Figure 1.

Figure 2.

Figure 3.

An overview of included studies evaluating efficacy of vitamin C supplementation in prevention of preterm birth
| Study ID (first author, year) | Country | Inclusion criteria regarding gestational age and if previous history of preterm birth was required | Age of the participants in years (mean±SD, unless otherwise specified) | Vitamin C dosage, commencement/duration of supplementation | Control, blinding | Frequency of preterm birth in vitamin C group (n/N) | Frequency of preterm birth in control group (n/N) | Study conclusions | |
|---|---|---|---|---|---|---|---|---|---|
| 1 | Casanueva et al. 2005 (27) | Mexico | <20 weeks, no |
| 100 mg daily, commenced after 20 weeks of gestation (duration not specified) | Placebo, double-blind | 7/52 | 14/57 | No significant difference (p=0.142). |
| 2 | Hajifoghaha et al. 2008 (17)/Haji Foghaha et al. 2009 (16) | Iran | 20 weeks, no |
| 100 mg daily, from 20 to 36 weeks of gestation | Placebo, single-blind | 3/57 | 7/60 | No significant difference (p=0.18). |
| 3 | Hans et al. 2010 (28) | Uganda | 4 to 12 weeks, no |
| 400 mg daily (two tablets of 100 mg two times a day), until delivery | No vitamin C, open-label | 15/187 | 18/197 | No significant difference (p=0.719). |
| 4 | Kiondo et al. 2014 (30) | Uganda | 12–22 weeks, no |
| 1000 mg daily, until delivery | Placebo, triple-blind | 47/415 | 51/418 | No significant difference (p=0.7). |
| 5 | Steyn et al. 2003 (29) | South Africa | Before 26 weeks, history of previous preterm birth |
| 500 mg daily (250 mg twice a day), until 34 weeks of gestation | Placebo, double-blind | 50/100 | 35/100 | Significantly more preterm birth in vitamin C group compared to control group (p=0.031). |
Characteristics of included studies which evaluated vitamin C levels
| Study ID (first author, year) | Country | Blood sample type, measurement method, time of taking the blood sample | Number of participants | Mean age ± SD in years; Mean gestational age ± SD in weeks | Mean ± SD of vitamin C levels | Conclusion | MINORS quality score and category | ||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| Preterm birth | Control | Preterm birth | Control | Preterm birth | Control | ||||||
| 1 | Agil et al. 2008 (9) | Spain | Plasma, HPLC, at delivery (within 30 minutes) | 40 | 30 | NR±NR; NR±NR | NR±NR; NR±NR | 59.33±16.32 μmol/L | 60.12±9.04 μmol/L | Comparable levels in both groups. | 17/24 Moderate |
| 2 | Al Rashedy et al. 2012 (10) | Egypt | Serum, HPLC, NS | 15 | 15 | 24.6±3.2; 33.9±1.6 (of neonates) | 25.5±3.4; 37.2±1.6 (of neonates) | 23.5±11.2 μmol/mL | 25.4±10.4 μmol/mL | No significant difference between groups. | 15/24 Moderate |
| 3 | Guajardo et al. 1995 (8) | USA | Plasma, HPLC, at delivery | 15 | 25 | 29.5±4.3; 31.4±3.4 (of neonates) | 26.2±5.3; 39.6±0.94 (of neonates) | 1.33±0.62 mg/dL | 0.72±0.69 mg/dL | Level was significantly higher in the preterm group than in the term group (p<0.05). | 16/24 Moderate |
| 4 | Joshi et al. 2008 (7) | India | Plasma, spectrophotometry, just after delivery | 40 | 100 | 23.6±3.3; 34.9±1.8 (of neonates) | 22.1±2.9; 38.9±1.0 (of neonates) | 257.8±54.1 μmol/L | 228.0±63.2 μmol/L | Level was significantly higher in the preterm group than in the term group (p=0.01). | 18/24 Moderate |
| 5 | Eryürek et al. 1991 (11) | Turkey | Blood, spectrophotometry (ascorbate + dehydroascorbate), immediately after the delivery | 7 | 10 | NR±NR; NR±NR | NR±NR; NR±NR | 6.08±1.75 μg/mL | 4.91±1.72 μg/mL | No significant difference between groups. | 14/24 Moderate |