Table 1.

Study characteristics

Author; Date	Study type	Exclusion criteria
Agabain; 2017	Case–control	Thyroid disease; hypertension; renal disease, diabetes; liver disease; medication for hypertension; gestational age < 20 weeks
Tokmak; 2014	Case-control	Early onset of preeclampsia; gestational age < 20 weeks
Birdir; 2014	Case-control	Gestational age < 11 or > 13 weeks
Shand; 2014	Retrospective cohort	Multiplepregnancy; birth at < 20 weeks gestation; pregnancy > first trimester