Table 1.
Characteristics of prenatal vitamin D trials included in systematic review and meta-analyses. Figures are number (percentage) unless stated otherwise
| Characteristic | All trials | Regular dose trials* | Bolus dose trials* |
|---|---|---|---|
| Trials | 43 | 37† | 8† |
| Intervention arms | 55 | 46 | 9 |
| Participants enrolled: | |||
| Total | 8406 | 7687 | 1031 |
| Median within individual trials (min-max) | 133 (16-1134) | 133 (16-1134) | 118 (50-235) |
| Meta-analyses to which each trial contributed: | |||
| Median maternal outcomes (min-max) | 2 (0-6) | 2 (0-6) | 2 (0-4) |
| Median newborn/infant outcomes (min-max) | 3 (0-17) | 3 (0-17) | 4 (0-7) |
| Trials that did not contribute data to any meta-analyses | 2 (4.7) | 2 (5.4) | 0 (0.0) |
| Geographical region: | |||
| Europe | 7 (16.3) | 7 (18.9) | 2 (25.0) |
| South East Asia | 6 (14.0) | 2 (5.4) | 4 (50.0) |
| Americas | 5 (11.6) | 5 (13.5) | 0 (0.0) |
| Eastern Mediterranean | 21 (48.8) | 19 (51.4) | 2 (25.0) |
| Western Pacific | 4 (9.3) | 4 (10.8) | 0 (0.0) |
| Health status at enrolment: | |||
| Generally healthy | 37 (86.0) | 33 (89.2) | 6 (75.0) |
| Gestational diabetes | 4 (9.3) | 2 (5.4) | 2 (25.0) |
| Hypocalcaemia | 1 (2.3) | 1 (2.7) | |
| Multiple sclerosis | 1 (2.3) | 1 (2.7) | 0 (0.0) |
| Baseline vitamin D status‡: | |||
| Mean 25(OH)D <30 nmol/L | 10 (23.3) | 9 (24.3) | 2 (25.0) |
| Mean 25(OH)D ≥30 nmol/L | 23 (53.5) | 20 (54.1) | 3 (37.5) |
| Not reported | 10 (23.3) | 8 (21.6) | 3 (37.5) |
| Type of vitamin D§: | |||
| D2 | 4 (7.3) | 3 (6.5) | 1 (11.1) |
| D3 | 48 (87.3) | 40 (87.0) | 8 (88.9) |
| Not reported | 3 (5.5) | 3 (6.5) | 0 (0.0) |
| Supplementation frequency§: | |||
| Daily | 31 (56.4) | 31 (67.4) | — |
| Weekly | 5 (9.1) | 5 (10.9) | — |
| Every 2 weeks | 6 (10.9) | 6 (13.0) | — |
| Monthly | 3 (5.5) | 3 (6.5) | — |
| Every 2 months | 1 (1.8) | 1 (2.2) | — |
| Single dose | 3 (5.5) | — | 3 (33.3) |
| Two doses | 4 (7.3) | — | 4 (44.4) |
| Other | 2 (3.6) | — | 2 (22.2) |
| Intervention dose§ ¶: | |||
| Median (min-max) | 2000 (200-7543) | 2000 (200-7543) | — |
| Regular dose <2000 IU/day | 24 (52.2) | 24 (52.2) | — |
| Regular dose ≥2000 IU/day | 22 (47.8) | 22 (47.8) | — |
| Control intervention type: | |||
| Placebo | 29 (67.4) | 23 (62.2) | 8 (100.0) |
| Active | 14 (32.6) | 14 (37.8) | 0 (0.0) |
| Median vitamin D dose in active control arms (min-max) | 400 (200-600) | 400 (200-600) | — |
| Timing of initiation of supplementation§: | |||
| 1st trimester | 19 (34.5) | 18 (39.1) | 1 (11.1) |
| 2nd trimester | 30 (54.5) | 24 (52.2) | 6 (66.7) |
| 3rd trimester | 5 (9.1) | 3 (6.5) | 2 (22.2) |
| Not reported | 1 (1.8) | 1 (2.2) | 0 (0.0) |
*Regular dosing refers to regimens in which supplement was administered at least three times at regular frequency throughout pregnancy (daily, weekly, every 2 weeks, monthly, or every 2 months ); bolus dosing refers to interventions administered either once or twice during intervention period.
†Two trials (Mallet 1986 and Yu 2009, see table A in appendix 5) are represented in both columns because they included both regular and bolus dose intervention arms.
‡Mean baseline maternal serum 25(OH)D concentration in control group (that is, at enrolment).
§Characteristic defined for each intervention arm; therefore, denominator is “intervention arms” rather than “trials.”
¶Regular doses given at frequencies other than daily expressed as equivalent daily doses. “Monthly” doses divided by 30.4 to estimate equivalent daily doses.