Vitamin D levels in pregnancies and neonatal outcomes. Perinatal Journal 2023;31(0):-
- Gazi University, School of Medicine, Ankara, Turkey Department of Obstetrics, Gynecology & Reproductive Sciences Ankara TR
Bilge Keskinsoy, Gazi University, School of Medicine, Ankara, Turkey Department of Obstetrics, Gynecology & Reproductive Sciences Ankara TR, [email protected]
Manuscript Received: April 06, 2022
Manuscript Accepted: November 02, 2022
Earlyview Date: November 02, 2022
Conflicts of Interest
No conflicts declared.
We aimed to evaluate the differences in obstetrics and neonatal outcomes, such as mode of delivery, gestational diabetes mellitus, preeclampsia, and infant birth weight between pregnancies with normal and insufficient vitamin D levels.
The study was designed as a retrospective study. 179 pregnant women who were followed up at our clinic and whose vitamin D levels were evaluated in each trimester were included. All patients were administered 1200 IU/day from the 12th week of gestation in accordance with the national guidelines. Vitamin D levels above 20 ng/ml were defined as sufficient, those below 20 ng/ml were defined as insufficient.
The median vitamin D level in the third trimester was significantly higher than that in the first and second trimester (p < 0.001). There was a positive, moderate correlation between vitamin D levels in infant cord blood at the time of birth and vitamin D levels in the third trimester (p < 0.001/R = 0.496). Birth weights for patients with insufficient vitamin D levels in the first trimester but with sufficient neonatal cord blood levels as a result of treatment, were significantly higher compared to those in patients with insufficient cord blood vitamin D levels (3327 g vs 3133 g; p = 0.030).
This study observed that, neonatal cord blood vitamin D level is a better indicator than antenatal vitamin D levels. Regardless of first trimester vitamin D levels, infant birth weight was significantly higher in the group with sufficient neonatal cord blood levels.
Fetal growth, pregnancy, perinatal outcome, vitamin D
Vitamin D receptors are present in many different types of cells and control mineral metabolism [1,2]. Maternal vitamin D levels during pregnancy affect maternal and fetal calcium homeostasis. The fetus receives vitamin D from the mother through the placenta . Vitamin D affects placental implantation, immune functions, inflammatory response and glucose homeostasis [4-7]. The total amount of calcium absorbed gradually increases during pregnancy and reaches a level of approximately 400 mg/day in the third trimester. Increased calcium absorption is provided by elevated concentrations of 1,25 dihydroxyvitamin D . In studies which monitored vitamin D levels, it was found that neonatal vitamin D levels were well correlated with maternal vitamin D levels .
Vitamin D deficiency is a common health problem around the world. Vitamin D has been associated with many diseases and numerous studies have been conducted on it. In terms of maternal results, there are studies that associate low vitamin D levels and obstetric complications such as preeclampsia, gestational diabetes, mode of delivery and low birth weight [10-14]. Therefore, standard vitamin D replacements are recommended by the Ministry of Health for pregnant women in Turkey. The recommended daily amount of vitamin D is 1200 IU.
In this study, we aimed to investigate the effect of standard treatment for pregnant women with normal or insufficient vitamin D level on vitamin D levels in the following pregnancy weeks and in the neonatal cord blood, in addition to evaluating the perinatal results.
The study was planned as a retrospective and single-center study. One hundred and seventy-nine patients who were followed up in the Gynecology and Obstetrics Clinic at Gazi University Faculty of Medicine between January 2016 and December 2017. Inclusion criteria were determined as evaluation of vitamin D levels in each trimester and gave birth in our clinic. All patients received vitamin D supplements in accordance with the national guidelines of the Ministry of Health. The exclusion criteria were pregnancies that did not use the recommended vitamin D replacement. Patients with multiple pregnancy were also excluded from study. Taking into account the reference value of our laboratory, vitamin D levels above 20 ng/ml were defined as sufficient and those below 20 ng/ml were defined as vitamin D deficiency. The Institute of Medicine (IOM) stated in 2011 that a blood level of 25 (OH) D 20 ng/mL and above was sufficient .
