Efficacy and Safety of Low-dose Calcium Supplementation in Pregnancy

This study is a prospective, randomized, double-blind controlled pilot trial designed to comprehensively evaluate the efficacy and safety of a targeted low-dose (300 mg/day) versus a standard high-dose (600 mg/day) calcium supplementation during mid-to-late pregnancy. The main research content is divided into the following four core dimensions: 1. Evaluation of Maternal Skeletal and Systemic Calcium Efficacy: The core efficacy evaluation focuses on the preservation of maternal bone mineral density throughout mid-to-late pregnancy. This is assessed by the change in the BMD Z-score (ΔZ-score) from baseline to delivery using quantitative ultrasound (QUS). Concurrently, by comparing pre- and post-intervention maternal serum total calcium concentrations, the study verifies whether the low-dose regimen is sufficient to maintain normal systemic calcium homeostasis and support fetal development. 2. Safety Assessment of Renal Filtration and Metabolic Burden: To investigate the potential metabolic toxicity of high-dose calcium, the study explicitly evaluates the maternal renal calcium excretion burden. By measuring the change in the fasting urinary calcium-to-creatinine ratio (ΔU-Ca/Cr) before and after the intervention, the research aims to determine whether the targeted low-dose regimen effectively prevents iatrogenic absorptive hypercalciuria induced by excessive calcium overload. 3. Evaluation of Clinical Gastrointestinal Tolerability: Given that gastrointestinal adverse events are the primary deterrent to prenatal supplement adherence, this study utilizes the internationally recognized Rome IV criteria to rigorously track and compare the time-to-onset and cumulative incidence of moderate-to-severe constipation between the two groups, thereby quantifying the difference in intestinal tolerability between the dosages. 4. Exploration of Adverse Pregnancy Outcomes and General Safety: As an exploratory clinical endpoint, the study records and compares the incidence rates of preeclampsia and preterm birth between the two intervention arms to ensure the low-dose regimen safely maintains prophylactic benefits against adverse obstetric outcomes. Furthermore, a comprehensive panel of maternal general laboratory parameters (e.g., hepatic and renal function, lipid profiles, and hematology) is monitored throughout to guarantee overall systemic safety.

Inclusion Criteria: * Pregnant women aged 18 to 45 years * Gestational age between 22 and 28 weeks * Body mass index (BMI) between 19.0\~28.0 kg/m2 * Willingness to comply with the study protocol and provision of informed consent Exclusion Criteria: * Known hypersensitivity or allergy to any active components or excipients of the study medications * Multifetal gestation * History of chronic constipation or functional defecation disorders prior to or during early pregnancy * Current clinical hypercalcemia, hyperuricemia, familial hyperlipidemia, or intestinal malabsorption disorders * History of prolonged extreme low-calcium diets or severe diagnosed nutritional deficiencies * History of illicit substance abuse, chronic alcoholism, heavy smoking, or excessive caffeine intake