Growth of Fetal Size

What should be considered more important for growth assessment than birth weight (at any gesta-tional age) is the genetic growth potential of the infant, which may or may not be limited by maternal size. Under usual conditions, the fetus grows at its genetic potential. Small fetuses of small parents or large fetuses of large parents do not reflect fetal growth restriction or fetal overgrowth, respectively; in fact, their rates of growth are normal for their genome. If the mother is unusually small, however, she might limit fetal growth by 'maternal constraint,' which represents a limited uterine size (primarily endometrial surface area) and thus the capacity to support placental growth and nutrient supply to the fetus. A clear example of maternal constraint is shown in Figure 1, showing the reduced rate of fetal growth of multiple fetuses in a species, i.e., human, that optimally supports only one fetus.

Fetal weight tends to increase exponentially in the middle part of gestation, producing the typical S-shaped curve of fetal weight versus gestational age that is derived from cross-sectional measurements of newborn weights at different known gestational ages (Figure 2). The length of gestation is more strongly related to the growth of neural tissue (range 0.015-0.033 g1/3/day - a 2.2-fold range) than to the growth of the fetal body (range 0.033 to 0.25 g1/3/day - a 7.6-fold range). The physiological significance of this relationship is not known, but intrauterine development of a large brain/body mass ratio in humans is favored in a single fetus and is made possible by a slow rate of somatic growth.

Figure 1 Mean birth weight of single and multiple fetuses related to duration of gestation. (Reproduced with permission from Ounsted M and Ounsted C (1973) On Fetal Growth Rate. Spastics International Medical Publications (Clinics in Developmental Medicine No. 46), p. 17. London: William Heinemann Medical Books Ltd.)

Figure 1 Mean birth weight of single and multiple fetuses related to duration of gestation. (Reproduced with permission from Ounsted M and Ounsted C (1973) On Fetal Growth Rate. Spastics International Medical Publications (Clinics in Developmental Medicine No. 46), p. 17. London: William Heinemann Medical Books Ltd.)

Fetal Weight Weeks

22 26 30 34 38 42 46 Weeks of gestation completed

Figure 2 Birth-weight percentiles for gestational age. Solid lines represent California total singleton live births, 1970-1976: dotted lines represent Colorado General Hospital (Denver, Colorado) live births, 1948-1960. (Reproduced with permission from Creasy R and Resnik R (1989) Intrauterine growth retardation. In: Creasy R and Resnik R (eds.) Maternal-Fetal Medicine, 2nd edn, pp. 549-564. Philadelphia: W.B. Saunders.)

22 26 30 34 38 42 46 Weeks of gestation completed

Figure 2 Birth-weight percentiles for gestational age. Solid lines represent California total singleton live births, 1970-1976: dotted lines represent Colorado General Hospital (Denver, Colorado) live births, 1948-1960. (Reproduced with permission from Creasy R and Resnik R (1989) Intrauterine growth retardation. In: Creasy R and Resnik R (eds.) Maternal-Fetal Medicine, 2nd edn, pp. 549-564. Philadelphia: W.B. Saunders.)

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