原来如此。。

孕期腰骨痛

妊娠进入中期后，孕妇体内开始分泌催乳和松弛骨关节的荷尔蒙，为分娩做好准备。这些荷尔蒙起到松弛关节和肌肉、促进顺利分娩的作用，但同时也使骨盆、脊椎等的结合变松，成为腰痛、背痛的原因。加上随着妊娠月份的进展，子宫不断增大，身体重心渐渐向前移，在站立或走路时，为保持重心平衡，孕妈咪只能将身体后倾，而这种姿势加重了腰背部的韧带和脊柱的负荷，也会导致或加重腰背痛。此外，增大的子宫对腰背部神经的压迫，也是造成腰背疼痛的另一个原因。进入妊娠末期，腰背痛的现象更明显。

(http://www.birthhealth.com/yunfubaojian/yunzaoqi/201112/65281.html)

What happens to the pelvis during pregnancy?

During pregnancy your body produces a hormone called relaxin, which softens the ligaments (the tough, flexible tissues that connect your bones) in your pelvis and other joints. This helps your baby to pass through your pelvis during birth. As a result, these joints move more during and just after pregnancy (Kristiansson 1997; Bjorkland et al 1999; 2000).

What causes pelvic pain?

It's thought that this softening combines with changes in your body to put too much stress on the joints, ligaments and muscles of your pelvis (Buyruk et al 1999; Damen et al 2001). These changes are in your posture, muscle strength and in the way the left and right joints in your pelvis work.
Normally, when you lie down, stand up or walk, your pelvis is in a locked, or stable, position. Women who suffer from pelvic pain are thought to do these activities with a less stable, or unlocked, pelvis (Hungerford et al 2004).

 

(http://www.babycentre.co.uk/pregnancy/antenatalhealth/physicalhealth/pelvicgirdlepain/)