The correct answer is choice A. Increased joint laxity contributes to increased spinal curvature in pregnant women.

Joint laxity is caused by the hormone relaxin, which loosens the ligaments and joints of the pelvis and spine to prepare for childbirth.

This can result in increased lordosis (inward curvature) of the lumbar spine and increased kyphosis (outward curvature) of the thoracic spine.

Choice B is wrong because increased weight gain does not directly affect spinal curvature, but it can cause postural changes such as anterior pelvic tilt and forward trunk lean.

Choice C is wrong because increased muscle strain is a consequence, not a cause, of spinal curvature changes.

Muscle strain can occur due to the altered biomechanics and balance of the spine and pelvis during pregnancy.

Choice D is wrong because decreased muscle strength is also a consequence, not a cause, of spinal curvature changes.

Muscle strength can decrease due to reduced physical activity, hormonal changes, or pain during pregnancy.

Normal ranges for spinal curvature vary depending on age, sex, and measurement method, but generally they are:

Sacral inclination: 30-50 degrees

Lumbar curvature: 20-45 degrees

Thoracic curvature: 20-40 degrees

Cervical curvature: 20-35 degrees