Vitamin D is essential for maternal calcium homeostasis and fetal skeletal growth and mineralisation. Serum 25-hydroxyvitamin D [25(OH)D] is the preferred biochemical indicator of vitamin D status. Although a concentration below 20 ng/mL is commonly used to define deficiency, the optimal pregnancy-specific concentration and the clinical benefits of correcting asymptomatic deficiency remain uncertain [1]

Vitamin D deficiency is common during pregnancy, particularly in South Asian populations. Indian studies have reported a substantial burden among pregnant women and their newborns despite abundant sunlight [2]. A systematic review and meta-analysis of studies conducted across India similarly demonstrated widespread deficiency, supporting the need for region-specific prevalence data [3]. Reduced sunlight exposure, dietary inadequacy, skin pigmentation, season and maternal adiposity may contribute to variations in vitamin D status.

Low maternal 25(OH)D concentrations have been associated with several adverse pregnancy outcomes. Observational evidence suggests possible relationships with gestational diabetes mellitus, pre-eclampsia, preterm birth and impaired fetal growth [4–8]. Early-pregnancy vitamin D status may be especially relevant because it is measured before the development of most obstetric complications and may therefore provide clearer temporal information than measurements obtained near delivery. Nevertheless, published findings remain inconsistent because of differences in study populations, gestational age at sampling, laboratory assays, deficiency thresholds, supplementation and adjustment for confounding variables.

Evidence from prospective studies in district-level Indian healthcare settings remains limited. Therefore, the present study was undertaken to determine the prevalence and severity of first-trimester vitamin D deficiency among pregnant women attending District Hospital, Chitradurga, and to evaluate its association with maternal outcomes—including gestational diabetes mellitus, hypertensive disorders, preterm delivery and mode of delivery—and neonatal outcomes such as birth weight, small-for-gestational-age birth, low Apgar score and neonatal intensive care unit admission.

Objectives

1. To estimate the prevalence and severity of vitamin D deficiency among women presenting during the first trimester of pregnancy.
2. To evaluate the association between first-trimester vitamin D status and maternal outcomes, including gestational diabetes mellitus, hypertensive disorders of pregnancy, preterm delivery and mode of delivery.
3. To assess the association between first-trimester vitamin D status and neonatal outcomes, including birth weight, low birth weight, small-for-gestational-age birth, low Apgar score and neonatal intensive care unit admission.

