Preeclampsia (PE) contributes substantially to maternal morbidity and mortality in Kenya, accounting for about one in five direct maternal deaths. Vitamin D, primarily measured as serum 25-hydroxyvitamin D (25(OH)D), has been proposed to modulate pathways implicated in PE including inflammatory responses and the renin-angiotensin system. Evidence from trials and meta-analyses suggests vitamin D supplementation may reduce PE risk, but few data are available from African populations. This study aimed to determine the association between maternal serum 25(OH)D concentrations and the risk of preeclampsia among pregnant women in a Kenyan referral hospital, and to describe the proportion of vitamin D deficiency in the study sample.
The investigators conducted a single-centre matched case-control study at Moi Teaching and Referral Hospital (MTRH) in Eldoret, Kenya, a tertiary facility at approximately 2,100 metres elevation with around 12,000 annual deliveries. Recruitment occurred between 1 August 2025 and 31 January 2026 and spanned both dry and wet seasons to mitigate seasonal variation in vitamin D status. Participants were screened consecutively on admission to the antenatal or labour wards.
Eligible women were aged ≥18 years with a confirmed singleton viable pregnancy at ≥28 weeks by ultrasound and planned delivery at MTRH. Cases were women diagnosed with preeclampsia according to International Society for the Study of Hypertension in Pregnancy criteria (new-onset hypertension with systolic ≥140 mmHg and/or diastolic ≥90 mmHg based on the average of two measurements). Controls were normotensive pregnant women (blood pressure <140/90 mmHg without proteinuria) recruited concurrently from the same wards. Exclusion criteria included pre-existing chronic kidney disease, parathyroid conditions, cardiac medication use, known thrombophilia, and participation in similar interventional studies.
The study was designed as a pilot to provide context-specific evidence for Kenya. Based on prior African studies, 59 matched case-control pairs were estimated to provide 80% power to detect a 25-percentage-point difference in deficiency prevalence (α = 0.05); the target sample was increased to 120 to allow for attrition. Cases and controls were matched on maternal age (±3 years), gestational age (±1 week), and parity (0, 1–3, ≥4) to control key confounders and PE risk factors.
Maternal serum 25(OH)D concentrations were measured at enrolment for all participants. Vitamin D status was categorised using a commonly applied threshold: suboptimal defined as <50 nmol/L and sufficient as ≥50 nmol/L, consistent with thresholds used in many studies and referenced guidelines cited by the authors.
The association between vitamin D status and preeclampsia was assessed using conditional logistic regression to account for the matched design. Results were presented as odds ratios (ORs) with 95% confidence intervals (CIs). The analysis examined both categorical status (suboptimal vs sufficient) and continuous effects per 10 nmol/L change in serum 25(OH)D.
A total of 118 women were analysed: 59 women with preeclampsia and 59 matched controls. Median serum 25(OH)D was significantly lower among preeclampsia cases compared with controls: 67.1 nmol/L (interquartile range 47.8–84.0) versus 77.6 nmol/L (62.4–99.7); P = 0.001. Suboptimal vitamin D (<50 nmol/L) was observed in 32.2% of cases and 6.8% of controls (P = 0.002).
In conditional logistic regression accounting for matching, women with suboptimal vitamin D had higher odds of preeclampsia (OR 8.0; 95% CI 1.84–34.78; P = 0.006). When modelled continuously, each 10 nmol/L decrease in serum 25(OH)D was associated with a 29% increase in the odds of preeclampsia (OR 1.29; 95% CI 1.09–1.54; P = 0.004).
The authors place their findings in the context of proposed biological mechanisms by which vitamin D could influence PE pathophysiology, including modulation of pro-inflammatory responses and effects on the renin-angiotensin system. They note that prior meta-analyses of randomized trials reported reduced PE risk with vitamin D supplementation but highlighted that many trials did not select participants by baseline vitamin D status or confirm achievement of optimal levels post-supplementation. There has been limited representation of African populations in prior syntheses; the present study addresses this gap by providing context-specific Kenyan data.
Strengths reported include the matched design controlling for maternal age, parity and gestational age; recruitment over both dry and wet seasons to reduce seasonal bias; and use of facility-based controls to ensure comparable healthcare access. The authors framed the work as a pilot study intended to generate preliminary, locally relevant evidence. Limitations inherent to observational case-control designs apply, and the source manuscript should be consulted for a full listing of study limitations and potential residual confounding.
In this Kenyan sample, lower maternal serum 25(OH)D and a higher prevalence of suboptimal vitamin D status were significantly associated with preeclampsia. The authors suggest potential value in antenatal screening for vitamin D status and recommend further research into vitamin D supplementation as a preventive intervention for preeclampsia, particularly in African settings.
The study was registered in the Pan African Clinical Trial Registry (PACTR202505516303679). Funding was provided by the Bill & Melinda Gates Foundation (grant INV-033705) through a Supporting Women in Science programme award. The de-identified dataset supporting the findings is available as S3 File reported by the authors. The source article includes author declarations, editorial and peer-review details.