As the fetus develops during pregnancy, your belly grows farther out in front of you. To avoid tipping over onto your face, your spine must extend backward to counter this new development. Therefore, the lordotic curve (inward curve) of your spine becomes more pronounced. This increased lordotic curve then changes how the spine bends forward. Now, because of its new shape, the vertebrae don't get to flex as much as they used to when bending forward; instead, they remain in some extension. Also, when you return from bending forward, your spine starts out in a more extended position and moves into even further extension while resuming its new, larger lordotic curve. So, the increased weight in your tummy has caused your lumbar spine to increase its lordotic curve, which then alters how you bend forward and straighten back up.
When the spine assumes an extension posture, it becomes more difficult for the vertebrae to recover their normal lordotic curve. Pain can result from this abnormal stress to the spine and surrounding tissues. This is what I refer to as an extension problem, meaning the spine is either extended too much, it's unable to flex well, or the abdominal muscles lack the necessary strength to stabilize the spine against extension-producing stresses.
Aside from extension problems, the spine can also become rotated. I call this a rotation problem. Rotation problems can exacerbate back pain due to extension problems.