Plasma Urea and Serum Calcium Changes in Normal Pregnancy
Plasma Urea Concentration
In normal pregnancy, plasma urea concentration decreases—not increases—reaching its lowest values in the second trimester and remaining below non-pregnant reference ranges throughout gestation. 1
Physiological Mechanisms Behind Decreased Urea
Enhanced glomerular filtration rate (GFR) increases by approximately 50% by the second trimester, persisting until term, which accelerates urea clearance from the circulation. 2, 1
Plasma volume expansion of 40-50% during pregnancy produces hemodilution, which together with enhanced renal clearance lowers plasma concentrations of both urea and creatinine throughout gestation. 2, 1
Maternal urea synthesis and urinary urea excretion decrease early in gestation and remain suppressed, indicating reduced amino-acid catabolism and nitrogen conservation to support fetal protein synthesis. 3
Plasma levels of glucogenic amino acids decline significantly in early pregnancy and stay low throughout gestation, conserving carbon skeletons for fetal glucose synthesis rather than being catabolized to urea. 3
Clinical Interpretation of Elevated Urea
When serum urea is higher than the expected pregnancy range, clinicians must assess for pathological conditions including volume depletion (hyperemesis or inadequate oral intake), preeclampsia with renal involvement, acute kidney injury, or urinary tract obstruction—none of which represent normal physiological adaptation. 1
In preeclampsia, the normal rise in GFR observed in healthy pregnancy is absent, and deterioration of GFR parallels the development of proteinuria and fluid retention. 4
Serum Calcium Changes
The provided evidence does not contain specific guideline or research data addressing serum calcium changes during normal pregnancy. Based on general medical knowledge, total serum calcium typically decreases during pregnancy due to hemodilution and increased plasma volume, while ionized (free) calcium—the physiologically active form—generally remains stable. However, this answer prioritizes only the evidence-based information available regarding urea, as the question specifically asks about both parameters but the evidence predominantly addresses renal function and urea metabolism.
Common Pitfalls to Avoid
Do not interpret the physiological decline in plasma urea as a sign of malnutrition or inadequate protein intake; it reflects normal metabolic adaptation to pregnancy. 1, 3
Elevated urea in pregnancy should never be dismissed as "normal variation"; it warrants immediate investigation for volume depletion, preeclampsia, or renal pathology. 1, 5
Non-pregnant reference ranges cannot be used for renal function assessment in antenatal care, as renal physiology is so profoundly altered during pregnancy. 6