A total of 569 cord blood samples were collected in DRC from Congolese mothers and their new-borns and were analyzed for 25(OH)D levels. The mean maternal serum 25(OH) D level was 35.63 ng/ml (SD 6.18, range 9.2–39.8). The classification of maternal 25(OH) D levels was done as per US Endocrine society in which 464(81.54%) mothers were found to have sufficient (≥ 30 ng/ml) and 105(18.45%) deficient Vitamin D levels (Table 1). Majority of mothers (81%) were having sufficient vitamin D levels. This finding is in contrast to many other studies which had found high prevalence of vitamin D deficiency during pregnancy[5, 25].
In present study, none of the neonates(n = 569) had sufficient 25(OH)D (≥ 30 ng/ml) as per US Endocrine society classification of vitamin D deficiency. Sachan[26] et al conducted a study to determine the prevalence of Osteomalacia and Hypovitaminosis D in Pregnant women and cord blood. They included 117 cord blood samples and observed high prevalence of Hypovitaminosis D(< 20 ng/ml) with 95.7% (mean 8.4 ng/ml) of neonates deficient in vitamin D. These findings corroborated with findings in our study. A prospective Cohort Study, conducted in Shanghai by Yu[27] et al with a birth cohort of 1071 infants to estimate the prevalence of vitamin D in cord blood and maternal blood found that 36.3%(388) were having 25(OH)D < 20 ng/ml and 84.1%(900) had < 30 ng/ml 25(OH)D. Bowyer’s[5] et al found 98(11%) of 901 neonates deficient in Vitamin D (< 25 nmol/l or 10 ng/ml) as compared to100.00% vitamin D deficiency observed in current study. These findings were different from our study which could be attributed to defining criteria for vitamin D deficiency in cord blood as < 10 ng/ml in Bowyer’s study against < 30 ng/ml in present study. Hence, high prevalence of vitamin D deficiency in newborns was observed in above studies similar to our study (99.99%) which affirms that vitamin D deficiency is a major health problem and can possibly adversely affect neonatal outcomes.
This study attempted to evaluate the association (if any) between the maternal vitamin D levels and neonatal birth weight. 104 (18.27%) newborns were low birth weight (LBW) and 465 (81.27%) were normal birth weight (NBW)(Table 2). The percentage of LBW corroborates with the prevalence of LBW in south Asia (20%) [28, 29]. The percentage of LBW babies born to VDD mothers was 18.09%, which was very similar to the percentage of LBW babies born to VDS mothers (18.31%) (p = 0.76456749). Hence, vitamin D status in maternal blood during peri-partum period was not associated with LBW as per our study. These findings were similar to an observational study CR Gale et al[30], in which 596 pregnant women were recruited and maternal 25 (OH)-vitamin D concentrations were measured in late pregnancy. There was no statistically significant association seen between maternal Vitamin D and weight at birth (P = 0.247). Morley and Carlin studied the relationship between maternal vitamin D and PTH concentrations at less than 16 and 28-week gestation and offspring birth size. 374 out of 475 (79%) women completed the study and they found no evident relationship between birth size and maternal vitamin D levels[31]. However, Alison D Gernand et al [32] reported higher birth weight in babies born to the mother with vitamin D status ≥ 37.5 nmol/L than the mothers with levels < 37.5 nmol/L. They further noticed a nonlinear relation between 25(OH)D and birth weight in which birth weight increased by 3.6 g per 1 nmol/liter increase in maternal 25(OH)D up to 37.5 nmol/liter and then leveled off thereafter. The results obtained in this study were incongruous to our findings. The possible explanation could be large sample size (n = 2146), maternal serum being analyzed in first trimester as compared to our study in which the samples were drawn during perinatal period and different cut off points to classify serum vitamin D levels.
The relationship between maternal vitamin D levels and mode of delivery was also studied. Out of the total 569 cases, 417 (73.28%) delivered vaginally and 152 (26.71%) underwent LSCS (Table 3). Out of 105 vitamin D deficient mothers, 61(58.09%) underwent LSCS in comparison to 91(19.61%) of the vitamin D sufficient mothers and this difference was highly significant (p < 0.00001). The rate of Cesarean deliveries was 2.96 times higher in mothers who had deficient vitamin D levels (< 30 ng/ml). Merewood A [33] (2009) obtained similar results with fourfold increase in rates of caesarean section in women with vitamin D levels below 37.5 nmol/liter (p = 0.012) after controlling for race, age, education level, insurance status and alcohol use. Another result similar to our study was reported by Scholl [34] during his analysis of a cohort of 1153 low-income pregnant women in which vitamin D deficiency was linked to a 2-fold increased risk of cesarean. However, Sakineh et al [35] (2015) in a triple blind randomized controlled trial on 126 pregnant ladies, found no relation between vitamin D and mode of delivery. This result is not consistent with the results of present study. The possible explanation for this finding is that over one third (34.1%) of caesarian deliveries in the study by Sakineh et al were due to previous caesarian surgery or elective caesarian delivery.
