The mean age of the study participants at recruitment was 27±5.4 years with a range of 16–44 years. The baseline maternal characteristics, including nutritional indicators, to compare the study groups are presented in Table . The mean gestational age at recruitment was 13.0±3.0 completed weeks. The study groups did not differ significantly in terms of their baseline characteristics, except for age where the iron-zinc group had older participants [F (1,598)=4.864, p=0.028)] (Table ).
Baseline characteristics and comparability of treatment groups
Follow-up and compliance with supplements
The number of participants recruited, enrolled, and loss-to-follow-up is shown in the .
A flow diagram showing the number of participants recruited, randomized, and lost-to-follow-up
The total loss-to-follow-up rate was 9.5%; this was less than 10.0% and was, therefore, not a threat to the validity and power of the study. In total, 9.0% (n=27) of the participants (n=299) were lost-to-follow-up in the zinc-supplemented group compared to 9.9% (n=30) of the participants (n=301) in the control group. The minimum total sample size required to detect the 150 g mean birthweight difference at 80% power was calculated to be 470, and 543 participants completed the study.
Pregnancy loss, i.e. miscarriages, spontaneous abortions, and stillbirths, accounted for 45.6% (n=26) of the participants who were lost-to-follow-up, and 1.3% (n=7) of the total deliveries (n=550) were multiple births. In addition, 4.0% (n=24) of the total number of randomized participants were lost at different stages of the trial as they travelled out of the study area and could not be traced.
There were no differences in the compliance levels between the study groups. Full compliance/adherence in the study was generally high and was measured in terms of dosage, time, and frequency of taking the supplements. Partial compliance meant that a participant did not take her supplement at the prescribed time, dosage, or frequency. The degree of compliance was measured in terms of the total number of capsules taken during the trial period.
Of the participants enrolled, 91.7% (n=550) delivered, and seven of the women delivered twins. There were, therefore, 543 singleton livebirths; 49.8% (n=269) were males, and 50.2% (n=271) were females. Of the 543 singleton livebirths, 91.5% (n=497) were by spontaneous vaginal delivery (SVD) and 8.5% (n=46) by caesarean section.
Effect of iron-zinc supplementation on pregnancy outcomes
Overall, there was no detectable difference in the mean birthweight between the study groups. The adjusted mean birthweight in the iron-zinc group was 3,105±490 g and in the control group was 3,120±486 g. However, adjusted for gestational weight gain between week 28 and week 36, diastolic pressure at 34–36 weeks of gestation, gender of baby, IUGR, preterm delivery, length of gestation, maternal weight, and gravidity through ANCOVA analysis, an interaction between maternal iron status at recruitment and treatment type manifested [F (1,179)=5.614, p=0.019]. Consequently, adjusted for diastolic pressure at 34–36 weeks of gestation, gender of baby, gestational weight gain between 28 and 36 weeks of gestation, IUGR, length of gestation, preterm delivery, and BMI, the adjusted mean birthweight of babies born to women in the iron-zinc group was 131 g higher than the adjusted mean birthweight of babies born to women who received the standard treatment [(3,223 g vs 3,092 g), F (1,121)=4.210, p=0.042] among anaemic women (Hb < 9.0 g/dL) and/or iron-deficient women (serum ferritin <35 µg/L). Among women who were iron-sufficient (serum ferritin ≥35 µg/L) at recruitment, the adjusted mean birthweight in the iron-zinc supplemented group was not different from that of the control group [(3,032 g vs 3,053 g), F (1, 94)=0.762, p=0.762].
There was no significant difference in the mean length of gestation between the study groups even after adjusting for the main determinants. The mean length of gestation in the iron-zinc and the control group were 37.1±1.8 and 37.2±1.7 respectively [F (1,541)=0.144, p=0.704]. The other adverse pregnancy outcomes and the risk of the event occurring in the iron-zinc compared to the standard treatment group are shown in Table .
Effect of iron-zinc supplementation on selected adverse pregnancy outcomes
Determinants of birthweight
Determinants of birthweight in the whole sample and among iron-deficient and iron-sufficient women are shown separately in Table , , and respectively. Variables entered in the regression analysis included malarial infection in early pregnancy, maternal age, type of treatment, use of sulphadoxine pyremethamine, and gravidity but these were excluded from the model for the whole sample because of their weak association with birthweight. This set of predictors in the separate models for birthweight explained 43.6% and 54.9% of variation in birthweight among iron-deficient and iron-sufficient women respectively. Malarial infection in early pregnancy was an important determinant of birthweight only among iron-sufficient women. Preterm delivery was not an important determinant of birthweight among iron-sufficient women but it was important among iron-deficient women. Type of supplement was an important determinant of birthweight among iron-deficient but not among iron-sufficient women. Among iron-deficient women, women who received standard treatment, i.e. iron-only, gave birth to babies whose mean birthweight was 0.141 standard units lower than women who received the iron-zinc supplement.
Determinants of birthweight (whole sample)
Determinants of birthweight among iron-deficient women
Determinants of birthweight among iron-sufficient women
Effect of iron-zinc supplementation on maternal zinc and iron status
The geometric mean concentrations of plasma zinc at recruitment and at 34–36 weeks of gestation were 26.3±2.5 µg/dL (95% confidence interval [CI] 24.1–28.6) and 40.1±2.7 µg/dL (95% CI 32.5–49.4) respectively. Treatment effect was different depending on the maternal zinc status at recruitment in the subgroup of the participants for whom plasma zinc was assessed. Adjusted for amount of supplement consumed, a strong interaction between the maternal zinc status and the type of supplement modified the treatment effect on geometric plasma zinc at 34–36 weeks [F (1,81)=4.765, p=0.032]. Consequently, among women who had low concentrations of plasma zinc at baseline, the adjusted geometric mean plasma zinc in the iron-zinc group was higher than that in the control group, although not significant [(41.2 µg/dL vs 31.2 µg/dL), F (1,56)=1.103, p=0.298]. The geometric mean concentration of plasma zinc was adjusted for IUGR, z-score plasma zinc concentrations at recruitment, and maternal age. Among women who were not zinc-deficient (plasma zinc ≥60 µg/dL) at recruitment, the adjusted geometric mean concentrations of plasma zinc at 34–36 weeks of gestation tended to be higher in the iron-zinc group compared to the control group [(69.8 µg/dL vs 44.4 µg/dL), F (1,22)=2.236, p=0.149]. Women with low levels of plasma zinc (<60.0 µg/dL) at recruitment also had significantly higher Hb compared to women who had relatively higher plasma concentrations at recruitment.
The adjusted geometric mean plasma zinc at 34–36 weeks of gestation was higher among women who had normal concentrations of plasma zinc (≥60 µg/dL) compared to women who were zinc-deficient in early pregnancy [(88.7 µg/dL vs 28.9 µg/dL), F (1,82)=10.010, p=0.002)]. Geometric concentrations of plasma zinc were adjusted for IUGR and z-score plasma zinc concentrations at baseline, gender.
Relationship of maternal plasma zinc with iron status parameters
The relationships between the maternal zinc status and the iron status indicators were assessed in a subsample of the study participants. Women who had low levels of plasma zinc (<60.0 µg/dL) at recruitment had also significantly higher Hb but lower serum ferritin and iron concentrations compared to women with normal values (≥60 µg/dL). Concentrations of plasma zinc in early pregnancy positively correlated with serum ferritin (r=0.137, p=0.069) and serum iron (r=0.261, p=0.001). Concentrations of plasma zinc correlated negatively with Hb (r=−0.165, p=0.001) in early pregnancy.