Context : Magnetohydrodynamical simulations show that the magnetic field can drive molecular outflows during the formation of massive protostars .
The best probe to observationally measure both the morphology and the strength of this magnetic field at scales of 10-100 au is maser polarization .

Aims : We measure the direction of magnetic fields at milliarcsecond resolution around a sample of massive star-forming regions to determine whether there is a relation between the orientation of the magnetic field and of the outflows .
In addition , by estimating the magnetic field strength via the Zeeman splitting measurements , the role of magnetic field in the dynamics of the massive star-forming region is investigated .

Methods : We selected a flux-limited sample of 31 massive star-forming regions to perform a statistical analysis of the magnetic field properties with respect to the molecular outflows characteristics .
We report the linearly and circularly polarized emission of 6.7 GHz CH _ { 3 } OH masers towards seven massive star-forming regions of the total sample with the European VLBI Network .
The sources are : G23.44-0.18 , G25.83-0.18 , G25.71-0.04 , G28.31-0.39 , G28.83-0.25 , G29.96-0.02 , and G43.80-0.13 .

Results : We identified a total of 219 CH _ { 3 } OH maser features , 47 and 2 of which showed linearly and circularly polarized emission , respectively .
We measured well-ordered linear polarization vectors around all the massive young stellar objects and Zeeman splitting towards G25.71-0.04 and G28.83-0.25 .
Thanks to recent theoretical results , we were able to provide lower limits to the magnetic field strength from our Zeeman splitting measurements .

Conclusions : We further confirm ( based on \sim 80 \% of the total flux-limited sample ) that the magnetic field on scales of 10-100 au is preferentially oriented along the outflow axes .
The estimated magnetic field strength of |B _ { || } | > 61 mG and > 21 mG towards G25.71-0.04 and G28.83-0.2 , respectively , indicates that it dominates the dynamics of the gas in both regions .