The outcome of obstetric brachial plexus palsy depends on the severity of the lesion of the nerve fibers. The aim of the prospective study is to evaluate if differences in force used in downward traction on the fetal head correlate to the number of nerve roots affected. At final neurological examination at 18 months of age, complete neurological recovery occurred in 80 of 98 children (82%). Downward traction of the fetal head was applied more often and with greater force in the group with persistent damage. There was a significant correlation between the force used to the number of nerve roots affected. The risk of persistent obstetric brachial plexus palsy at age 18 months depended on obstetric management and increased significantly with increasing force used in downward traction of the fetal head.