Calcium (Ca), potassium (K), sodium (Na), and magnesium (Mg) are the elements responsible for the fundamental metabolic and biochemical processes in the cells of the body. The demand for these elements increases significantly during pregnancy, where an adequate supply protects women from the hypertension common in pre-eclampsia and preterm labor. This study aimed to evaluate the association between macro-elements (Ca, Mg, Na, and K) in the placenta, fetal membrane, and umbilical cord and the morphometric parameters of newborns from multiple pregnancies. The study involved 57 pregnant European women with healthy uncomplicated twin pregnancies (
n = 52) and triple pregnancies (
n = 5); 40 pairs of dichorionic diamniotic twins, 11 pairs of monochorionic diamniotic twins, 1 pair of monochorionic monoamniotic twins, 3 trichorionic triamniotic triplets, and 2 dichorionic triamniotic triplets. Placentas (
n = 107), umbilical cords (
n = 114), and fetal membranes (
n = 112) were collected immediately following delivery, and then weighed and measured. The levels of Ca, K, Na, and Mg were determined using inductively coupled plasma atomic emission spectroscopy (ICP-OES) in a Thermo Scientific ICAP 7400 Duo (Waltham, MA, USA). The respective mean concentrations of Ca, K, Na, and Mg (mg/kg
−1 dry mass) were: 2466, 8873, 9323, and 436 in the placenta; 957, 6173, 26,757, and 326 in the umbilical cord, and 1252, 7460, 13,562, and 370 in the fetal membrane. In the studied materials from northwestern Poland, we found strong positive correlations between Ca and Mg concentrations in both the umbilical cord (r = 0.81,
p = 0.00) and the fetal membrane (r = 0.73,
p = 0.00); between K and Mg concentrations in the umbilical cord (r = 0.73,
p = 0.00); between Ca and K concentrations in the fetal membrane (r = 0.73,
p = 0.00), and we found moderately positive correlations between placental Ca concentration and placental weight (ρ = 0.42,
p = 0.00) and between umbilical cord Mg concentrations and the length of the pregnancy (ρ = 0.42,
p = 0.00). Negative correlations were found between Na and Ca concentrations in the fetal membrane (r = −0.40,
p = 0.00) and Na concentrations in the fetal membrane and Mg concentrations in the placenta (r = −0.16,
p = 0.02). Negative correlations were confirmed between the length of pregnancy and head circumference (ρ = −0.42; p = 0.00), infant weight (ρ = −0.42;
p = 0.00), infant length (ρ = −0.49;
p = 0.00), shoulder width (ρ = −0.49;
p = 0.00); and between the infant weight and head circumference (ρ = −0.62;
p = 0.00), weight before delivery (ρ = −0.36;
p = 0.00), infant length (ρ = −0.45;
p = 0.00), shoulder width (ρ = −0.63;
p = 0.00), and weight gain during pregnancy (ρ = −0.31;
p = 0.01). We found statistically significant correlations between cigarette smoking before pregnancy and the women’s weight before delivery (ρ = 0.32,
p = 0.00), and a negative correlation between the women’s ages and infant head circumference (ρ = −0.20,
p = 0.02). This is probably the first study to evaluate Ca, Na, K, and Mg concentrations in the afterbirth tissues of multiple pregnancies. It adds to the knowledge of elemental concentrations in multiple pregnancies and their possible effects on fetal morphometric parameters.
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