Laboratory values, pregnancy information, pregnancy outcomes, pregnancy complications (GDM, preeclampsia) and infant birth weight measurements, were retrieved from archives and electronic data analyses and recorded. Small for gestational age (SGA) was defined as a birth weight less than 2500 g. IBM SPSS 20 (Statistics Programme for Social Scientists) (USA) was used for statistical analysis. Kolmogorov–Smirnov test was used to analyze the normal distribution of the data. Continuous data with normal distribution were expressed as mean ± standard deviation; continuous data without normal distribution were expressed as median and min–max, and categorical data were expressed as percent (%). Student’s t-test was used to compare the data of two groups with normal distribution between independent groups. Friedman test was used to compare more than two dependent variables without normal distribution, and when there was a difference between variables, the groups were compared in pairs using the Wilcoxon test to determine the group that caused the difference. Spearman’s correlation analysis was used to determine correlation between variables. Chi-squared or Fisher’s exact test was used to compare independent categorical variables, and p < 0.05 was considered statistically significant.
179 patients were enrolled in the study. All patients used 1200 IU/day vitamin D starting from the 12th gestational week. The characteristics of patients such as age, gravida, parity and birth mode are shown in Table 1.
The median vitamin D level of each trimester was determined and compared. The median vitamin D level measured in the third trimester was significantly higher than in the first and second trimester (p < 0.001). Vitamin D levels measured in the first and second trimester were similar (p = 0.961). There was a positive moderate correlation between neonatal vitamin D level and third trimester maternal vitamin D level (p < 0.001; R = 0.496).
The first trimester D vitamin level was sufficient for 73 patients. The first and third trimester D vitamin level was sufficient for 34 patients. Vitamin D levels were insufficient in the first and third trimesters of 73 patients. Approximately 52% of 73 patients with vitamin D levels above 20 ng/dl in the first trimester had a vitamin D level below 20 ng/dl in the third trimester despite treatment. Of 106 patients with vitamin D levels below 20 ng/dl in the first trimester, only 31% were able to reach a sufficient level in the third trimester after treatment, and 69% remained at an insufficient level. The rate of patients whose vitamin D level was sufficient in the first trimester and continued adequately in the cord blood was 79%. The rate of patients whose vitamin D level is sufficient in the first trimester and whose cord blood vitamin D is insufficient is 21%. In patients with sufficient vitamin D levels in the first trimester who received treatment, SGA was significantly lower in the group with sufficient neonatal cord blood vitamin D level, compared to that in the group with insufficient neonatal cord blood vitamin D level (p = 0.046)( Table-2). In the group with insufficient vitamin D levels in the first trimester, preeclampsia rate (p = 0.049), and birth rate below 2500 g (p = 0.016), was significantly lower as shown in Table-2. The mean birth weight of 34 patients with sufficient first and third trimester vitamin D levels was 3401 g, and the mean birth weight of 73 patients with insufficient, first and third trimester vitamin D levels was 3227 g (Table-3).
There was a significant difference between the birth weights of these two groups (p = 0.03). Although there was no significant difference in terms of small for gestational age (SGA) in the subgroups categorized to obtain secondary results, birth weights were higher in women who had sufficient vitamin D levels in the first trimester (p = 0.057 and p = 0.096). Birth weights for patients with insufficient vitamin D levels in the first trimester but with sufficient neonatal cord blood levels as a result of treatment, were significantly higher compared to those in patients with insufficient cord blood vitamin D levels (3327 g vs 3133 g; p = 0.030) as shown in Table-3.
The main purpose of our study was to determine vitamin D levels measured in all trimesters of pregnancy in patients who received vitamin D replacement in accordance with the national health guidelines, to monitor the change between trimesters. The secondary purpose of the study is to compare perinatal results. The importance of controlled and regular vitamin D replacement in pregnant women has been demonstrated. Despite replacement therapy, vitamin D levels did not increase to desired levels in a significant number of patients during the following gestational weeks. In this study, there was a correlation between third trimester vitamin D levels and neonatal cord blood vitamin D levels. Birth weights were lower in the group with insufficient vitamin D level in the beginning of the pregnancy and insufficient neonatal cord blood vitamin D level. It was observed that the standard dose of vitamin D replacement therapy used in Turkey was insufficient.
There are many studies in the literature that evaluate the effect of vitamin D deficiency in pregnancy on poor prenatal results [16-18]. Increased levels of vitamin D deficiency in cord blood are clinically important due to the risk of health complications in newborns. In the study by Dangard et al.  in 2016, the relationship between 25(OH)D vitamin levels in the third trimester and fetal development was investigated. They observed infant birth weight and height measured in the first 14 days. It was demonstrated that maternal third trimester 25(OH)D vitamin level in maternal third trimester was correlated with infant height measurement but not with infant weight. It was shown that the concentration of vitamin D in maternal blood was higher in the late stages of pregnancy than that in the cord blood. In a study involving 7098 pregnant women in which Miliku et al.  measured 25(OH)D vitamin levels in the second trimester in Netherlands, they found that low vitamin D concentrations were associated with proportional fetal growth restriction, SGA, and an increase in the frequency of preterm births. In the meta-analysis including 13 randomized controlled trials published in 2015, it was shown that vitamin D supplementation increased 25(OH)D vitamin levels, birth weight, and birth height . In our study, the birth weight was lower in the group with insufficient vitamin D levels at the beginning of the pregnancy and insufficient neonatal cord blood vitamin D levels. Fetal growth is a complex process. Many factors affect the development, such as genetic structure, trophoblast implantation, placental development, nutrition and physical activity.
