We have demonstrated a reversible decrease in brain size in pregnancy that has a prolonged recovery, taking months to return to normal after delivery (Oatridge et al. 2002). The mechanism and functional significance is speculative so this study was designed to investigate the metabolic changes that may be part of this normal adaptation to parturition.

After Ethics Committee approval consenting women were scanned at term, 6 weeks and, if possible, 6 months after delivery. Control women had scans a month apart. All measurements were obtained using a 1.5 T Eclipse▓trade│ MR scanner (Philips Medical Systems Inc., Cleveland, Ohio, USA). ³¹P MR spectra (TR = 10 s, 64 signal averages) were localised on a 70 X 70 X 70 mm voxel in the brain using an ISIS sequence. The MR spectra were analysed by an observer blinded to the clinical status of the subjects using prior knowledge (Hamilton et al. 2002) in the AMARES algorithm included in the MRUI software program. The peak areas of phosphomonoester (PME), inorganic phosphate (P_i), phosphodiester (PDE), phosphocreatine (PCr), and nucleoside triphosphate (λ, α and βNTP) resonances were calculated as a percentage of the total peak areas. Further the intracellular pH was calculated using:

pH = 6.77 + log ((A – 3.29)/(5.68 – A)),

where A = chemical shift difference in p.p.m. between P_i and PCr (see arrow in Fig. 1).

Thirteen pregnant (10 at 6 months) women completed the study. The mean (S.E.M.) pH was 7.029 (± 0.010), 7.072 (± 0.017) and 7.017 (± 0.016) at term, 6 weeks and 6 months, respectively, with P < 0.05 from term to 6 weeks and 6 weeks to 6 months using ANOVA. In five controls, at both times, pH was similar to 6 months after delivery. The metabolic change in the ³¹P spectra was between term and 6 weeks, with P_i 4.59 (± 0.27) % and 5.39 (± 0.26) % of the total peak area, respectively.

Pregnancy is associated with a mild arterial alkalaemia secondary to hyperventilation. MR spectrum of erythrocytes has demonstrated an intracellular acidosis (Bardicef et al. 1995) to explain this respiratory stimulation. However, CSF is more alkalotic during pregnancy (Hirabayashi et al. 1996). We have demonstrated that prolonged metabolic perturbations may be induced that persist after delivery.