PTHLH and gestational diabetes: However, limited samples and the lack of a complete evaluation of PTH–vitamin D–calcium axis in these pregnancies (including PTH, PTH-rP, and PTH-r1 expression in maternal serum or umbilical cord blood) make it difficult to clearly understand the role of PTH-rP and PTH-R1 in the placental pathophysiology in pregnancies complicated by GDM.