Table 3

Assessment of study quality through the Newcastle–Ottawa Scale

Case-control study	Selection	Comparability of cases and controls	Exposure assessment
Bengtsson et al (1973)	****	–	*
Bertuccio et al (2007)	***	– (history of miscarriage)	*
Bertuccio et al (2007)	***	** (recurrent)	*
Kharazmi et al (2010)	****	**	**
Winkelstein et al (1958)	***	–	**
Winkelstein et al (1964)	**	–	**
Cohort study	Selection	Comparability of cases and controls	Outcome assessment
Cooper et al (1999)	*	*	**
Gallagher et al (2011)	**	*	**
Kharazmi et al (2011)	***	**	**
Pell et al (2004)	**	**	**
Smith et al (2003)	**	**	**

The Newcastle–Ottawa Scale assesses study quality in three different areas, selection criteria, comparability of cases and controls, and exposure or outcome assessment. A maximum score of nine stars is allocated to each study: four for selection, two for comparability and three for exposure or outcome.