentation in which the fetal head is tilted forward and close to the chest, and is also called flexed presentation. In contrast, parietal, brow, and face presentations are abnormal cases in which the fetal head is tilted backward and away from the chest, and are called retroflexed presentations.

The transverse position is divided into the narrow transverse position, where the two sides cross at a right angle, and the oblique position, where the two sides cross at an angle. In the oblique position, the fetus is positioned at an angle, and is further called the dorsal position when the fetal back faces forward, the dorsal position when the fetal back faces backward, the first oblique position when the fetal head is on the mother's left side, and the second oblique position when the fetal head is on the right side. Natural childbirth is not possible if the fetus remains in the oblique position, but it will naturally change to a longitudinal position after the onset of labor pains.

[Masao Arai]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

子宮腔(くう)内における胎児の位置関係をいい、胎児長軸と子宮縦軸との方向関係で示される。両者の方向が一致するものを縦位、交差するものを横位という。

　縦位は、正常胎位を含む頭位（頭部が先進する）と、異常胎位である骨盤位（骨盤部が先進する）とに分けられ、頭位はさらに頭蓋(とうがい)位（後頭位と頭頂位）、額位、顔位などに分けられる。このうち後頭位は、児頭が前屈して頭部が胸部に接近する正常胎位で、屈位ともよばれる。これに対し、頭頂位、額位、顔位などは、逆に、児頭が後屈して頭部が胸部から離れた異常な場合であり、反屈位とよばれる。

　横位は、直角に交差する狭義の横位と斜めに交差する斜位に分けられる。斜位は胎児が斜めに位置するもので、さらに児背が前方に向かう場合を背前位、児背が後方に向かう場合を背後位、また児頭が母体の左側にある場合を第一斜位、右側にある場合を第二斜位とよぶ。斜位のままでは自然分娩(ぶんべん)ができないが、陣痛発来後には自然に縦位となる。

［新井正夫］

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