Bone Mineral Content at Birth Is Determined Both by Birth Weight and Fetal Growth Pattern

Abstract

Adult peak bone mass is related to birth weight, suggesting it could be affected by fetal growth pattern. Small-forgestational-age (SGA) newborns have lower bone mineral content (BMC), but what about adapted-for-gestational-age (AGA) newborns with fetal growth restriction? The purpose of the study was to determine the respective role of birth weight and fetal growth pattern on BMC. Full-term newborns from SGA high-risk pregnancies were included (n ϭ 185). Estimated fetal weight percentiles were measured monthly from mid-gestation to birth, and restricted fetal growth (FGR) was defined as a loss by more than 20 percentiles. BMC was measured at birth, using dual x-ray absorptiometry. Newborns were SGA (n ϭ 56) or AGA (n ϭ 129). Newborns with FGR (n ϭ 111) were AGA (n ϭ 71) or SGA (n ϭ 41). BMC was significantly lower in SGA than AGA (1.48 Ϯ 0.02 vs. 1.87 Ϯ 0.04 g/cm) and lower when FGR irrespective of birth weight (1.66 g/cm Ϯ 0.03 vs. 1.89 g Ϯ 0.05). In multivariate analysis, FGR and SGA were significant and independent predictors of low BMC. In conclusion, fetal growth pattern affects BMC not only in SGA infants but also when birth weight is maintained in the normal range. T he peak bone mass of an individual depends upon growth and mineralization during the first two decades of life. However, a proportion of the variance of bone mineral content (BMC) found in the general population cannot be explained by genetic factors or childhood environment (1,2). Epidemiologic studies have suggested that part of this residual variation might be explained by the growth pattern in infancy (3) and probably during fetal development. Linear fetal growth is high during the last two trimesters of pregnancy, and fetal bone mineralization increases toward the end of pregnancy (4). Previous studies on a small number of newborns have shown that being born small for gestational age (SGA) was for a strong determinant of bone metabolism. BMC is lower in SGA infants at birth and is associated with a decrease in osteocalcin plasma level, suggesting that fetal mineralization is affected by fetal growth pattern (5,6). Moreover, prospective data in adult subjects indicate that bone mass and BMC are associated with birth weight after adjustment for environmental factors and body size at the time of investigation (7). However, birth weight results from fetal growth in utero. In addition, to gestational age and gender, other pregnancy characteristics, such as maternal height and weight before pregnancy, parity and ethnicity account for a large part of variation in fetal growth velocity and weight at birth. On the one hand, small babies who are small simply as a result of adaptation to maternal size can be separated from those who have suffered from restricted fetal growth (FGR). On the other hand, infants with appropriate for gestational age (AGA) birth weight can fail to reach their genetic potential of growth because of a real FGR. Birth weight by itself is not sufficient to identify FGR then. It has recently been shown that customi...

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