text: **White matter microstructure changes tied to gestation**
  In contrast to decreasing global GMV, correlational tractography of 
  white matter, which tests for linear trends in the data, revealed increas- 
  ing microstructural integrity across the whole brain during gestation 
  (Fig. 4a), concomitant with the rise in 17β-estradiol and progesterone 
  (all*q*< 0.001; Supplementary Fig. 9). Tracts displaying robust corre- 
  lations with gestational week included the corpus callosum, arcuate 
  fasciculus, inferior fronto-occipital fasciculus and inferior longitudinal 
  fasciculus (Fig. 4b), as well as the cingulum bundle, middle and superior 
  longitudinal fasciculus, corticostriatal, corticospinal and corticopon- 
  tine tracts (see Supplementary Table 9 for complete list). 
  
  **Comparing brain changes across pregnancy against controls**
  We then compared the changes in GMV across gestation to that of typi- 
  cal variability over time, derived from eight densely-sampled controls23. 
  The GMV changes we see across pregnancy far exceed normative brain 
  variability (Supplementary Fig. 11). On average, change in cortical GMV 
  was nearly three times higher than controls scanned over a similar 
  duration (Supplementary Fig. 11a,b). This extends to MTL subfields, 
  wherein change in volume was three to four times greater across gesta- 
  tion than normative brain variability (Supplementary Fig. 11c,d). We 
  contextualized these findings further by comparing gestational GMV 
  change against our participant’s preconception brain volumes; average 
  GMV change during pregnancy was six times (cortical) and three times 
  (MTL) higher than the variability observed between baseline sessions. 