1. Quality of the included studies according to the NHS Centre 2001 table.

Trials	Entry point	Confounding of cases	Confounding of controls	Follow‐up length	Dropouts < 10%	Data clear	Endpoints clear	Same population
Cooney 1996	+	Unclear		+	Unclear	+	Unclear	‐
Díaz 1988a	‐	Unclear	Unclear	+	+ Cases	‐	+	‐
Egbuonu 2005	+	‐		+	+	+	‐	+
Kazi 1995	+	‐		+	+	+	+	‐
Labbok 1997	+	‐		+	+	+	Unclear	‐
Peréz 1991	+	Unclear	Unclear	+	+ Cases	‐	+	‐
Ramos 1996	+	‐		+	+	+	+	‐
Ravera 1995	+	Unclear		+	Unclear	+	+	‐
Rodríguez 1993	‐	Unclear		+	+	‐	+	‐
Shaaban 2013	+	‐	‐	+	+	‐	Unclear	+
WHO 1999a	+	Unclear		+	Unclear	+	+	‐

LAM: lactational amenorrhoea method
 WHO: World Health Organization