Study design and setting This prospective observational cohort study was conducted in the Department of Obstetrics and Gynaecology, District Hospital, Chitradurga, Karnataka, India, from December 2025 to May 2026. Pregnant women enrolled during the first trimester were followed until delivery to document maternal and neonatal outcomes. Study population A total of 200 pregnant women attending the antenatal clinic during the study period were enrolled through consecutive sampling after obtaining written informed consent. Eligibility criteria Pregnant women aged 18 years or older with a viable singleton pregnancy at less than 14 completed weeks of gestation who intended to continue antenatal care and deliver at the study hospital were eligible. Women with multiple pregnancy, pre-existing diabetes mellitus, chronic hypertension, chronic renal or hepatic disease, parathyroid disorders, malabsorption syndromes, major fetal anomalies, or use of therapeutic-dose vitamin D before enrolment were excluded. Women who could not complete follow-up until delivery were excluded from the outcome analysis. Data collection At enrolment, information was recorded regarding maternal age, residence, socioeconomic status, parity, gestational age, body mass index, dietary pattern, sunlight exposure, vitamin supplementation, medical history and previous obstetric outcomes. Gestational age was determined from the last menstrual period and confirmed by first-trimester ultrasonography wherever available. Assessment of vitamin D status A maternal venous blood sample was collected at enrolment for measurement of serum 25-hydroxyvitamin D [25(OH)D]. Serum 25(OH)D concentrations were determined using a chemiluminescent immunoassay on the ADVIA Centaur XP automated immunoassay system (Siemens Healthineers, Germany). For the primary analysis, vitamin D deficiency was defined as a serum 25(OH)D concentration below 20 ng/mL (50 nmol/L). Vitamin D status was further categorized as severe deficiency (<12 ng/mL), deficiency (12 to <20 ng/mL), and adequate status (≥20 ng/mL). These thresholds were prespecified because consensus regarding the optimal serum 25(OH)D concentration during pregnancy remains limited, while concentrations of at least 20 ng/mL are generally considered adequate for most individuals. All participants received routine antenatal care according to institutional protocols. Any vitamin D supplementation prescribed after enrolment was documented, including the formulation, dose, duration, and adherence Follow-up and outcome measures Participants were followed during routine antenatal visits and at delivery. The primary outcome was the prevalence of first-trimester vitamin D deficiency. Maternal outcomes included gestational diabetes mellitus, gestational hypertension or pre-eclampsia, preterm delivery before 37 completed weeks, induction of labour and mode of delivery. Diagnostic criteria for gestational diabetes and hypertensive disorders should be specified according to the institutional protocol used during the study. Neonatal outcomes included gestational age at birth, birth weight, low birth weight below 2500 g, small-for-gestational-age birth below the 10th percentile, macrosomia of at least 4000 g, five-minute Apgar score below 7 and admission to the neonatal intensive care unit. Stillbirth and neonatal death, where present, were also recorded. Statistical analysis Data were analysed using SPSS version 24. Continuous variables were summarized as mean ± standard deviation or median with interquartile range, according to their distribution. Categorical variables were presented as frequencies and percentages. The prevalence of vitamin D deficiency was reported with a 95% confidence interval. Maternal and neonatal outcomes were compared between women with and without vitamin D deficiency using the independent-samples t test or Mann–Whitney U test for continuous variables and the chi-square or Fisher exact test for categorical variables. Multivariable logistic regression was used to evaluate associations between vitamin D deficiency and categorical outcomes after adjustment for clinically relevant variables such as maternal age, body mass index, parity, socioeconomic status, season of sampling and vitamin D supplementation after enrolment. Adjusted odds ratios with 95% confidence intervals were reported. A two-sided p value below 0.05 was considered statistically significant. Ethical considerations The study was conducted after approval from the Institutional Ethics Committee Written informed consent was obtained from all participants, and confidentiality of participant information was maintained throughout the study.

Participant characteristics and vitamin D status

A total of 200 women were enrolled at a mean gestational age of 10.4 ± 1.9 weeks. First-trimester vitamin D deficiency was identified in 150 (75.0%; 95% CI 68.6–80.5), including 48 (24.0%) women with severe deficiency and 102 (51.0%) with deficiency; 50 (25.0%) had adequate vitamin D status. Women with vitamin D deficiency had a higher body mass index, lower sunlight exposure, and less frequent baseline prenatal vitamin D use, while the remaining baseline characteristics were comparable between groups (Tables 1 and 2; Figure 1).

Table 1. Baseline characteristics according to first-trimester vitamin D status

Data are mean ± SD, median [IQR], or n (%). Welch independent-samples t test, Mann–Whitney U test, Pearson chi-square test, or Fisher exact test was used as appropriate.

Table 2. Prevalence and severity of first-trimester vitamin D deficiency (n=200)

Prevalence confidence intervals were calculated using the Wilson method. 25(OH)D, 25-hydroxyvitamin D; CI, confidence interval; IQR, interquartile range.

Maternal outcomes

Delivery outcomes were recorded for 192 (96.0%) women, comprising 143 with vitamin D deficiency and 49 with adequate status. Gestational diabetes mellitus, hypertensive disorders, preterm delivery, and cesarean delivery occurred more frequently among deficient women, although none of these associations reached statistical significance. Gestational age at delivery, induction of labour, and the overall mode of delivery distribution were similar between groups (Table 3; Figure 2).

