Effects of Age, Season, and Pregnancy History on Vitamin D Status in Women Seeking Preconception Care

Introduction

Preconception care is crucial for optimal childbirth and postnatal development, focusing on health monitoring and risk evaluation to reduce adverse pregnancy outcomes [1,2]. Maternal nutritional profiles, metabolic conditions, and chronic disease control during the preconception phase have been shown to directly influence embryonic development, placental function, and the long-term health of mother and infant [3,4]. In recent years, preconception care has received increasing attention as a public health priority, reflecting societal concerns about fertility quality and reproductive outcomes [5,6].

Vitamin D, a fat-soluble micronutrient, plays a pivotal role in maintaining skeletal health via calcium-phosphorus homeostasis and has pleiotropic effects on immune modulation, glucose metabolism, angiogenesis, and gene regulation [7,8]. Vitamin D deficiency is a widespread global public health issue, with regional disparities [9,10]. The global prevalence of deficiency among women of reproductive age is reported to be 53.3%, and up to 72.86% among pregnant individuals [11]. Within the domain of female reproductive health, emerging evidence suggests that vitamin D contributes to female reproductive health by regulating follicular maturation, enhancing endometrial receptivity, and facilitating embryo implantation through receptor-mediated pathways [12,13]. An extensive body of literature has indicated associations between inadequate vitamin D levels and various conditions, including impaired ovarian reserve function [14], polycystic ovary syndrome, endometriosis [13,15,16], hypertensive disorders in pregnancy, preterm birth, and low birth weight [17], along with possible links to autoimmune diseases, autism, and asthma in offspring [18,19]. Given the scarcity of definitive evidence, the precise role of vitamin D in adverse pregnancy outcomes, as well as the necessity of supplementation for their prevention, remains unclear [20,21]. A retrospective analysis of the association between vitamin D levels and pregnancy history can provide a preliminary insight. Furthermore, while most existing research has concentrated on vitamin D status during pregnancy, comparatively little attention has been directed toward women in the preconception period [22–24]. This gap highlights the need for region-specific data on preconception vitamin D status and its determinants.

Exposure of the skin to ultraviolet B (UVB) light, dietary intake, and consumption of fortified foods are the primary sources of vitamin D [25]. Studies have indicated that risk factors for vitamin D deficiency include extremes of age, female sex, winter and spring seasons, high latitudes, sedentary jobs, lack of sun exposure, obesity, and low income [11,26]. Given the limitations in data collection, this economical and pragmatic cross-sectional study retrospectively analyzed specific variables that could be fully obtained from our electronic medical records system, including age, serum 25(OH)D level, season, and pregnancy history. The aim was to assess the distribution of serum 25(OH)D levels among women receiving preconception care in Foshan, China, and to examine its associations with these variables, thereby providing preliminary data for subsequent studies.

Material and Methods

STUDY DESIGN AND SETTING:

This was a retrospective cross-sectional analysis of women who received preconception care at the Affiliated Foshan Women and Children Hospital, Guangdong Medical University (Foshan, Guangdong Province, China; approximate latitude 23.0°N), between January 1 and December 31, 2023. The study received approval from the Ethics Committee of Foshan Women and Children Hospital (FSFY-MEC-2023-054), and was granted a waiver of informed consent as it involved the retrospective analysis of routinely collected clinical data.

PARTICIPANTS AND SAMPLING:

Women aged 20 years or older who voluntarily underwent serum 25(OH)D testing for assessment during their routine preconception care were eligible. Participants were selected from the full clinic registry for 2023. Subjects were eliminated from the study if they presented with previous chronic hepatic or renal conditions, thyroid or parathyroid dysfunction, disorders of bone metabolism, prior gastrointestinal resection, malignant neoplasms, or serious infections. These conditions were excluded because they can affect vitamin D metabolism, absorption, or serum levels [27]. All were ascertained through review of electronic medical records.

SAMPLE SIZE JUSTIFICATION:

A sample size calculation for estimating a prevalence was performed a priori. Using an expected vitamin D deficiency prevalence of approximately 65% [28] with a 95% confidence level and a margin of error of 3%, the required sample size was approximately 971 participants. Our final sample (n=1050) exceeded this target and therefore provided sufficient precision for prevalence estimates and adequate power for multivariable analyses.

