Objective: To investigate associations between plasma oxytocin and vasopressin concentrations and renal water and sodium excretion during normal pregnancy in comparison with gestational hypertension.
Design: A prospective open trial conducted in the 12th, 24th, and 36th weeks of gestation.
Settings: Seven antenatal clinics in Sweden.
Participants: Thirty-seven normotensive women, 15 women with gestational hypertension, and five women with mild preeclampsia.
Main outcome measures: Hormones were analyzed with radioimmunoassay. Albumin, osmolality, sodium, and urea were analyzed by routine methods.
Results: Blood pressure was elevated in the hypertensive women and body mass index in mild preeclampsia from week 12. Renal sodium excretion did not differ between groups or weeks and mean renal free water clearance was negative. In normotensive women, the vasopressin concentration was 1.1+/-0.2 (week 12) and 0.7+/-0.1 pmol/L (week 36: p = 0.053). In hypertensive women, vasopressin concentration was 1.7+/-1.0 pmol/L, week 12, and 0.7+/-0.1 pmol/L in week 36 (ns). In normotensive women, oxytocin concentration increased from 23+/-1 pmol/L in week 12 to 48+/-3 pmol/L in week 36 (p<0.001). Corresponding values in hypertensive women were 36+/-11 (week 12) and 55+/-5 pmol/L (week 36: ns). In all groups, plasma estradiol concentration increased. Plasma progesterone increased until week 24 in normotensive and hypertensive women with further increase in normotensive women.
Conclusions: The low plasma vasopressin and increasing plasma oxytocin concentrations with unchanged water and sodium excretion indicate that oxytocin assists vasopressin in concentrating urine during pregnancy.