Table 1 Overview of included studies
Environmental risk factor | Number of studies |
 Air pollution | 28 |
 Persistent organic pollutants (POP) | 18 |
 Arsenic | 11 |
 Phthalate | 9 |
 Seasonality | 8 |
 Bisphenol A (BPA) | 8 |
 Cadmium | 6 |
 Ambient temperature | 5 |
Locations | Number of studies |
 China | 32 |
 USA | 25 |
 Taiwan | 5 |
 UK | 4 |
 Canada | 4 |
 Australia | 4 |
 Iran | 3 |
 Spain | 2 |
 Greece | 2 |
 France | 2 |
 Sweden | 2 |
 Guadeloupe | 1 |
 Argentina | 1 |
 Chile | 1 |
 Japan | 1 |
 Israel | 1 |
 Denmark | 1 |