Previous studies have shown conflicting findings regarding the relationship between maternal vitamin D deficiency (VDD) and foetal growth restriction (FGR). We hypothesized that parathyroid hormone (PTH) may be an underlying factor relevant to this potential association. In a prospective birth cohort study, descriptive statistics were evaluated for the demographic characteristics of 3407 pregnancies in the second trimester from three antenatal clinics in Hefei, China. The association of the combined status of VD and PTH with birth weight and the risk of small for gestational age (SGA) was assessed by a multivariate linear and binary logistic regression. We found that declined status of 25(OH) D are associated with the lower birth weight (for moderate VDD : adjusted β=-49.4 g, 95% CI: -91.1, -7.8, P <0.05; for severe VDD: adjusted β=-79.8 g, 95% CI: -127.2, -32.5, P <0.01), as well as ascended levels of PTH (for elevated PTH: adjusted β=-44.5 g, 95% CI: -82.6, -6.4, P <0.05). Compared to the non-VDD group with non-elevated PTH, pregnancies with severe VDD and elevated PTH had the lowest neonatal birthweight (adjusted β=-124.7 g, 95% CI: -194.6, -54.8, P <0.001) and the highest risk of SGA (adjusted RR=3.36, 95% CI: 1.41, 8.03, P <0.01). Notably, the highest risk of less calcium supplementation were founded in severe VDD group with elevated PTH (adjusted RR=4.67, 95% CI: 2.78, 7.85, P <0.001).
In conclusion, elevated PTH induced by less calcium supplementation would further aggravate the risk of FGR in pregnancies with severe VDD through impaired maternal calcium metabolism homeostasis.