development and growth: birth and infancy

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development and growth: birth and infancy The Latin word infans means ‘not speaking’ and by convention the word infancy has come to mean the first year of childhood. There are very great differences between mammalian species in the degree of maturity of their newborn young. Deer, cattle, sheep, and guinea pigs are described as precocial for they are born with a protective covering of hair and are able to stand and walk almost immediately after birth. In contrast mice, rabbits, and humans — born naked, helpless, and vulnerable — are described as non-precocial or altricial (from the Latin for nurses or feeders). The human infant is known as neonatal for the first month after birth.

Comparison of the gestation period in man (266 days from the date of conception, or 280 days from the last menstrual period, to delivery) with that of other animals shows that whilst a number of species have longer periods of gestation they are all born in a state of greater functional maturity. The human is unusual in having a long gestation period without obtaining great size or maturity at the time of birth. Another peculiarity is the size of the brain which already weighs about 350 g at birth or 10% of the average total body weight of about 3.5 kg.

Adaptation from complete dependence upon the maternal uterine environment and placenta to the extra-uterine environment requires major changes in the infant body organs. Within a minute of cessation of placental blood supply and delivery from a watery to a gaseous environment, the infant lungs, heart, skin, and the alimentary, renal, and nervous systems undergo a series of dramatic functional changes.

By means of the exchange of substances between their respective bloodstreams, the fetus has depended on the mother for obtaining oxygen and nutrients, and for excretion of carbon dioxide, heat, and other metabolic waste products. Now it must fend for itself. More blood flow must be directed through the lungs for gas exchange, to the gut for nutrient absorption, to the kidneys for urine formation. But first and foremost, breathing must begin.

Breathing

Fetal breathing movements are necessary for normal lung development in the womb. The patterns of these movements are related to the ‘sleep’ and ‘awake’ states of the fetus but can be affected by extrinsic factors such as maternal smoking. Fetal lung produces a fluid which is normally swallowed although some may be expelled into the amniotic fluid. Nearer to the time of delivery this lung fluid contains surfactant which at delivery is critically important for the stability of the alveolar air spaces once they are cleared of fluid and filled with air sucked into the chest by the first breath. Lack of this surfactant material is one of the major causes of breathing difficulty in infants born prematurely.

Normal vaginally-delivered infants make their first breathing movements within 20 to 30 sec from the emergence of the nose. Within 90 sec of complete delivery most infants have started to breathe rhythmically. The strong initial inspiratory efforts and subsequent rhythmic activities of respiratory muscles are largely dependent upon neurones in the brain stem, in turn stimulated by the carbon dioxide and hydrogen ion concentration in the blood and in the cerebrospinal fluid. Breathing is also stimulated by a low level of oxygen in the blood, acting on chemoreceptors in the carotid bodies; this hypoxic drive may assume great importance if the brainstem centres have been depressed by the infant becoming hypoxic during delivery or by anaesthesic and analgesic drugs given to the mother.

Circulation

At birth the circulation of the blood is drastically re-routed. In the fetus there was relatively little blood flow through the lungs. Oxygenated blood reached the fetus from the placenta in the umbilical vein and joined the blood entering the right side of the heart. Most of this blood bypassed the lungs — shunted through to the left side through a hole between the right and left atria of the heart (foramen ovale), and through a channel from the pulmonary artery to the aorta (ductus arteriosus); thence to the rest of the body and the umbilical arteries. Now, after birth, the right ventricle must pump all the blood it receives through the lungs. This change is assisted by the onset of breathing itself. The expansion of the lungs with air reduces the resistance to flow in their blood vessels. Flow through the foramen ovale stops mainly as a result of altered pressures. Constriction of the ductus arteriosus and dilation of the lung vessels is mediated by the increased oxygen content of the blood in them, and by locally released substances that act on the muscle in the vessel walls. Anatomical closure of the fetal channels ensues. Closure of the umbilical vessels is accompanied by increased blood flow to the abdominal organs.

Nutrition and metabolism

Newborn polar bears weigh about 550 g, which is 0.3% of the mother's weight, whereas a newborn lesser horseshoe bat weighs 2 g, 30% of its mother's weight. The human lies between these extremes at 5–6%. In spite of these great species differences in prenatal vs. postnatal growth there is in all mammals a continuum of nutrient supply by the mother from conception until after complete weaning. Even after weaning in most human societies and other mammalian species, the mother is primarily responsible for helping the immature offspring to obtain adequate nutrition. The importance of optimal nutrition in human fetal and neonatal life has been highlighted by Barker and his colleagues in Southampton who have noted the association of disordered growth in early life with an increased incidence of hypertension, strokes, diabetes, and coronary artery disease in later life.

