Maternal hemoglobin level, prematurity, and low birth weight

Abstract

Birth weight is affected by several maternal factors. Levels of 10.5 g/dl of hemoglobin in the second trimester and 11 g/dl in the first and third trimester are considered the lowest normal levels for pregnant women, and some authors report these levels as ‘physiologic anemia’ [1]. A decrease in red blood cell numbers during pregnancy can be the result of several conditions, the most common of which being ‘true anemia,’ induced by nutritional deficiencies (iron intake is too low) [1]. Maternal anemia and high maternal hemoglobin levels during gestation are reported to be associated with low birth weight and prematurity, respectively [2]. The present study was undertaken to investigate the possible effects of abnormal maternal hemoglobin level on the birth weight of their offspring. From August 2001 to September 2002, the records of 2000 pregnant women (age, number of previous gestations and deliveries, hemoglobin levels, and smoking habits of the mother and gestational age and birth weight of the newborns) in two hospitals of Shiraz University of Medical Sciences were studied. Women with inherited or acquired hematological disorders and receiving medications were excluded from the study. Included women were classified according to anemia (hemoglobin levels below 10.5 g/dl), abnormally high hemoglobin levels (≥14 g/dl), and term or preterm delivery (delivery at or after the 37th week of gestation and delivery before it). A weight of 2500 g was considered the minimum normal birth weight. As shown in Table 1, 9% of women were anemic, 70% had normal hemoglobin levels, 21% had abnormally high hemoglobin levels, 2.7% had a preterm delivery, and 97.4% had a delivery at term. Among the newborns, 86.8% had a birth weight of at least 2500 g, while 13.2% had low birth weights. None of women had any history of smoking or hematological disorders, and none was taking medications. The relations between maternal hemoglobin level, gestational age at delivery, and birth weight was statistically significant. Newborns delivered before the 37th week whose weight was below 2500 g were considered premature, while those born at or after the 37th week but with low birth weights were regarded as small for gestational age. The low prevalence (9%) of anemia can be attributed to the routine prescription of supplemental iron for pregnant women. Higher-than-normal hemoglobin concentrations should be regarded as an indicator of possible pregnancy complications, not necessarily as a sign of adequate iron nutrition. Iron supplementation cannot increase the hemoglobin level higher than the optimal concentration needed for oxygen at delivery [3]. Results show that abnormal maternal hemoglobin levels are associated with low birth weight and preterm delivery. Many studies performed in various areas on this subject have demonstrated the same relation between abnormally low or high maternal hemoglobin levels and poor pregnancy outcome [2]. Similar to the results of Scanlon et al. [2], our results showed that both low and high hemoglobin levels are associated with low birth weight and prematurity, although, in our study, women with anemia gave birth to premature newborns more frequently than small-for-gestational-age newborns, while this ratio was reversed in women with high hemoglobin level. There does not seem to be a causal relation between abnormally high maternal hemoglobin and poor pregnancy outcome. This may be related to hypertensive disorders during pregnancy and to pre-eclampsia. A pathophysiologic mechanism of pregnancy can explain the increase in hemoglobin concentration. The decrease in plasma volume may cause fetal stress because of the reduced placental-fetal perfusion [3]. However, none of the women with high hemoglobin levels had any report of gestational hypertensive disorders. It seems that the reduction in plasma volume can result in an increase in hemoglobin levels in conditions other than gestational hypertension and pre-eclampsia. The relation between iron deficiency and poor pregnancy outcome is not clearer. Several potential biological mechanisms were identified through which anemia or iron deficiency could affect pregnancy outcome. Anemia and iron deficiency can induce maternal and fetal stress, stimulating the synthesis of corticotropin-releasing hormone (CRH). Elevated CRH concentration is a major risk factor for preterm labor, pregnancy-induced hypertension, eclampsia, and premature rupture of membranes; it can also increase fetal cortisol production, and cortisol can inhibit longitudinal growth of the fetus. By an alternative mechanism, iron deficiency increases oxidative damage to erythrocytes and the fetoplacental unit [4]. The authors thank the office of the Vice Chancellor for Research and the Center of Clinical Research Development of Nemazi Hospital of Shiraz University of Medical Sciences for financial support.