Table 3. Methodological quality of case-control studies included in the meta-analysis^*.

First author (reference), publication year	Adequate definition of cases	Representativeness of cases	Selection of control subjects	Definition of control subjects	Control for important factor or additional factor†	Exposure assessment	Same method of ascertainment for all subjects	Non-response Rate‡
Luo, 2012	⚝	⚝	—	⚝	⚝⚝	⚝	⚝	⚝
Fantuzzi, 2007	⚝	⚝	⚝	⚝	⚝	⚝	⚝	⚝
Ewko, 1993	⚝	⚝	—	⚝	⚝⚝	⚝	⚝	⚝

* A study could be awarded a maximum of one star for each item except for the item Control for important factor or additional factor. The definition/explanation of each column of the Newcastle-Ottawa Scale is available from (http://www.ohri.ca/programs/clinical_epidemiology/oxford.asp.).

† A maximum of 2 stars could be awarded for this item. Studies that controlled for maternal age received one star, whereas studies that controlled for other important confounders such as body mass index, parity received an additional star.

‡ One star was assigned if there was no significant difference in the response rate between control subjects and cases by using the chi-square test (P>0.05).