Research published in Obstetrics and Gynecology found that vitamin D is associated with incidence of preterm birth and that this relationship is similar in spontaneous and medically indicated preterm births as well as preterm births before 34 weeks gestation.
Preterm birth refers to the birth of a baby less than 37 weeks gestational age. Every year, 15 million babies are born preterm and of these, 1.1 million die as a result of preterm complications and even more suffer from complications related to preterm birth.
In many cases, the cause of preterm birth is unknown. However, there is strong evidence that infections in the uterine are involved in causing early births.
Vitamin D may be linked to preterm births due to its ability to reduce bacterial infections by inducing production of the antibacterial compound, cathelicidin, in placental cells.
Past research, mostly in the form of epidemiological studies, have found conflicting results regarding the relationship between maternal vitamin D levels and risk of preterm birth.
A past study found that vitamin D deficiency was associated with increased rates of spontaneous preterm births (SPTBs) among black and Puerto Rican mothers. SPTBs are preterm births that are not planned, as opposed to medically indicated preterm births (MIPTBs), which are preterm deliveries intentionally performed in order to ensure the health of the mother and her child.
Findings from a randomized controlled trial found no effect from vitamin D supplementation with 2,000 or 4,000 IU/day at 16 weeks gestation compared to placebo in reducing the preterm birth rates. But this trial was only designed to assess safety of supplementation in pregnant mothers and did not have the statistical power to test the causative role of vitamin D in preterm birth rates.
Researchers conducted the current study to provide more insight into the association between maternal vitamin D status and preterm births to see if the association differed among SPTB and MIPTB rates.
To do this, they gathered data from the Epidemiology of Vitamin D Study (EVITA). EVITA was designed to evaluate associations between maternal vitamin D status and adverse pregnancy outcomes in 65,867 infants delivered at Magee-Women’s Hospital between 1999 and 2010.
The researchers selected data from 3,453 mother-infant pairs, with 1,126 of these pairs being cases of preterm birth (621 cases were SPTBs and 505 were MIPTB). Data on vitamin D status was retrieved from samples taken by the Center for Medical Genetics and Genomic during the second trimester of the mothers’ pregnancies as part of a screening test.
They wanted to see if maternal vitamin D status was related to preterm birth rates and if a relationship existed, whether it differed according to subtype of preterm birth.
Here is what the researchers discovered:
- The prevalence of vitamin D status less than 20 ng/ml, between 20 ng/ml and 29.9 ng/ml, and greater than or equal to 30 ng/ml was 21.4%, 36.7%, and 41.9%, respectively.
- The incidence of preterm birth was 11.3%, 8.6%, and 7.3% among mothers with vitamin D levels less than 20 ng/ml, between 20 ng/ml and 29.9 ng/ml, and greater than or equal to 30 ng/ml, respectively.
- After adjusting for confounders, the risk of preterm birth was 1.8 times and 1.4 times higher among mothers with vitamin D levels less than 20 ng/ml and between 20 ng/ml and 29.9 ng/ml compared to mothers with levels greater than or equal to 30 ng/ml.
- The risk of preterm birth with increasing vitamin D levels plateaued around 36 ng/ml.
- The risk of preterm birth was similar among cases of SPTB, MIPTB, and cases occurring at less than 34 weeks of gestation.
The research team summarized the results by stating,
“We found that the confounder-adjusted risk of preterm birth was highest when serum 25-hydroxyvitaminDwas less than [20 ng/ml], declined as 25-hydroxyvitamin D increased to approximately [36 ng/ml], and then plateaued. Findings were similar for spontaneous or medically indicated preterm birth and preterm birth at less than 34 weeks of gestation.”
It is interesting to note that data from the study indicates a dose-response association between vitamin D and preterm birth.
Evidence is increasingly showing that other markers of vitamin D status and metabolism, such as vitamin D binding protein and bioavailable vitamin D, may be better predictors of health outcomes than circulating vitamin D status in some cases. The researchers noted that one major limitation of their study was a lack of any of these additional markers.
These findings support the crucial need for randomized controlled trials to be performed. But before these trials are conducted, the researchers recommended that,
“More research is needed into whether the intervention should be given universally, in a selected population, or based on screening; the most effective mode of intervention; patient preferences regarding screening and route of vitamin D administration; and fundamental pharmacokinetic and pharmacodynamic data regarding vitamin D replacement and supplementation in pregnancy.”