Table 3. Maternal outcomes according to first-trimester vitamin D status

Data are mean ± SD or n (%). Effect estimates are risk ratios for categorical outcomes and mean differences for continuous outcomes, calculated as deficient minus adequate. CI, confidence interval.

Neonatal outcomes

Mean birth weight was lower in the vitamin D-deficient group, and low birth weight, small-for-gestational-age birth, low five-minute Apgar score, and neonatal intensive care unit admission were numerically more frequent. However, the corresponding confidence intervals included the null value and no neonatal outcome differed significantly between groups. Stillbirth occurred in 2 (1.4%) deficient pregnancies, and neonatal death occurred in 1 (0.7%) liveborn infant in the deficient group (Table 4; Figure 2).

Table 4. Neonatal outcomes according to first-trimester vitamin D status

Birth weight, low birth weight, small-for-gestational-age birth, macrosomia, and stillbirth were assessed among 143 deficient and 49 adequate pregnancies with delivery outcomes. Apgar score, respiratory distress, NICU admission, and neonatal death were assessed among 141 deficient and 49 adequate live births. Continuous data are mean ± SD or median [IQR]. Effect estimates are risk ratios for categorical outcomes and mean differences for continuous outcomes. CI, confidence interval; NICU, neonatal intensive care unit.

Figure 1. Distribution of first-trimester vitamin D status. Error bars represent 95% confidence intervals.

Figure 2. Risk ratios for selected maternal and neonatal outcomes in women with vitamin D deficiency compared with adequate vitamin D status. Points represent risk ratios and horizontal lines represent 95% confidence intervals. NICU, neonatal intensive care unit.

