Am J Perinatol 2021; 38(S 01): e367-e369
DOI: 10.1055/s-0040-1709494
Letter to the Editor

Vitamin D Status May Help Explain Maternal Race and Ethnic Factors in Primary Cesarean Section Delivery

1  Sunlight, Nutrition, and Health Research Center, San Francisco, California
› Author Affiliations
Funding The author receives funding from Bio-Tech Pharmacal, Inc. (Fayetteville, AR).

The paper by Stark et al reported that race or ethnicity had a significant impact on the risk of primary cesarean delivery.[1] Compared with non-Hispanic Whites (NHWs), non-Hispanic Blacks (NHBs) had a 53% increased risk while Hispanics had a 30% increased risk. The authors identified maternal obesity, advanced maternal age, pregestational diabetes, gestational diabetes, chronic hypertension, and hypertensive disorders of pregnancy as significant factors related to failure to progress. For HNBs, nonreassuring fetal status was a significant factor but failure to progress was not. They also suggested that unmeasured factors might be driving the observed racial and ethnic differences.

A factor not considered by the authors is vitamin D status. In the period 2001 to 2004, based on the U.S. National Health and Nutrition Examination Survey data, the mean serum 25-hydroxyvitamin D [25(OH)D] concentrations for women aged 20 to 39 were: NHBs, 14 ng/mL; Mexican Americans, 20 ng/mL; and NHWs, 28 ng/mL.[2]

Several articles have reported that the risk of primary cesarean delivery was significantly correlated with 25(OH)D concentration. A study from Boston for the period 2005 to 2007, in a multivariable logistic regression analysis controlling for race and other factors, maternal 25(OH)D concentration <15 ng/mL was associated with an increased cesarean delivery rate of 284%.[3] A study in Singapore based on 25(OH)D concentrations measured at 26 to 28 weeks gestation found risk of emergency cesarean delivery for <30 ng/mL versus >30 ng/mL was increased by 90% for Chinese women and by 141% for Indian women.[4] A study in Spain found that maternal 25(OH)D concentration >30 ng/mL was associated with a 40% reduction in cesarean delivery by obstructed delivery compared with <20 ng/mL.[5]

The role of vitamin D in reducing risk of emergency cesarean-section delivery may be due primarily to the reducing risk of low blood flow to the placenta. A recent article noted that an important risk factor for emergency cesarean section births is fetal distress due to hypoxia resulting from reduced uteroplacental blood flow.[6] The authors conducted a trial of sildenafil citrate, a vasodilator also known as Viagra, given to women in early labor or undergoing scheduled induction of labor. Sildenafil citrate reduced the risk of emergency operative birth by 51% (relative risk = 0.51 [95% confidence interval, 0.33–0.73]).[6]

A search for factors linked to the use of cesarean-section delivery found additional support for diagnosis of hypertension, preeclampsia, and overweight/obesity as important risk factors.[7] High-dose vitamin D supplementation has been found to reduce arterial stiffness, a risk factor for hypertension, in overweight African Americans (AAs).[8] An open-label study found that high-dose vitamin D supplementation raising serum 25(OH)D concentrations above 40 ng/mL could reduce blood pressure among hypertensive participants by 12 to 14 mm Hg, which was enough to lower their blood pressure below the cutoffs for hypertension.[9] However, a review of clinical trials of blood pressure lowering medication during pregnancy found a 4-mm Hg reduction but no effect on risk of preeclampsia.[10] Lowering blood pressure to a greater extent might have a beneficial effect on preeclampsia and cesarean section delivery.

Another article reported that for pregnant women with type 2 diabetes mellitus, elevated hemoglobin A1c (HbA1c) was associated with increased risk of preeclampsia and cesarean section delivery.[11] Blacks have higher HbA1c than White Americans, perhaps due to environmental factors such as diet and 25(OH)D concentration as well as to genetics.[12] Vitamin D deficiency is a risk factor for type 2 diabetes mellitus based on secondary results of a vitamin D randomized controlled trial (RCT) involving prediabetics taking 4,000 IU/d vitamin D3.[13]

Preeclampsia reduces blood flow to the uterus.[14] A meta-analysis of observational studies found that 25(OH)D concentrations <20 ng/mL are associated with preeclampsia.[15] An observational study conducted in South Carolina found that pregnant women with early onset preeclampsia had lower 25(OH)D concentrations than those without: the concentrations were 17 versus 22 ng/mL for AAs and 30 versus 42 ng/mL for Whites.[16]

Obesity during pregnancy is associated with gestational diabetes, preeclampsia, induced birth, and cesarean section delivery.[17] An RCT conducted in Iran found that calcium plus vitamin D3 supplementation significantly reduced the cesarean section delivery rate for women who developed gestational diabetes (23 vs. 63%, p = 0.002).[18] A vitamin D supplementation study in Iran found that supplementing vitamin D-deficient women with at least 50,000 IU vitamin D3/month greatly reduced risk of preeclampsia and gestational diabetes.[19]

Of course other factors also increase the risk of requiring primary or emergency cesarean section delivery. Metabolic syndrome is a cluster of different risk factors including abdominal obesity, insulin resistance, high blood pressure, and high cholesterol. AA women (AAW) are prone to metabolic syndrome. A study in the United States found that in a study of 1,918 AAW, prevalence of metabolic syndrome was 47%.[20] “Older age, lower education level, low socioeconomic status, unmarried status, low physical activity level, and smoking were associated with higher prevalence of metabolic syndrome (p < 0.001). The prevalence of borderline hypertension, hypertension, diabetes, stroke, and cardiovascular diseases was significantly higher in AAW with metabolic syndrome (p < 0.001).”[20] Regardless of the cause of metabolic syndrome, vitamin D supplementation may be an efficient way to counter some of the effects of metabolic syndrome.

Vitamin D supplementation studies in South Carolina have found that supplementing pregnant women of all races/ethnicities with 4,000 IU/d vitamin D3 can raise serum 25(OH)D concentrations to >40 ng/mL with no adverse effects[21] and can significantly reduce risk of preterm delivery.[22]

Thus, there is a strong evidence that vitamin D supplementation during pregnancy sufficient to raise 25(OH)D concentrations above 30 to 40 ng/mL would have significant benefits in reducing adverse pregnancy outcomes. While some of the studies were conducted on AAs, additional studies with AAs should be conducted. Meanwhile, since there are multiple benefits of vitamin D supplementation during pregnancy and few, if any, adverse effects, all women should be advised to take vitamin D3 during pregnancy as well as before and after. Taking vitamin D while nursing helps ensure that the infant gets vitamin D.[23]

Publication History

Received: 18 December 2019

Accepted: 06 March 2020

Publication Date:
24 April 2020 (online)

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