Table 3.

Characteristics of included studies.

Reference	Publication date	Period of intervention	Study design	Sample size	Study period	Country of study	Study aim
Hofmeyr et al.	2019	Prepregnancy into pregnancy	Multicentre, parallel arm, double-blind, randomised, placebo-controlled trial	N = 1355	2011–2016	South Africa, Zimbabwe, Argentina	To test whether calcium supplementation before and in early pregnancy (up to 20 weeks’ gestation) prevents the development of pre-eclampsia
Baba Dizavandy et al.	1998	Pregnancy	Double-blind randomised trial	N = 143	Not reported	Iran	To determine the effect of calcium supplementation in the incidence of hypertensive disorders of pregnancy (gestational hypertension and pre-eclampsia) in nulliparous and high risk women
Herrera et al.	1998	Pregnancy	Randomised double-blind placebo-controlled trial	N = 89	1995–1996	Colombia	To determine the effect of low doses of linoleic acid and calcium on prostaglandin (PG) levels and the prevention of pre-eclampsia
Niromanesh et al.	2001	Pregnancy	Double-blind placebo randomised controlled trial	N = 30	Not reported	Iran	To study the effect of calcium supplementation on reduction of pre-eclampsia in Iranian women at high risk of pre-eclampsia
Sanchez-Ramos et al.	1994	Pregnancy	Randomised, double-blind, placebo-controlled clinical trial	N = 67	Not reported	USA	To evaluate the efficacy of oral supplemental calcium in reducing the incidence of pregnancy-induced hypertension (gestational hypertension or pre-eclampsia) in angiotensin sensitive nulliparas
Behjat et al.	2017	Pregnancy	Randomised double-blinded controlled clinical trial	N = 142	Not reported	Iran	To evaluate if vitamin D supplementation prevents pre-eclampsia in women with history of pre-eclampsia
Karamali et al.	2015	Pregnancy	Randomised double-blind placebo-controlled trial	N = 60	2014	Iran	To assess the beneficial effects of high-dose (cholecalciferol) vitamin D supplementation on metabolic profiles and pregnancy outcomes among pregnant women at risk for pre-eclampsia
Samimi et al.	2015	Pregnancy	Prospective, double-blind, placebo-controlled randomised trial	N = 60	2014–2015	Iran	To examine the effects of vitamin D plus calcium administration on metabolic profiles and pregnancy outcomes among women at risk for pre-eclampsia
Chappell et al.	1999	Pregnancy	Randomised controlled trial	N = 283	Not reported	UK	To assess the effect of supplementation with vitamin C and E in women at increased risk of the disorder on plasma markers of vascular endothelial activation and placental insufficiency and the occurrence of pre-eclampsia
Poston et al.	2006	Pregnancy	Randomised controlled trial	N = 2404	2003–2005	UK and Holland	To assess whether supplementation with vitamin C and vitamin E prevents pre-eclampsia in women at increased risk
Spinatto et al.	2007	Pregnancy	Randomised controlled trial	N = 739	2003–2006	Brazil	To evaluate whether antioxidant supplementation will reduce the incidence of pre-eclampsia among patients at increased risk
Kalpdev et al.	2010	Pregnancy	Randomised controlled trial	N = 50	2005–2007	India	To investigate whether vitamin C and vitamin E prophylaxis will reduce the incidence of superimposed pre-eclampsia in chronic hypertensive women
Beazley et al.	2005	Pregnancy	Double-blind randomised controlled trial	N = 109	Not reported	USA	To determine the effect of supplemental antioxidant vitamins C and E on the rate of pre-eclampsia in high-risk pregnant women
Vadillo-Ortega et al.	2011	Pregnancy	Randomised controlled trial	N = 628	2001–2005	Mexico	To test the hypothesis that deficiency in L-arginine and antioxidant supplementation would reduce the development of pre-eclampsia in a population at high risk
Villar et al.	2009	Pregnancy	Multi-centre, randomised, double-blind controlled trial	N = 1365	2004–2006	India, Peru, South Africa, Vietnam	To determine if vitamin C and E supplementation in high-risk pregnant women with low nutritional status reduces pre-eclampsia
Zheng et al.	2020	Prepregnancy into pregnancy	Randomised controlled trial	N = 1576	Not reported	China	To investigate whether supplementation with high doses of folic acid would reduce the subsequent development of pre-eclampsia and its adverse outcomes
Wen et al.	2018	Pregnancy	Randomised, phase III, double-blind, international multi-centre clinical trial	N = 2464	2011–2015	Argentina, Australia, Canada, Jamaica and UK	To determine the efficacy of high dose folic acid supplementation for prevention of pre-eclampsia in women with at least one risk factor: pre-existing hypertension, prepregnancy diabetes (type 1 or 2), twin pregnancy, pre-eclampsia in a previous pregnancy, or body mass index ≥35
De Araujo et al.	2020	Pregnancy	Randomised double-blinded controlled clinical trial	N = 911	2014–2017	Brazil	To evaluate magnesium citrate to prevent adverse perinatal and maternal outcomes among women at higher risk
Azami et al.	2017	Pregnancy	Randomised controlled trial	N = 100	Not reported	Iran	To investigate the effect of multimineral-vitamin D supplements (calcium, magnesium, zinc and vitamin D) and vitamins (C + E) in the prevention of pre-eclampsia
Parrish et al.	2013	Pregnancy	Randomised, placebo-controlled double-blind trial	N = 684	2004–2011	US	To evaluate if phytonutrient supplementation initiated in the first trimester of pregnancy and continued throughout gestation prevents pre-eclampsia