This prospective cohort study found that 75% of women attending District Hospital, Chitradurga, had first-trimester vitamin D deficiency, including 24% with severe deficiency. Deficient women had higher body mass index, lower sunlight exposure and less frequent prenatal vitamin D use. Gestational diabetes mellitus, hypertensive disorders, preterm delivery and caesarean birth were more frequent among deficient women, while their infants had lower mean birth weight and higher frequencies of low birth weight, small-for-gestational-age birth and neonatal intensive care unit admission. However, none of these outcome associations reached statistical significance, and the confidence intervals were wide. The high prevalence observed in this study is consistent with regional evidence. In a cohort of 559 pregnant women from Mysore, Farrant et al. reported that 66% had serum 25-hydroxyvitamin D [25(OH)D] below 50 nmol/L and 31% had concentrations below 28 nmol/L, despite 156 women using calcium and vitamin D preparations. Maternal vitamin D status was not associated with gestational diabetes or neonatal size in that study [10]. More recently, the Bengaluru MAASTHI cohort reported deficiency in 77.4% of 230 women, closely matching the present prevalence. In that cohort, gestational diabetes occurred in 19.1% and low birth weight in 27.8%; women in the lowest vitamin D quartile had higher adjusted odds of gestational diabetes, while deficiency below 20 ng/mL was associated with approximately twice the odds of low birth weight [18]. These findings indicate that vitamin D deficiency is widespread among pregnant women in Karnataka, although its clinical consequences remain variable. In the present study, gestational diabetes occurred in 16.8% of deficient women compared with 8.2% of women with adequate vitamin D, corresponding to a risk ratio of 2.06. Baker et al., however, found no association in a nested case-control study involving 60 gestational-diabetes cases and 120 controls: deficiency was present in only 5 cases and 8 controls, median 25(OH)D was approximately 89 nmol/L, and 73% of participants had concentrations of at least 75 nmol/L [11]. The relatively vitamin D-replete population may partly explain the null result. Conversely, Xia et al. studied 107 cases and 214 controls and found that deficiency at 10–14 weeks was associated with a 2.82-fold higher risk of gestational diabetes. Persistent deficiency through 15–26 weeks was associated with a 4.46-fold higher risk after adjustment for maternal age, body mass index, ethnicity, parity, season and family history [12]. The present effect estimate is therefore directionally consistent with studies reporting increased risk, but the limited number of events reduced statistical precision. Hypertensive disorders occurred in 9.1% of deficient women and 4.1% of women with adequate status. Bodnar et al. found that women who later developed pre-eclampsia had lower early-pregnancy 25(OH)D concentrations than controls, 45.4 versus 53.1 nmol/L. A concentration below 37.5 nmol/L was associated with an adjusted odds ratio of 5.0, while each 50-nmol/L reduction was associated with a 2.4-fold increase in pre-eclampsia risk [13]. In contrast, Schneuer et al. evaluated 5,109 pregnancies and found no independent association between first-trimester vitamin D and pre-eclampsia or gestational diabetes. A concentration below 25 nmol/L showed poor discrimination for severe pregnancy outcomes, with an area under the curve of 0.51, and did not improve prediction beyond established maternal factors [14]. These contrasting findings suggest that vitamin D may partly reflect adiposity, diet, sunlight exposure and socioeconomic conditions rather than functioning as an isolated causal factor. Preterm delivery occurred in 13.3% of deficient pregnancies compared with 8.2% of adequate pregnancies. In a case-cohort analysis including 1,126 preterm births, Bodnar et al. reported a graded decline in preterm-birth incidence from 11.3% among women with 25(OH)D below 50 nmol/L to 8.6% at 50–74.9 nmol/L and 7.3% at 75 nmol/L or above. Risk decreased until approximately 90 nmol/L and then plateaued [15]. Monier et al., however, found only a borderline overall association in 2,813 women: first-trimester deficiency was present in 45.1%, preterm birth occurred in 6.7%, and the adjusted odds ratio comparing the lowest with the highest quartile was 1.53. A stronger association was observed among women with darker skin, whereas no overall relationship with small-for-gestational-age birth was detected [17]. The current risk ratio of 1.63 is comparable in magnitude but remains inconclusive because of the smaller sample. Neonates of deficient women weighed an average of 126 g less, and low birth weight occurred in 11.9% compared with 4.1% among women with adequate vitamin D. Leffelaar et al. studied 3,730 pregnancies and reported that infants of vitamin D-deficient women initially weighed 114.4 g less. After adjustment for maternal, ethnic, seasonal and infant factors, birth weight remained 64 g lower, and the adjusted odds of small-for-gestational-age birth were 1.9-fold higher [16]. The similarity in the direction and magnitude of the birth-weight difference supports a possible relationship, although the present confidence interval included no effect. The higher frequencies of low Apgar score and neonatal intensive care admission should also be interpreted cautiously because these outcomes were uncommon and may have been mediated by prematurity, fetal growth restriction or maternal complications. Differences across studies may reflect variation in gestational age at sampling, assay methods, vitamin D thresholds, skin pigmentation, adiposity, supplementation and adjustment for confounding factors. A single first-trimester measurement may also inadequately represent vitamin D exposure throughout pregnancy, particularly when supplementation is initiated after enrolment. The strengths of this study include its prospective design, first-trimester vitamin D measurement, assessment of both maternal and neonatal outcomes and 96% follow-up to delivery. Its principal limitations were the single-centre design, small adequate-vitamin-D group, limited number of adverse events, single 25(OH)D measurement and potential residual confounding. Post-enrolment supplementation may additionally have reduced differences between exposure groups.

First-trimester vitamin D deficiency was highly prevalent in this cohort and was associated with less sunlight exposure, higher body mass index and lower prenatal supplement use. Adverse maternal and neonatal outcomes were consistently more frequent among deficient women, but the study lacked sufficient precision to establish independent associations. Larger multicentre cohorts with repeated vitamin D measurements, standardised assays and appropriately adjusted analyses are required to clarify whether early-pregnancy deficiency is a causal and modifiable risk factor.

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