DATA COLLECTION AND DEFINITIONS:

Demographic and clinical information (age, serum 25(OH)D level, testing period, pregnancy history, and relevant medical history) was retrospectively collected from the institutional electronic medical records system. Information on vitamin D supplementation and lifestyle factors (eg, diet, outdoor activity) was not available in the electronic medical records and therefore was not included in the analyses. Utilizing obstetric documentation, pregnancy history was classified in 2 ways. First, participants were categorized into 2 groups: those with pregnancy history and those with no pregnancy history. Second, all participants were further categorized into 2 groups: those with adverse pregnancy history and those with no adverse pregnancy history. Adverse pregnancy history was defined as any documented instance of spontaneous abortion, preterm delivery, perinatal death, congenital anomalies, ectopic pregnancy, or gestational trophoblastic disease, or any prior pregnancy-associated complications and comorbidities. Vitamin D status was classified as: sufficiency (>30 ng/mL), insufficiency (20–30 ng/mL) and deficiency (<20 ng/mL) per Endocrine Society guidance and widely used cutoffs in the literature [29,30]. Seasonal classification followed meteorological definitions: spring (March–May), summer (June–August), autumn (September–November), and winter (December–February).

LABORATORY METHODS:

Serum 25(OH)D levels were measured using a liquid chromatography–tandem mass spectrometry (LC-MS/MS) on the ACQUITY UPLC I-Class/Xevo TQD (Waters, USA) at the Medical Laboratory Center of our hospital, which is accredited under ISO 15189 and recognized by the Chinese vitamin D external quality assessment scheme. The assay had a detection range of 2.0–100.0 ng/mL, with intra- and inter-assay coefficients of variation (CVs) of ≤15.0%.

STATISTICAL ANALYSIS:

Continuous variables are presented as mean±standard deviation (SD) or median (interquartile range) according to distribution; categorical variables are presented as counts and percentages. Group comparisons used t tests or Mann-Whitney U tests for continuous variables and χ² tests for categorical variables as appropriate. To assess the associations of age, season, pregnancy history and adverse pregnancy history with serum 25(OH)D (continuous outcome), mixed-effects linear regression models were constructed adjusting for the covariates listed above, and results are presented as regression coefficients (β) with 95% confidence intervals (CIs) to indicate the magnitude and direction of effects. To assess determinants of vitamin D deficiency (binary outcome), multivariable logistic regression models were fitted and results are reported as adjusted odds ratios (OR) with 95% confidence intervals (CIs). Statistical analyses were performed using SPSS 22.0 (IBM Corp., Armonk, NY). Two-sided P<0.05 was considered statistically significant.

Results

SERUM 25(OH)D LEVELS IN WOMEN UNDERGOING PRECONCEPTION CARE:

A total of 1050 women receiving preconception care were included in this investigation, with a mean age of 29.97±4.02 years (range: 20–44 years). The median serum 25(OH)D level was 17.97 (14.47, 21.84) ng/ml. The distribution of vitamin D status revealed that 685 individuals (63.07%) were classified as deficiency, 332 (31.62%) as insufficiency, and 33 (3.14%) as sufficiency. The distribution of participants across seasons was examined by age group and pregnancy history, and no significant differences were observed.

SERUM 25(OH)D LEVELS AND VITAMIN D DEFICIENCY RATES IN WOMEN UNDERGOING PRECONCEPTION CARE OF DIFFERENT AGE GROUPS:

The lowest serum 25(OH)D concentration was recorded in the 20–29 years age group [16.98 (13.96, 20.96) ng/ml], and values demonstrated a progressive increase with advancing age. Statistically significant variations in serum 25(OH)D levels were identified among the diverse age groups (P<0.05), and all pairwise comparisons also yielded statistically significant differences (P<0.05). The prevalence of vitamin D deficiency was highest in the 20–29 years age group (70.09%) and declined with increasing age. Significant intergroup differences in vitamin D deficiency rates were observed (P<0.05), and pairwise analyses revealed significant differences between each group (P<0.05) (Table 1).

SERUM 25(OH)D LEVELS AND VITAMIN D DEFICIENCY RATES IN WOMEN UNDERGOING PRECONCEPTION CARE ACROSS DIFFERENT SEASONS:

Serum 25(OH)D levels reached their peak in autumn [18.99 (15.43, 23.83) ng/mL], with levels in both summer and autumn markedly surpassing those observed in winter and spring (P<0.05). Moreover, no statistically significant differences were detected between winter and spring (P=0.672) or between summer and autumn (P=0.188). The lowest deficiency rate was identified in autumn (56.61%), and markedly reduced rates were noted in summer and autumn compared to winter and spring (P<0.05). Pairwise analyses revealed no substantial distinction between winter and spring (P=0.323) or between summer and autumn (P=0.090) (Table 2).

