Background Increased hypothalamic-pituitary-adrenal (HPA) axis activity has been associated with low birthweight and preterm labour. Maternal obesity is associated with macrosomia (birthweight ≥4000 g) and prolonged pregnancy (≥41 weeks). We hypothesised altered HPA axis activity may underlie these pregnancy outcomes and aimed to characterise HPA axis hormones in obese pregnancy.
Method Fasting serum cortisol was measured by radioimmunoassay at 16, 28 and 36 weeks in 286 obese (BMI 44.05 ± 3.98 kg/m2) and 137 lean (BMI 22.71 ± 1.66 kg/m2) pregnant women. In subsets (n = 20 obese, 20 lean) we measured corticosteroid-binding-globulin (CBG) and corticotrophin-releasing-hormone (CRH) by radioimmunoassay. Free cortisol was calculated using Coolen’s equation.
Results Cortisol was significantly lower throughout pregnancy in obese (Table 1), and lower free cortisol at 16 weeks was associated with higher birthweight (r = -0.46, p < 0.05). Lower CRH at 28 weeks correlated with later gestation at delivery in obese (r = -0.56, p < 0.05) and remained significant as an independent predictor of gestation at delivery (beta = -3.19; p < 0.05). Lower CRH at 36 was associated with delivery ≥41 weeks (236.4 ± 172.7 pmol/L vs. 271.9 ± 233.8 pmol/L; p < 0.05) and with macrosomia (108.68 ± 31.7 pmol/L vs. 291.09 ± 216.32 pmol/L; p < 0.05).
Conclusion Decreased HPA axis activity in obese pregnancy may underlie macrosomia and prolonged pregnancy.
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