De Quervain's tenosynovitis often occurs in mothers who have newborn babies, so it is also called 'Mother's Thumb'. In particular, as the hormone 'Relaxin' is secreted during pregnancy, soft tissues (muscles, tendons, and ligaments) are stretched and weakened over time, and when they keep lifting the baby repeatedly or complete housework, the already weak muscles and tendons get overloaded and tight.

In the left image above, the thumb muscles are naturally activated to support the baby. Since the baby's weight creates a downward force due to gravity, the thumb muscles have to elongate a bit while contracting at the same time. However, if the muscles are too weak to do so, they tend to contract strongly (Green arrows in the right image), which pulls the tendon away from its attachment (Red arrow)

Then, the tendon sheath, a thin layer that protects the tendon, starts to get rubbed against the extensor retinaculum that wraps around the whole wrist joint. This may cause swelling or inflammation, which is called 'De Quervain's tenosynovitis'. This condition can occur in anyone who has weakness in their shoulder/arm muscles which causes overloading in their hand/thumb instead.

Also, if you have rounded shoulders, your thumb tends to face your thigh. In this case, the thumb muscles in the figure above will be placed in a lengthened position. Stretched muscles gradually weaken and become overloaded, which can cause thumb pain due to the mechanisms described above.