SERUM 25(OH)D LEVELS AND VITAMIN D DEFICIENCY RATE IN WOMEN UNDERGOING PRECONCEPTION CARE WITH ADVERSE PREGNANCY HISTORY:

Elevated serum 25(OH)D levels (P<0.05) and a reduced prevalence of deficiency (P<0.05) were detected in women with a history of adverse pregnancy compared to those without such a history (Table 3).

SERUM 25(OH)D LEVELS AND VITAMIN D DEFICIENCY RATE IN IN WOMEN UNDERGOING PRECONCEPTION CARE WITH DIFFERENT PREGNANCY HISTORIES:

Elevated serum 25(OH)D levels (P<0.05) and a decreased rate of deficiency (P<0.05) were identified in women with a history of pregnancy compared to those without pregnancy history (Table 4).

MIXED-EFFECTS LINEAR REGRESSION ANALYSIS OF SERUM 25(OH)D LEVELS WITH AGE, SEASON, ADVERSE PREGNANCY HISTORY, AND PREGNANCY HISTORY IN WOMEN UNDERGOING PRECONCEPTION CARE:

Mixed-effects linear regression analysis demonstrated that age, pregnancy history, and season were all significant determinants of serum 25(OH)D concentrations. After adjustment for covariates, age (β=0.116, 95% CI: 0.073–0.246, P<0.001) and pregnancy history (β=0.156, 95% CI: 0.874–2.671, P<0.001) were positively associated with serum 25(OH)D levels. Compared with autumn, significantly lower serum 25(OH)D concentrations were observed in spring (β=−0.193, 95% CI: −3.328 - −1.395, P<0.001), summer (β=−0.096, 95% CI: −2.057 - −0.261, P=0.011), and winter (β=−0.120, 95% CI: −2.606 - −0.649, P=0.001). Adverse pregnancy history was not significantly associated with serum 25(OH)D concentrations (P>0.05) (Table 5).

MULTIVARIABLE LOGISTIC REGRESSION ANALYSIS OF VITAMIN D DEFICIENCY WITH AGE, SEASON, ADVERSE PREGNANCY HISTORY, AND PREGNANCY HISTORY IN WOMEN UNDERGOING PRECONCEPTION CARE:

Multivariable logistic regression analysis was further performed to examine determinants of vitamin D deficiency. Increasing maternal age was associated with a lower risk of deficiency (OR=0.945, 95% CI: 0.913–0.978, P=0.001), while women without a pregnancy history had a significantly higher risk compared with those with a history of pregnancy (OR=1.717, 95% CI: 1.209–2.439, P=0.003). Seasonal variation remained significant: compared with autumn, the risk of vitamin D deficiency was higher in spring (OR=2.145, 95% CI: 1.478–3.111, P=0.001) and winter (OR=1.743, 95% CI: 1.811–2.574, P=0.005). No significant association was observed between adverse pregnancy history and vitamin D deficiency (P=0.835) (Table 6).

Discussion

LIMITATIONS:

Key limitations include the single-center, retrospective design; absence of prospectively collected data on sun exposure, dietary vitamin D intake, supplement use, sunscreen practices, and physical activity; and lack of follow-up data on subsequent pregnancy outcomes in the current dataset. We have added explicit recommendations for future research: prospective, multicenter cohort studies that collect detailed behavioral and dietary data, measure 25(OH)D longitudinally, and evaluate clinical outcomes (eg, spontaneous abortion, preterm birth, hypertensive disorders of pregnancy, gestational diabetes mellitus, and fetal growth restriction) are needed to inform evidence-based preconception supplementation policies.

Conclusions

Vitamin D deficiency was common among women receiving preconception care in Foshan. Serum 25(OH)D concentrations were associated with age, season, and pregnancy history. These findings support consideration of vitamin D assessment and counseling during preconception care, particularly for younger women anticipating their first pregnancy during winter or spring. Future prospective, multicenter research with detailed exposure measurement is required before definitive public health recommendations can be made.