Out of the total of 569 deliveries, 501(88.04%) occurred at term (≥ 37 weeks period of gestation) and 68 (11.95%) were preterm (< 37 weeks period of gestation) (Table 4). The incidence of preterm births have been reported to range from 5–7% of live births in developed countries, but are estimated to be substantially higher in developing countries [36]. Beck S[37] et al estimated that 9.6% of all births worldwide in 2005 were preterm and approximately (85%) of these preterm births were concentrated in Africa and Asia. The findings in these studies are close to figures in current study (11.95% preterm births). The incidence of preterm births was 10.77% in the vitamin D sufficient group (≥ 30 ng/ml) as compared to 17.14% in the vitamin D deficient group (p = 0.038). The rate of preterm delivery was 1.59 times higher in mothers with vitamin D deficiency and the difference was statistically significant. Perez-Ferre[38] et al. also observed that maternal vitamin D deficiency increased the risk of premature delivery with Odds Ratio of 3.31 (95% CI 1.52–7.19). A systematic review and meta-analysis consisting of 24 studies reported the association between maternal blood vitamin D levels and adverse pregnancy outcomes including preterm birth, preeclampsia and gestational diabetes mellitus (GDM)[39]. The results revealed that women with circulating vitamin D level less than 20 ng/mL (50 nmol/L) in pregnancy had an increased risk of preterm births [OR 1.58 (1.08–2.31)], preeclampsia [odds ratio (OR) 2.09 (95% confidence intervals 1.50–2.90)] and GDM [OR 1.38 (1.12–1.70)]. The cut off levels for vitamin D deficiency in both the above studies was taken at 20 ng/mL (50 nmol/L) in contrast to current study (vitamin D deficiency < 30 ng/ml) despite which the results were similar and statistically significant (p = 0.038). This indicates that even vitamin D levels between 20–30 ng/ml are associated with incresed risk of preterm births. A systematic review by De-Regil and Luz Maria,[40] et al from the cochrane database reported that vitamin D supplementation during pregnancy reduces the risk of preterm births compared to no intervention or placebo (8.9% versus 15.5%; RR 0.36; 95% CI 0.14 to 0.93). The results corroborate with current study, however current study did not take vitamin D supplementation during pregnancy into account. Various other studies have shown that both maternal and neonatal 25(OH)D concentrations do not have any association with the risk of preterm births. In a randomized controlled trial by Yu [41] et al, no significant difference in gestational age at delivery was found. In an RCT by Hollis BW [42] et al, vitamin D supplementation with 50–100 mcg/day did not alter gestational duration compared with vitamin D supplementation of 10 mcg/day. A prospective study conducted by Hossain[43] et al which included 75 mothers alongwith their newborns. Maternal vitamin D levels for sufficiency was taken as 30 ng/mL (75 nmol/L) similar to current study. Hossain et al found higher maternal and cord blood vitamin D status to be associated with shorter gestational periods (r = 0.33, p = 0.003). This finding is in contrast to current as well as most other studies which could be explained by relatively small sample size (75 vs 569 in current study).
The percentage of SGA babies born to mothers with VDD was 13.97%, which was very similar to the percentage of SGA babies born to mothers with Vitamin D sufficiency at 17.24% (p value = 0.40) (Table 5). The findings of current study concurred with Farrant, Hannah JW, et al[44]. However Leffellaar[45] et al in a large multiethnic study found higher risk of SGA in women with deficient vitamin D levels (OR = 2.4,95%CI1.9-3.2.
The possible reasons for the contrasting results in the relation between vitamin D and various neonatal outcomes can be due to several variables such as pre-pregnancy BMI, arbitrary cut-off values for vitamin D, smoking status (including second hand smoke), socioeconomic status, physical activity (pre-pregnancy as well as during pregnancy), ethnicity, geographical location, season of birth, emotional distress [46] etc. However, potential role of vitamin D cannot be undermined in mitigating risk for Caesarean section and preterm births as seen in current study.
The strength of this study is that a large sample size (569) with relatively uniform population in terms of socioeconomic status was studied. Secondly, sampling for vitamin D was done during the peripartum period when levels of vitamin D are not falsely high as compared to first and second trimester [47]. Thirdly, this study was done exclusively on Congolese ladies, to find race-specific correlation.
The prevalence of vitamin D deficiency is high in Africa, and DRC is no exception, and our study reiterates this. Public health strategies in DRC should escalate efforts to prevent, detect, and treat vitamin D deficiency, particularly in new-borns, women, and urban populations.
LIMITATION
The limitation of the study was that factors such as pre-pregnancy BMI, vitamin D supplementation, smoking/other substance use etc. were not taken into account.