The present invention relates to mammography, e.g. tomosynthesis mammography. In order to provide a mammography X-ray imaging with improved data quality, a mammography X-ray imaging system (10) for tomosynthesis mammography is provided that comprises an X-ray source (12), an X-ray detector (14), a support structure (22), and a breast support (18) with a breast support surface (20). The X-ray source and the X-ray detector are mounted on an upwardly extending scan arm (24); the X-ray source is mounted on the scan arm above the breast support and the X-ray detector is mounted below the breast support. The scan arm is movably mounted to the support structure to perform a swivelling motion about a rotation axis (26) located below the breast support. During the swivelling motion, the scan arm swivels about the rotation axis such that the X-ray source and the X-ray detector perform a scan motion and an object on the breast support is radiated from different angular directions. A motion adapting mechanism (34) is provided that, during the scan motion, moves the X-ray detector along an adapted trajectory (36) that follows the breast support surface. In an example, the adapted trajectory is in alignment with the breast support surface.