Energy

The human infant born at term (37–42 week's gestation) has relatively large stores of lipid, carbohydrate, and important nutrient elements such as iron. After birth, fat and lactose supplied in the mother's milk are the major sources of energy, whereas before birth glucose supplied by the placenta provided the energy for fetal growth. This abrupt transition in nutrient supply causes major challenges to the digestive, absorptive, and metabolic processes of the infant. Until lactation is established stores of glycogen in the liver and muscles, and triglyceride fat from adipose tissue, help to maintain the infant body temperature, metabolic activity, and tissue growth. Release of these energy stores is mediated through signals from the hypothalamus which change the ratio of glucagon to insulin released from the pancreas. When the infant blood reaching the hypothalamus is low in glucose a higher ratio of glucagon to insulin stimulates the release of more glucose from the glycogen stores and of triglycerides from subcutaneous fat stores.

Temperature

If the infant's temperature falls, neural thermostats stimulate the sympathetic nervous system to release heat and fatty acids from brown fat. Brown fat looks brown because its cells are full of mitochondria, which are cellular power-houses for the release of energy from fat; it is located mainly between the shoulder blades in the newborn infant and there is relatively little in later life. (Animals which hibernate retain brown fat stores in adult life and it is from these stores, replenished in the summer months, that they maintain their body temperatures during winter.) Maternal body heat, and covering the head and body of the infant with clothing to reduce heat and fluid loss, greatly reduce the energy and fluid needs of the newborn. Throughout life the skin is an important organ for temperature regulation by means of heat and fluid loss or retention, and the relatively high surface-to-volume ratio of the infant enhances its importance. Temperature receptors in the hypothalamus acting through the sympathetic nervous system regulate blood flow to the skin and thus help to control heat and water loss, but the infant at first lacks the additional mechanisms of sweating and shivering.

Colostrum and milk

Once the immediate needs for an adequate supply of oxygen have been met the infant normally within minutes begins to ‘seek’ a supply of water and nutrients at the mother's breast. During the first few days the mother supplies colostrum, which is specifically designed for her own infant in that it contains antibodies, cells, and other protective substances which will safeguard her infant from virtually all of the infections to which she has been previously exposed. Thereafter, the volume of milk produced by the mother increases to meet the demands of the infant. It may take 4–6 weeks to achieve a complete balance of maternal milk output and a satisfied infant. To meet the demand of one infant the mother will produce 700–750 ml/day, but she can easily double this volume to meet the needs of twins. The same levels of protection from infections, particularly of the gastrointestinal tract and upper airways, that were provided in the first few days by colostrum, are provided by these larger volumes of milk.

There are a series of brainstem-mediated reflexes which together with help from a supportive mother ensure that the infant finds, attaches, and obtains nutrition from her breasts. Rooting reflexes are initiated usually by contact of the infant's cheek with the mother's nipple. They are mediated by the trigeminal nerve (which carries sensory nerves from the face and activates the jaw muscles) and they ensure that the infant's head turns towards the stimulus and the mouth opens to accept the nipple. If the infant is hungry these reflexes are enhanced so that a stimulus applied anywhere on the face will elicit the rooting responses. Once the infant has located the nipple it is drawn well into the mouth so that outer margin of the nipple (areola) lies at the level of the infant's gums. By closing the gums (fixing) and with rolling movements of the tongue, milk is squeezed from areolar milk ducts into the pharynx where a series of reflexly co-ordinated muscle actions stop the infant breathing while milk is directed down the oesophagus and into the stomach. The gums and tongue are relaxed whilst the infant breathes through the nose, the milk ducts refill and a further cycle commences. Newborn infants are obligatory nose breathers which is why nasal obstructions can interfere with normal feeding. Suckling is the major stimulus to further milk production in the glandular tissue of the breasts, mediated by a reduction in the maternal hormone oestrogen and increase in prolactin. Muscle sphincters at the nipple prevent leakage of milk between feeds. Reflex propulsion of milk from the alveoli and relaxation of the nipple sphincters is mediated by maternal oxytocin release from the posterior pituitary gland. The main stimulus for this so-called ‘draught’ reflex is suckling by the infant but it can be initiated even by the cry of the hungry infant. Once the draught reflex has been initiated milk may continue to flow from both breasts even when the infant is not suckling. This reflex greatly helps the infant obtain milk. If the mother cannot relax during feeding the draught reflex may be delayed and the infant becomes frustrated. These early feeding interactions between mother and infant are important determinants for the subsequent behaviour, communication, and language development of the infant.

