Technically, back labor occurs when the fetus is in a posterior position, with its face up and the back of its headpressing against your sacrum, or the back of your pelvis.
从技术上来说，背阵痛发生在胎儿处于背对的位置，它的脸朝上而后脑勺紧靠你的骶骨或者骨盆后方。
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It did so indeed, and much sooner than she had expected: before she had drunk half the bottle, she found her headpressing against the ceiling, and had to stoop to save her neck from being broken.