Table 30. The effect of maternal vitamin D status in gestation on Caesarean section (C-section) – Observational studies.

First Author and year	Bias score	Study details	Study type	Confounders/ adjustments	Number of weeks gestation when 25(OH)D was measured	Mean (SD) or median (IQR) 25(OH)D concentration (nmol/l) in cases of C-section		Mean (SD) or median (IQR) 25(OH)D concentration (nmol/l) in vaginal deliveries	Odds ratio/Relative risk of C-section from univariate analysis			Odds ratio of C-section from multivariate analysis	Conclusion
Ardawi, 1997 87	5 (low)	Jeddah, Saudi Arabia Cohort size=264 women	Cohort	nil	Delivery	Not given C-section incidence of 12.5% (n=3) if 25(OH)D <20 nmol/l c-section rate of 9.59% (n=23) if 25(OH)D >20 nmol/l		Not given	Not given			Not given	25(OH) <20 nmol/l was associated with an increased rate of C-section but results not significant (p>0.05).
Brunvand, 1998 140	1 (med)	Pakistan Cases=37 women Controls=80 women All nulliparous Pakistani women of low social class Cases all had emergency C-sections due to mechanical dystocia	Case-control	Cases had higher maternal age, lower maternal height, lower maternal weight , longer length of gestation and higher neonatal birth weight. Maternal height and birth weight included in logistic regression model	Just before delivery**	26 (15-37)**		19 (11-27)**	Not given	1.03 (0.99,1.06)			No significant association seen between maternal 25(OH)D concentration and risk of emergency C-section due to obstructed labour
Merewood, 2009 139	6 (low)	Boston, USA Cohort=277 women Cases=67 women All cases were women having primary C-sections	Cross- sectional	No significant difference in season of birth, maternal age, maternal BMI, maternal education, maternal insurance status, marital status, prenatal vitamin use and calcium supplementation, milk in pregnancy or sunscreen in pregnancy. Race/ethnicity, alcohol in pregnancy (yes/no), maternal educational status, maternal insurance status and maternal age included in multivariate analysis	Within 72 hours of delivery	Unadjusted = 45.0 (36.5-62.0)		Unadjusted = 62.5 (57.4-68.2)	If 25(OH)D <37.5 nmol/l, OR= 2.43 (1.20,4.92)	If 25(OH)D <37.5 nmol/l, adjusted OR= 3.84 (1.71,8.62)			25(OH)D <37.5 nmoll/l is significantly associated with an increased risk of primary C-section
Scholl, 2012 138	5 (low)	Camden cohort, New Jersey, USA Cohort=1153 women Cases=290 women (173 primary C-sections)	Cohort	Age, parity, ethnicity, gestation at entry to study, season at entry to study used to calculate adjusted OR1. Adjusted OR2 used the same confounders with the addition of maternal BMI	At entry to study. Mean (SD) 13.73 (5.6) weeks	Not given		Overall mean not given	Not given	25(OH)D conc.	OR1 (95% CI)	OR2 (95% CI)	Serum 25(OH)D <30 was associated with a significantly increased risk of overall C-section in both regression models. Regarding primary C-section, if BMI is not included in the model (OR1), serum 25(OH)D <30 was associated with a significantly increased risk of primary C-section, When maternal BMI is included (OR2) in the model the trend remains but the relationship no longer remains significant (p=0.054) Risk of overall C-section and primary C-section due to prolonged labour was significantly higher iif 25(OH)D <30 nmol/l even after adjusting for maternal BMI (OR2 = 2.24 (1.17,3.98) for primary C-section)
										<30	1.70 (1.12,2.58)	1.66 (1.09,2.52)
										30-49.9	0.89 (0.63,1.25)	0.83 (0.59,1.17)
										50-125	Ref	Ref
										>125	0.59 (0.17,2.08)	0.90 (0.49,1.66)
Savvidou, 2012 141	7 (low)	London, UK Cohort=1000 women Cases=199 women (111 emergency)	Cohort	Maternal age, racial origin, smoking, method of conception, season of blood sampling	Between 11-13 weeks	Elective= 58.40 (28.12-78.89) Emergency=42.53 (22.91-72.1)		46.4 (28.25-69.01)	Not given	OR not given. Result presented as multiples of the median after adjustments			No significant association seen between maternal 25(OH)D concentration and risk of either elective or emergency C-section
										Indication		MoM (IQR)
										Vaginal		0.99 (0.71,1.33)
										Elective		0.96 (0.73,1.27)
										Emergency (total)		0.99 (0.71,1.46)
										Emergency due to failure to progress		0.95 (0.71,0.25)
										Emergency due to fetal distress in labour		0.95 (0.71,1.27)
Fernandez-Alonso, 2012 115	3 (med)	Almeria, Spain Cohort=466 women Cases=105 women (61 emergency)	Cohort	Nil	Between 11-14 weeks**	Overall mean not given		Not given	Not given	Not given			No significant association between C-section rates as a function of first trimester 25(OH)D status Overall C-section, p=0.65 Emergency C-section p=0.47 Elective C=section p=0.06
						25(OH)D conc	N
						<50	23
						50-75	41
						>75	41

^**Measured 25(OH)D₃