Table 3.
Quality of included studies
| Author | Measurement(s) | Sample | Fasting sample | Separation of sample | Sample storage | Sample characteristics | Variables controlled for in analyses |
|---|---|---|---|---|---|---|---|
| Jackson & Mathur (1991) | Haematocrit | Whole blood | Not stated | – | Not stated | Random sample; not stated whether participants took supplements; stage of pregnancy not stated | No variables controlled for in analysis of nutritional status. |
| Wolfe et al. (1994) | ZincAlkaline phosphatase | Plasma & hairPlasma | Overnight fast | Within 3‐h of taking sample | Frozen | Convenience sample; 4 had pre‐eclampsia and 1 had gestational diabetes | Pearson correlation analyses to test associations between nutrient intake from supplements, number of cigarettes smoked, chronological age and gynaecologic age with biochemical markers. |
| Gadowsky et al. (1995) | FolateVitamin B12 Methylmalonic acidHomocysteineFerritinHaemoglobinMean Cell Volume | Plasma & RBCPlasmaPlasmaPlasmaPlasmaWhole bloodWhole blood | Overnight fast | Sample transported on ice then separated | Frozen at −75°C | Convenience sample | Logistic regression analysis for associations between prenatal supplement use, oral contraceptive use, smoking status, chronological and gynaecologic age with biochemical markers. ANOVA used to test effect of living arrangement and main source of income on biochemical markers. |
| Chang et al. (2003a) | HaemoglobinHaematocrit | Whole blood | Non‐fasting samples | – | Not stated | 10‐year retrospective review of medical charts. All adolescents were prescribed daily prenatal supplements, and anaemic adolescents were prescribed additional iron supplements. However, data on prenatal supplement use or compliance were unavailable | Analysis of variance used to test effects of parity, pre‐pregnancy BMI and adequacy of prenatal care on Hb concentration. T‐tests used to test effects of sexually transmitted diseases, pre‐eclampsia, smoking status and drug use on Hb concentration. |
| O’Brien et al. (2003) | Calcium absorptionCalcium‐related hormones | UrineBlood | 24‐h urine postdosing & 3 spot daily urinesOvernight fast | Not stated | Not stated | Convenience sample. Only assessment of calcium intake included contribution of prenatal supplements | Simple regression analysis used to examine association between age and calcium intake with calcium absorption. T‐tests used to test effects of race on calcium‐related hormone levels. |
| Iannotti et al. (2005) | HaemoglobinHaematocritFerritinTransferrin receptorTfR : ferritin ratioBody ironFolic acidVitamin B12 | Whole bloodWhole bloodSerumSerum––SerumSerum | Non‐fasting samples | Not stated | Not stated | Convenience sample; not stated whether participants took supplements | Linear and logistic regression analyses for associations between maternal age, parity, pre‐pregnancy BMI, pregnancy weight gain, leptin concentration, infant birthweight, genitourinary infections, prenatal care visits, smoking status and iron status. |
BMI, body mass index; Hb, haemoglobin; RBC, red blood cell; TfR, transferrin receptors.