World Health Organization (WHO) recommends 200 IU vitamin D supplement per day for pregnant women with vitamin D deficiency . While American College of Obstetricians and Gynecologists (ACOG) recommends routine supplementation at a standard vitamin D dose, it states that it is safe to use 1000–2000 IU vitamin D per day in accordance with expert opinions in case of vitamin D deficiency during pregnancy. Although there is no consensus on optimal vitamin D levels, there is an opinion that blood vitamin D levels should be at least 20 ng/dl to avoid bone problems . The Institute of Medicine (IOM) states that pregnant women should take 600 IU of vitamin D, that the daily intake for pregnant women with vitamin D deficiency can increase up to 4000 IU, and that they should continue the same dose during breastfeeding in order to maintain their own health and the health of their infants. In addition, it is stated that blood levels of 25(OH)D vitamin of 20 ng/mL and above are sufficient . As per the protocol of the Ministry of Health in Turkey, it is recommended to start vitamin D support in pregnant women regardless of their blood level. Approximately 1200 IU of vitamin D is recommended to be taken as a single daily dose for pregnant women in the prenatal period and for postpartum period for 6 months . Although this standard treatment was administered in our study, third trimester and neonatal cord blood values did not reach desired levels in most patients.
There are many possible biological mechanisms that underlie the potential contribution of maternal vitamin D deficiency to the pathophysiology of preeclampsia. Abnormal placental implantation, vascular endothelial dysfunction at the maternal–fetal interface, and excessive inflammation characterize preeclampsia . In our study, preeclampsia rate was significantly lower in the group with sufficient neonatal vitamin D levels. In the review conducted by Callaghan and Kiely  in 2018, 37 systematic reviews and observational studies were discussed. The purpose of their review was to demonstrate the effect of maternal vitamin D status or dietary vitamin D replacement on the prevalence of hypertensive diseases in pregnancy. Systematic studies have not reach definite conclusions regarding the potential of vitamin D to protect against gestational hypertensive diseases. Observational cohort studies show a positive association between vitamin D deficiency and increased risk of preeclampsia; however, the results cannot be relied upon to draw conclusions due to suboptimal clinical phenotyping, incomplete subject characterization, and large heterogeneity between studies.
Possible biological mechanisms of the relationship between vitamin D and gestational diabetes mellitus (GDM) are based on insulin-sensitive tissues, calcium pool in the pancreas, genetic diversity and inflammation . In a meta-analysis of 29 observational studies by Hu et al. , 28,982 patients were included. Although GDM diagnostic criteria and vitamin D cut-off values differ between studies, 25(OH)D vitamin levels were significantly lower in patients with GDM. As in our study, in the study by Eggemoen et al.  with 745 pregnant women of different ethnic origins, it was shown that vitamin D deficiency had no effect on GDM and glucose metabolism. Based on literature review, vitamin D deficiency has been associated with GDM more frequently. In this study, vitamin D deficiency was not associated with GDM, mode of delivery, and gestational age. It was observed that the third trimester 25(OH)D vitamin level was correlated with cord blood. It was also shown that there was a relationship between vitamin D deficiency and SGA. In our study, there was no significant difference in the group with insufficient 25(OH)D vitamin levels in all trimesters, although the number of births with C/S was higher. There are publications in the literature showing that birth rate with C/S is higher in case of low blood vitamin D levels .
Some limitations of the study were that it was retrospective in nature, was designed in a clinic where risky pregnancies are followed up intensively, and had insufficient number of patients. The strengths of the study were that it was a single-center study and all patients received a standard treatment.
Vitamin D levels in neonatal cord blood predicted SGA better compared to third trimester cord blood vitamin D levels. There was a moderate correlation between third trimester and neonatal cord blood levels. Regardless of first trimester vitamin D value, infant birth weight was significantly higher and the SGA rate was significantly lower in the group with sufficient neonatal cord blood level.
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Demographic and Clinical Characteristics of Patients
Comparison of perinatal results
Comparison of birth weights