This in turn prevents effective uterine contractions, and leads to uterine atony.

胎盘组织残留会阻碍子的有效，而导致乏力。

The amount of bleeding can vary, and it can be intermittent or continuous, sometimes increasing during labor because of uterine contractions and cervical dilation.

出量不等，可能是间续出，有时在生产时由于颈扩张，出量会增多。

Power refers to forceful uterine contractions, passenger refers to the fetus, and passage refers to the route that the fetus has to travel through the bony pelvis.

产力指有力的子，娩出物指胎儿，产道指胎儿穿过骨盆的路径。

It can also occur when a woman is unable to empty her bladder, since a full bladder can push against the uterus and interfere with uterine contractions.

不仅如此，膀胱排空障碍也能引起产后子乏力，因为充盈的膀胱将推挤子，进而阻碍子的有效。

Finally, some commonly used obstetric medications such as anesthetics (especially halothane), magnesium sulfate, nifedipine, and terbutaline can all interfere with uterine contractions and increase the risk of uterine atony.

此外，一些产科常用药物如麻醉药（特别是氟烷）、硫酸镁、硝苯地平和特布他林等也会影响产后子的正常，增加乏力的风险。