Table 2 Characteristics of included reports

 August et al., USA, 1992 [88]

Cross-sectional

11 – women with PE

3rd trimester

9 – chronic HTN

12 – normotensive pregnant controls

 Fernandez- Alonzo et al., Spain, 2012 [97]

Cross-sectional

466 – pregnant women (7 had PE)

1st trimester

 Pena HR, et al. Brazil 2015 [83]

Cross-sectional

179 -- pregnant women recruited before delivery

Near delivery

 Abedi et al., Iran, 2014 [84]

Case Control

59 – women with PE

At delivery

59 – healthy pregnant controls

 Achkar et al., Canada, 2015 [81]

Case Control

169 – women with PE

<20 weeks gestation

1975 – healthy pregnant controls

 Anderson et al., USA, 2015 [111]

Case control

11 -- gestational HTN (10/11 with PE)

At delivery

37 – healthy pregnant controls

 Baker et al., 2010 [80]

Nested Case Control

51 – developed PE

2nd trimester

204 – healthy pregnant controls

 Bodnar et al., USA, 2007 [18]

Nested Case Control

55 – developed PE

<22 weeks

219 – healthy pregnant controls

 Bodnar et al., USA, 2014 [79]

Case Control

717 – developed PE

<26 weeks gestation

560 mild; 157 severe

2986 – healthy pregnantcontrols

 Gidlof S, et al. Sweden, 2015 [112]

Nested Case Control

39 – developed preeclampsia

12th week of gestation

120 – healthy pregnant controls

 Halhali, Mexico, 2004 [95]

Case Control

10 -- developed PE, 40 – healthy pregnant controls

Median 20.7 weeks gestation

 Halhali et al., Mexico, 2007 [113]

Case Control

26 -- women with PE 26 – healthy pregnant controls

3rd trimester

 Lechtermann C, et al. Germany, 2–14 [85]

Case Control

20 – women with PE

At delivery

43 – healthy pregnant controls

 Mohaghegh et al., Iran, 2015 [89]

Case Control

41 -- women with PE 50 – healthy pregnant controls

Time of labor

 Powe, USA, 2010 [86]

Nested Case Control

39 -- developed PE 131 – healthy pregnant controls

1st trimester

 Robinson et al., USA, 2010 [25]

Case Control

50 -- women with EOSPE

29 weeks gestation

100 – healthy pregnant controls

 Schneuer et al., Australia, 2014 [114]

Nested Case Control

5109 pregnant women (223 with PE and 29/223 with EOSPE)

10–14 weeks gestation

 Singla et al., India, 2015 [87]

Case Control

74 -- nulliparous women with PE

>30 weeks gestation

100 -- healthy nulliparous controls

 Ullah et al., Bangladesh, 2013 [82]

Case Control

33 -- women with PE

>20 weeks gestation

79 – normal pregnancy controls

 Wetta et al., UK, 2013 [96]

Case Control

100 -women with PE

15–21 weeks gestation

200 – healthy pregnant controls

 Woodham et al., USA, 2011 [15]

Nested Case Control

41 – women with severe PE

2nd Trimester

121 – uncomplicated birth controls

 Xu et al., USA, 2014 [53]

Case Control

100 – women with PE

≤ 24 weeks gestation

100 – uncomplicated birth controls

 Yu et al., UK, 2012 [115]

Case Control

60 –late PE

11–13 weeks gestation

30 –early PE

1000 – healthy controls

 Alvarez-Fernandez et al., Spain, 2014 [90]

Retrospective Cohort

257 -- women attending obstetric triage with suspicion of PE

1st Trimester and 20 weeks of gestation

 Scholl et al., USA, 2013 [78]

Retrospective cohort

1141 -- low income and minority pregnant women

Entry to care (mean 13.7 ± 5.7 weeks)

 Burris et al., USA, 2014 [94]

Prospective Cohort

1591 -- pregnant women

16.4–36.9 weeks gestation

 Haugen et al., Norway, 2000 [77]

Prospective Cohort

23,423 -- pregnant women

Vitamin D intake at weeks 15, 22, and 30 gestation

 Shand et al., Canada, 2010 [37]

Prospective cohort

221 -- women at risk for PE

10–20 weeks gestation

 Wei et al., Canda 2012 [60]

Prospective Cohort

697 -- pregnant women receiving vitamin C and E supplementation for the prevention of PE

12–18 weeks gestation, 24-26 weeks gestation

 Wei, Canada, 2013 [14]

Prospective Cohort

697 -- pregnant women receiving vitamin C and E supplementation for the prevention of PE

24–26 weeks gestation

 Zhou, China, 2014 [116]

Prospective Cohort

74 – women with PE

16–20 weeks gestation