Digestion

Over 90% of the fat present in human milk can be digested and absorbed by the infant intestine even though the activity of the pancreatic fat-digesting enzyme (lipase) is low relative to adult values, and bile salt production (which helps absorption of fats) is also low. Fat digestion is possible because lipases are present in the milk, and are also released from glands in the infant tongue. These enzymes remain active in the environment of the stomach.

Lactose, the sugar present in milk, is split in the small intestine by the enzyme lactase into its constituent glucose and galactose molecules, allowing absorption of the glucose. Absence of this enzyme, from an inherited defect, or through damage to the intestinal mucous membrane from viral or bacterial infection, can result in intractable diarrhoea.

There are no digestive enzymes for protein in human milk or (unlike the adult) in the infant's stomach and duodenum. This is significant because there are important proteins in the milk — immunoglobulins and growth factors — which might otherwise be damaged before they can be absorbed from the intestine.

Weaning

is the process of expanding the diet to include foods and drinks other than breast milk or infant formula. A Department of Health working group in 1994 recommended that most infants should not be given solid foods before the age of 4 months and that a mixed diet should be offered by the age of 6 months. Cow's milk is not recommended as a main drink during infancy but during the second year it can make an important contribution to the intakes of several different nutrients and energy. By the end of the first year the child should be able to drink from a cup with help and have been introduced to most of the range of foods eaten by the family, with the exception of highly spiced foods.

Growth

If comparison is made between the Madonna and Child Enthroned of Cimabue (1240–1302) and the Castelfranco painting of the same subject by Giorgione (1477–1510), the change from the conception that the human infant is a diminutive adult to the reality of early infant body proportions is very obvious. The change in relative size of head, body, and limbs, constructed from photographs in which height has been standardized, is shown on page 209.

Factors which influence growth are genetic, nutritional, endocrine, and psychosocial. Diseases of essential organs or generalized disease will also have secondary effects on growth. On average, girls are slightly smaller than boys at birth and there remains a slight average difference in height and weight throughout childhood. In general, when both parents are above average height and come from tall families the children are likely to be tall, and the converse is also true. Undernutrition, specific nutritional deficiencies, and disease can prevent children achieving their genetic growth potential. In all countries in which the standards of living have improved during this century there has been a progressive increase in the average heights and weights of the children.

Growth in length is most rapid during the first 6 months of postnatal life and gradually decelerates during the pre-school years. By convention, body ‘length’ is measured up to the age of 2 years and ‘height’ thereafter. Measurements of height and weight of individual children are of most value when carried out at regular intervals and when compared with data obtained from large groups of children with similar age and development. Since children may be small or large depending on their inheritance and prenatal growth much greater importance should be attached to their growth increments than to their actual size. No individual child should be expected to conform to the mean childhood population growth data throughout the whole growth period.

Neuroendocrine function

Neural inputs or stimuli from eyes, ears, nose, skin, muscles, tendons, joints, and from most body organs through various types of sensory receptors enter the brain stem and higher centres of the infant brain where they are integrated, interpreted, and responded to. Chemical messengers pass signals between nerve cells and between nerves and other tissues such as muscles. Nerves of the sympathetic and parasympathetic systems link with blood vessels, muscles, intestine, the central nervous system, and endocrine glands to regulate the many automatic functions of the infant body. The brain also links with endocrine gland chemical messengers through the hypothalamus and pituitary gland.

At birth much of the underlying brain and neuroendocrine system development is equipped to integrate newborn infant body functions, but it is becoming evident that if there is failure during the first year of life to use and develop a good pattern of response to a given stimulus from the environment, then there may be significant impairment in the ability to respond in later life to stresses both physical and emotional.

Forrester Cockburn

Bibliography

Rennie, J. M. and Roberton, N. R. C. (ed.) (1999). Textbook of neonatology, (3rd edn). Churchill Livingstone, Edinburgh,
Department of Health (1994). Wearing and the wearing diet. Health and social subjects No. 45. HMSO.


See also antenatal development; infant feeding.