figure 2-3 The 50th centile of triceps and subscapular skinfolds. (Source: Tanner J. Growth at Adolescence, 2nd ed. Oxford: Blackwell Scientific Publications, 1962.)
against age.4 The slopes of the curve at different ages are similar to those expressed by the growth of total body fat (see Chapter 13). Fat tissue exhibits an important increase during the first 9 months of postnatal life. The baby is born with a relatively small amount of body fat, about 14%, which rapidly increases to about 25% by 6 months and stays at this level throughout infancy but has decreased to about 15% by 5 years. After birth, the infant is thus equipped with an energy reserve that helps it cope with one of the most dangerous periods for nutrition scarcity in human evolution, weaning. After the first 6 months of postnatal life, breast milk does not fulfill the increasing nutritional needs of the infant. Therefore, weaning is initiated, and from an evolutionary perspective, the infant enters into competition with other human groups and other species for appropriate food sources.
Weaning should not be considered a single event, as it is actually a process of gradual incorporation of new, "solid" foods into the diet from approximately the age of 6 months. Initially, this complements breast milk, but after several months it replaces it. The process involves the activation of a multiplicity of what could be called family functions. These include the acquisition and selection of adequate food (not only in quantity but in quality), the development of certain food preparation and presentation technologies, the development of psychomotor skills in the baby to enable it to eat, changes in family routines, and so on. When these functions are not met or in areas of food scarcity where these functions break down, there is a high risk of growth delay during the weaning process. Growth during the weaning period should be specially surveyed in pediatric programs.
The second year of life coincides with a decline in skinfold thickness, and the baby changes its initial plump "babylike" appearance toward a leaner, slender body build. This is a period coincident with important progress in psychomotor skills: The child is now able to walk and run and develops an extraordinary amount of physical activity and energy expenditure. During childhood (5-10 years old), the percentage body fat remains quite stable, as do many other growth indicators, to the extent that childhood growth velocity has been characterized as a plateau, reflecting a quite stable period of somatic growth. Body fat continues to be greater in girls than boys, a genetic difference that holds true in skinfold thickness and percentage of total body fat. By 5 years old, the sex difference is about 2% (14.6% for boys and 16.7% for girls) but this difference increases to almost 6% by 10 years old.
some particular features of growth during infancy and childhood
Growth is the result of three forces: the genetic program, the action of environmental factors, and the interaction between the two. One simple example of the influence of genetics on physical growth is that tall parents tend to have tall children and short parents tend to have short children. The relationship between the height of children and their parents is a useful tool for growth assessment, especially in childhood and puberty. However, in the first 2 years of postnatal life, this relationship is not as close as at later ages. Physical size during the first 2 years of life is related more to prenatal growth than the size of parents, and in turn, prenatal growth cannot be solely explained in terms of the genotype of the fetus.
Figure 2-4 shows the correlation coefficients between the height of the child as an adult and the height of the same child at each age from birth onward.5 Size at birth is poorly correlated with adult size; the values rise steeply during the first 2 years to a level of about 0.80, then become quite stable with very small variations until adulthood. The single major variation during adolescence is due to differing rates of maturation (see Chapter 3). This means that not all genes regulating body size are fully expressed at birth. After birth, they gradually become more influential, until they can express their full influence after the age of 2 years. This is not the only example showing that some genes have a particular period of time in which they are active. At other times, they are either inactive or their effect is masked by other (environmental) factors. In the case of infants, their size in the first 2 years of postnatal life at a given age is related more to size at birth (and, consequently, to fetal growth) than the height of parents. The knowledge of those factors regulating fetal growth is then crucial for evaluating growth in infancy.
One of the factors proven to be very important in the modulation of fetal growth is maternal size. Small mothers tend to have small babies, and large mothers tend to have large babies, independent of their genotype. Avery illustrative experiment
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The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.