Pregnant women are at risk of developing anaemia because of the extra iron required by the growing fetus, the placenta and the increased maternal red cell mass.1 2 Prepartum maternal anaemia, a condition characterised by impaired haeme synthesis and hypoplastic erythropoiesis,3 is diagnosed when the haemoglobin concentration falls below 11 g/dl during gestation.4 Despite recommendations on iron prophylaxis, maternal anaemia is still a major antenatal health problem due to its high incidence and potential medical consequences.5 6 In Europe, the estimated number of pregnant women with maternal anaemia is approximately 2.5 million.4 A recent study showed a maternal anaemia incidence of approximately 16% at delivery in pregnant westerners receiving no iron supplements.6 The potential health problems associated with maternal anaemia are of importance as they include increased fatigue and decreased work performance, cardiovascular stress due to inadequate haemoglobin and low oxygen saturation, impaired resistance to infection and poor tolerance to heavy blood loss and surgical interventions at delivery.6

The number of pregnant women engaging in regular exercise is growing,7 a tendency supported by recent scientific evidence. In healthy gravida, regular exercise does not affect the main gestational outcomes, eg, gestational age or Apgar scores.7 8 9 10 11 12 Recent guidelines by the American College of Obstetricians and Gynaecologists do in fact promote regular exercise during pregnancy for its overall health benefits, including a decreased risk of hypertension and gestational diabetes.13 Whether regular exercise influences the risk of maternal anaemia, however, remains to be determined.

Especially during the second half of gestation, regular exercise may superimpose increasing iron and oxygen transport demands. In sportswomen, additional iron losses can result from exercise-related mechanisms such as haemolysis or sweating.14 Exercise-induced acute elevations in cytokine levels increase the liver production of hepcidin.15 Hepcidin upregulation has a negative impact on the iron transport and absorption channels within the body.15 The purpose of this study was to determine whether regular exercise performed over the second half of pregnancy is associated with an increased risk of maternal anaemia. To this end, we conducted a randomised controlled trial (RCT) in which the intervention consisted of regular, supervised exercises and the main outcome was the incidence of maternal anaemia (as defined above) and changes in haematological variables over pregnancy.

A complete description of the design and methods of this RCT, which was in accordance with CONSORT guidelines16 was published recently.12 The research protocol was reviewed and approved by our ethics committee and followed the ethical guidelines of the Declaration of Helsinki. All women provided written informed consent. We initially contacted 480 Spanish (white) pregnant women from a primary care medical centre (Madrid, Spain). A total of 160 healthy gravida (25–35 years old) who were sedentary (exercising ⩽20 minutes on ⩽3 days/week) and had no medical contraindication for exercise practice during gestation13 was randomly assigned to either a training or control group (n  =  80 each).12

Women in the training group were enrolled in three sessions per week of individually supervised resistance exercise training performed over the second and third trimester of pregnancy as detailed elsewhere (in total 26 weeks, approximately 80 training sessions).12 In brief, each session consisted of 35–40 minutes of light intensity resistance exercises, which were performed with barbells (⩽3 kg per exercise) or low-to-medium resistance bands (Therabands). They included one set of 10 or less to 12 repetitions of abdominal curls, biceps curls, arm extensions, arm side lifts, shoulder elevations, seated bench press, seated lateral row, lateral leg elevations, leg circles, knee extensions, knee (hamstring) curls and ankle flexion and extensions. Heart rate was carefully and individually controlled (⩽80% of age-predicted maximum heart rate value, 220 minus age in years) through a heart rate monitor (Accurex Plus; Polar Electro OY, Finland) during the training sessions. Women in the non-exercise control group maintained their level of activity during the study period.

We found no between-group differences12 at the start of the study in potential confounding variables such as habitual physical activity and occupational activities,17 smoking habits, alcohol intake and previous parity.12 There is no worldwide consensus regarding iron prophylaxis during gestation, although the European Comission’s report recommends that daily iron supplements (with no dose specification) are given in the latter half of pregnancy.18 In Spain, there are no official guidelines, but iron supplementation (50–70 mg) is commonly administered by obstetricians from the first maternal visit. Accordingly, all the participants were told to ingest a tablet containing 60 mg ferrous iron daily (at bedtime) from the first maternal visit to the end of gestation. All participants took folic acid (4 mg) during the first trimester.

We determined haematological variables and iron (see table 1) in the first maternal visit (baseline) and at the beginning of the second and third trimesters with Advia 2120 (Bayer Health Care, Tarrytown, New York, USA) and Hitachi 717 analysers (Roche Diagnostics, Barcelona, Spain). We compared the aforementioned variables with repeated-measures analysis of variance. The Tukey test was used post hoc. The frequency of maternal anaemia in the two groups was compared with the χ² test. We conducted all analyses using the intent-to-treat principle. All statistical analyses were performed using SPSS, with the significance level at less than 0.05.

A total of 72 (training) and 70 (control group) gravida were included in the analysis. There were no protocol deviations from study as planned.

The frequency of participants with maternal anaemia in the two groups did not differ at baseline (χ²  =  2.058, p = 0.151), second (χ²  =  0.350, p = 0.554) or third trimesters (χ²  =  0.115, p = 0.735; fig 1). The proportion of participants with maternal anaemia in both groups (⩽10%) was within the range previously reported in women treated with a similar iron dosage.19 Haematological variables are shown in table 1. The pattern of haematological adaptations was similar in both groups, reflecting the typical pattern observed in supplemented gravida, ie, a steady fall in haemoglobin concentrations over the second trimester due to haemodilution and a rise later in gestation due to haemoconcentration.5 Except mainly for a slightly higher haemoglobin and iron concentration in the intervention group at baseline, we observed no major between-group differences (either in absolute values or in over time relative changes).

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Figure 1

Frequency of prepartum maternal anaemia in the control and intervention group at different time points.

The novel topic we addressed here is of medical relevance given the potential medical complications and the deterioration of quality of life associated with maternal anaemia.6 Although pregnancy is a physiological, naturally occurring process, it is also unique in that most body control systems are transiently modified in an attempt to maintain both maternal and fetal homeostasis.11 Regular exercise could thus be a cause of concern as it could potentially challenge the homeostasis of the maternal–fetal unit by inducing changes in visceral blood flow, body temperature, shear stress or iron and erythrocyte turnover.7 20 21 22 The present findings, however, together with other recently published outcomes from the same RCT12 support the notion that regular exercise is a safe option for pregnant, healthy gravida. Whether exercise may impair iron metabolism in non-supplemented women, as can be the case in developing countries, remains to be elucidated. However, it is biologically plausible that exercise of light intensity, as that performed in the present study, does not increase the risk of prepartum maternal anaemia.