...I didn't use Marvelous Designer to make the clothing - I am so Blenderised now, I didn't even think of using it. As to the pose - her zero pose is a kind of A-pose,  but the arms are slightly bent and forward, and there is a lot of curvature in the back. The pose she loads up in, is not the zero pose. You need to explicitly zero this figure.

Hmm. so Blenderized you did not even think about MD? Oh dear! We really must have a serious chat with you to get you back in the fold, LOL!.

Anyway, thank you for the info on the pose. T pose is disastrous but light arm bending may even be an advantage. It is the natural pose of an arm and when the sleeve is cut straight it will give some nice natural folds to the mesh in the elbow area. 

I had noted the back curvature which indeed may cause problems.

The images by shvrdavid are they zero pose? If not are there images of the zero pose?