Estimation of the Variability of Requirements. Several approaches regarding components of variation could be considered in estimating the CV for iron needs in pregnancy:
variability of basal requirement based on prepregnancy body weight; this would then need to be matched with the estimates of basal losses in nonpregnant females;
variability of iron in the fetus based on variation in fetal weight at term; basing variability on birth weight alone would be a conservative (low) approach;
variability of blood iron based on variation in hemoglobin concentration (SD of about 9 g/L) ignoring variation in blood volume; and
variation based on the responses to level of iron supplementation.
The most conservative approach is based on variation in basal loss and assumes a CV of body weight of 21 percent (see “Adults Ages 19 Years and Older”) and a CV of hemoglobin concentration in iron-supplemented women during the third trimester of about 7 percent (9 g/L/135 g/L) (Beaton et al., 1989). When these assumptions are applied, with basal losses based on prepregnancy weight, the iron need for products of conception is 315 ± 66.2 (SD), and the iron need for hemoglobin mass expansion is 500 ± 35 (SD). For the total pregnancy, this model yielded an estimated requirement of 1,055 mg ± 99.2 (SD) (CV, 9.4 percent). Table 9-16 summarizes the average requirement for absorbed and dietary iron for each trimester.
To estimate the needs of pregnant adolescents, the approach described above was followed with the notable exception that for adolescents the factorial model included basal losses and iron deposition in tissue as computed for adolescents. The fact that birth weights for adolescent mothers tend to be lower than for older women was ignored. In adolescents, the ninety-seven and one-half percentile of requirement was estimated for each trimester from simulation models rather than deriving one CV estimate and applying it to all three trimesters.
The EAR and RDA are established by using estimates for the third trimester to build iron stores during the first trimester of pregnancy.