Characteristics of the included studies in the present meta-analysis
| Author | Region | Year | Age at baseline (mean, year) | Pre-pregnancy BMI (mean, kg/m2) | Gestational week of blood sampling | Measurement of vitamin D | SGA criteria | Cut-off values | Ethnicity group | OR (95% CI) | Adjusted | NOS score | Sample size |
| Leffelaar15 | The Netherlands | 2010 | NA | NA | 12–14 weeks | enzyme immunoassay | <10th | <15 ng/mL | Dutch (60.3%), Surinamese (6.7%), Turkish (4.0%), Moroccan (6.3%), other non-western (14.2%), other western (8.6%) | 1.90 (1.40 to 2.70) | yes | 8 | 3730 |
| Burris24 | USA | 2012 | 32.5 | 24.8 | 26–28 weeks | CLIA and RIA | <10th | <10 ng/mL | White (83.6%), black (16.4%) | 3.17 (1.16 to 8.63) | yes | 7 | 1133 |
| Zhou25 | China | 2014 | 29.5 | 20.3 | 16–20 weeks | ECLIA | <10th | <20 ng/mL | Asian | 2.46 (0.71 to 8.46) | no | 8 | 1923 |
| Choi26 | Korea | 2015 | 32.0 | 20.2 | first or second or third trimester | LC-MS/MS | <10th | <20 ng/mL | Asian | 0.448 (0.149 to 1.351) | yes | 6 | 220 |
| Ong18 | Singapore | 2016 | 30.5 | 26.1 | 26–28 weeks | LC-MS/MS | <10th | <20 ng/mL | Asian | 1.00 (0.56 to 1.79) | yes | 8 | 910 |
| Kiely27 | Ireland | 2016 | 30.5 | 24.9 | 14–16 weeks | LC-MS/MS | <10th | <20 ng/mL | White (98%), others (2%) | 0.88 (0.60 to 1.28) | yes | 6 | 1768 |
| Scholl28 | USA | 2014 | 22.8 | 26 | 13.8±5.6 weeks | HPLC | <10th | <20 ng/mL | Hispanic (51.4%), non-Hispanic black (34.4%), non-Hispanic white (14.2%) | 0.930 (0.568 to 1.523) | no | 8 | 1045 |
| Chen4 | China | 2015 | 27.5 | NA | first or second or third trimester | RIA | <10th | <20 ng/mL | Asian | 6.47 (4.30 to 9.75) | yes | 6 | 3658 |
| Boyle29 | New Zealand | 2016 | 30.3 | 24.8 | 15 weeks | LC-MS/MS | <10th | <20 ng/mL | NZ European (83.8%), other ethnicities (16.2%) | 1.33 (0.91 to 1.96) | yes | 7 | 2065 |
| Berg30 | The Netherlands | 2013 | NA | NA | 12.9 weeks | enzyme immunoassay | <10th | <20 ng/mL | NA | 1.57 (1.03 to 2.39) | yes | 7 | 2274 |
| Gerand16 | USA | 2013 | NA | 22.3 | 20.6 weeks | LC-MS/MS | <10th | <15 ng/mL | White (52.1%), Black (41.6%), Puerto Rican (6.3%) | 1.284 (1.026 to 1.608) | no | 6 | 2146 |
| Miliku31 | The Netherlands | 2016 | 29.7 | 23.7 | 20.3 weeks | LC-MS/MS | <15th | <10 ng/mL | European (57.3%), Cape Verdean (4.4%), Dutch Antillean (3.5%), Moroccan (6.6%), Surinamese (9.1%), Turkish (9.2%), other (9.9%) | 2.07 (1.33 to 3.22) | yes | 7 | 7176 |
| Nobles17 | USA | 2015 | NA | >25 | first or second or third trimester | ECLIA | <10th | <20 ng/mL | White (75.6%), black (13.5%) | 2.14 (0.67 to 6.88) | yes | 8 | 237 |
CI, confidence interval; CLIA, chemiluminescence immunoassay; ECLIA, electrochemiluminescence immunoassay; HPLC, high-performance liquid chromatography; LC-MS/MS, liquid chromatography tandem mass spectrometry; NA, not available; OR, odds ratio; RIA, radioimmunoassay; SGA, small for gestational age.