Summary of findings for quantitative analysis from the case–control and cohort studies, with unadjusted and adjusted results
| Outcome | Overall sample | Case–control study (unadjusted effect size) | Case–control study (adjusted effect size) | Cohort study (unadjusted effect size) | Cohort study (adjusted effect size) | Certainty of evidence | |||||||||||||
| Outcomes (n) | Sample size | Studies (n) | Sample size | OR | I2 (%) | Studies (n) | Sample size | aOR | I2 (%) | Studies (n) | Sample size | RR | I2 (%) | Studies (n) | Sample size | aRR | I2 (%) | ||
| All adverse pregnancy-related outcomes* | 160 | 2 876 075 | |||||||||||||||||
| Stillbirth | 8 | 415 704 | 4 | 610 | 5.05 (2.95 to 8.6) | 33.70 | N/A | N/A | N/A | N/A | 4 | 415 094 | 1.28 (1.09 to 1.5) | 19 | 3 | 408 152 | 0.95 (0.87 to 1.0) | 64.20 | Low |
| Infant death | 3 | 10 494 | 2 | 6239 | 1.86 (1.11 to 3.1) | N/A | 1 | 6156 | 1.80 (1.14 to 2.9) | N/A | 1 | 4255 | 1.87 (0.76 to 4.5) | N/A | 1 | 4255 | 1.02 (0.37 to 2.8) | N/A | Very low |
| Infertility | 39 | 581 309 | 24 | 8049 | 2.72 (2.37 to 3.1) | 82.70 | 2 | 732 | 1.91 (1.29 to 2.8) | 61.70 | 5 | 573 250 | 1.84 (1.77 to 1.9) | 90.90 | 2 | 572 446 | 1.37 (1.28 to 1.4) | 0.00 | Low |
| Tubal infertility | 14 | 574 323 | 7 | 1872 | 4.20 (3.24 to 5.4) | 89.70 | 1 | 244 | 3.16 (1.53 to 6.5) | N/A | 2 | 572 446 | 1.86 (1.79 to 1.9) | 96.50 | 2 | 572 446 | 1.37 (1.28 to 1.4) | 0.00 | Low |
| Spontaneous abortion | 23 | 13 429 | 14 | 11 694 | 1.30 (1.14 to 1.4) | 81.10 | 2 | 2858 | 0.97 (0.79 to 1.1) | 0 | 4 | 1529 | 1.47 (1.16 to 1.8) | 50.60 | 1 | 615 | 0.70 (0.40 to 1.2) | N/A | Very low |
| Preterm labour | 25 | 606 793 | 11 | 7792 | 1.29 (1.11 to 1.5) | 81.70 | 4 | 4106 | 1.15 (0.95 to 1.3) | 84.40 | 13 | 599 000 | 1.54 (1.48 to 1.6) | 98.00 | 6 | 586 317 | 1.09 (1.03 to 1.1) | 34.10 | Low |
| Ectopic pregnancy | 31 | 619 271 | 25 | 14 573 | 3.24 (2.95 to 3.55) | 87.00 | 10 | 10 480 | 2.82 (2.52 to 3.1) | 83.20 | 5 | 604 697 | 1.67 (1.64 to 1.7) | 89.40 | 4 | 604 578 | 1.31 (1.27 to 1.3) | 94.30 | Low |
| Low birth weight | 11 | 211 515 | 3 | 1296 | 1.80 (1.20 to 2.7) | 35 | 2 | 1213 | 2.36 (1.41 to 3.9) | 0 | 7 | 210 218 | 1.33 (1.27 to 1.4) | 49 | 4 | 202 151 | 1.13 (0.94 to 1.3) | 26.2 | Low |
| Small for gestational age | 6 | 400 895 | 2 | 426 | 1.64 (0.81 to 3.3) | 59 | N/A | N/A | N/A | N/A | 4 | 400 670 | 1.19 (1.12 to 1.2) | 0.00 | 2 | 393 600 | 0.97 (1.13 to 1.3) | 0.00 | Low |
| Premature rupture of membranes | 8 | 6882 | 2 | 718 | 2.34 (1.19 to 4.6) | 0 | 1 | 534 | 2.40 (1.10 to 5.2) | N/A | 5 | 6163 | 1.53 (1.20 to 1.9) | 51.30 | 1 | 4227 | 1.50 (0.90 to 1.0) | N/A | Very low |
| Postpartum endometritis | 4 | 8310 | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | 4 | 8310 | 1.61 (1.24 to 2.0) | 58.90 | N/A | N/A | N/A | N/A | Very low |
| Pre-eclampsia | 2 | 1473 | 2 | 1473 | 1.56 (0.9 to 2.9) | 0 | 2 | 1473 | 1.73 (0.88 to 3.38) | 88 | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | Very low |
*Include stillbirth, infant death, spontaneous abortion, preterm labour, low birth weight, small for gestational age, premature rupture of membranes, postpartum endometritis and pre-eclampsia.
aOR, adjusted OR; aRR, adjusted risk ratio; N/A, not available; RR, risk ratio.