Reduced postpartum hippocampal volume is associated with positive mother-infant caregiving behavior

Maternal caregiving is a complex set of behaviors that can be impacted by early life stress (ELS), yet human neurobiological mechanisms are not well understood.

Young mothers (n=137) were enrolled into a neuroimaging substudy of the longitudinal Pittsburgh Girls Study (PGS). Using data collected annually while subjects were ages 8-16, ELS was calculated as a composite score of poverty, trauma, and difficult life circumstances. At 4 months postpartum, mothers underwent neuroimaging and filmed mother-infant interaction. Maternal caregiving was coded along 6 dimensions yielding "positive" and "negative" components of caregiving. Participants' MPRAGE images were subjected to preprocessing and voxel-based morphometry (VBM) to quantify vmPFC, amygdala and hippocampus gray matter (GM) volume. We used hierarchical linear regression to investigate the relationship between GM volume and maternal caregiving, covarying for ELS as well as maternal age, weeks postpartum, race and postpartum depression score.

Hippocampal GM volume was inversely associated with independent observations of positive maternal caregiving. Similar findings in the vmPFC did not remain significant after correction for multiple comparisons. ELS, particularly physical assault, was associated with reduced GM volumes but was unrelated to observed maternal caregiving.

Our single-timepoint MRI-based GM volume method was not able to demonstrate time-related intra-individual perinatal neuroplasticity, nor could it resolve neural subregions involved in caregiving-related plasticity.

Our findings shed light on the putative plasticity of the human maternal extra-hypothalamic stress-circuitry underlying positive maternal caregiving behavior. Whether reduced hippocampal GM volume represents pruning or represents neural resilience in the face of ELS, remains to be studied.