Human growth is a wonderful, dynamic process taking place in an orderly sequence, keeping continuity with time. It applies to all aspects of human organism from conception to maturity and designates an essential feature of the various phases of life of a growing child which distinguishes him or her from an adult. The term growth refers to an increase in size of various parts and organs of the human body resulting from multiplication of cells and increase in the intercellular substances: This increase is limited by pre-established constitutional hereditary factors and influenced by exogenous factors such as race, climate, diet, environment etc. (Comas, 1960). On the other hand, development refers to maturation of structures and functions of various parts of the human body that are associated with the process of growth. In explicit terms development signifies maturation of organs and systems, acquisition of skills, ability to adapt more readily to stress and ability to assume maximum responsibility and to achieve freedom of creative expressions (Silver et aI, 1980). The changing structure and function in a child are inseparable as growth and .development usually .proceed concurrently, so that there can be no sharp line between the two terms. The whole process of growth and development is extremely complex and a mass of processes interacting with each other.
EMBRYONIC PHASE OF LIFE
After fertilization by 6 days of post conceptional life, implantation of embryo begins. At this stage embryo consists of a spherical mass of cells, with a central cavity, the blastocyst. By second week implantation gets complete and utero-placental circulation begins. At this point of life embryo possesses two distinct layers i.e., endoderm and ectoderrn and the amnion begins to form. The third primary germ, layer i.e. mesoderm along with primitive -neural tube and blood vessels appear by third week of life. Paired heart tubes also start to pump blood. In addition to budding of arms and legs, human like shape appears by 4-8 weeks. Thereafter, mandible, maxilla, palate, external ear, and other head and neck structures appear. Lens placodes appear” marking the site of future eyes and the brain grows rapidly. By the end of 8 weeks the embryonic period closes and the rudiments of all major organ systems have developed. The average embryo weighs 9 g(m) and has a crown rump length ofS cm.
FETAL PHASE OF LIFE
From the 9th week onwards fetal period begins and somatic changes besides, experiencing increase in cell size also undergo number of structural modifications in the organ systems and body proportions. By 10m week, the face appears human like. The mid gut returns into the abdomen and rotates to bring the .stomach, small and large intestine into their normal biologic positions. By 12 weeks of fetal life, the gender of the external genitals becomes clearly distinguishable. After that lung development proceeds arid by 20-24 weeks, primitive alveoli have formed. During the 3rd trimester, weight triples and length doubles and body stores of protein, fat, iron, and calcium increase in magnitude.
NEUROLOGIC AND BEHAVIORAL DEVELOPMENT
Besides, a number of somatic changes occurring during prenatal life, the human organism also undergoes through a series of neurologic and behavioral transformations. During the 3rd week, a neural plate appears on the ectodermal surface of the embryo. By the 5th week, the 3’main subdivisions of brain (forebrain, midbrain, the hindbrain) become evident, the dorsal and, ventral horns of the spinal cord begin to form along with the peripheral motor and sensory nerves. Towards the close of embryonic period (8 weeks), gross structures of the nervous system get established and at birth, the structure of brain becomes complete.
Some of the important behavioural transformations experienced by growing embryo/ fetus include muscle contractions. Which first appear around 8 weeks of prenatal life. By 13-14 weeks, breathing and swallowing motions appear. Eye opening occurs around 26th week By midgestion, the full range of neonatal movements can be observed. During the’ 3rd trimester, fetus starts responding to external stimuli with heart rate elevation and body movements. Individual differences in the level of fetal activity are commonly experienced by mothers. Fetal behavior is often affected by maternal medications and diet. Besides, the above brief discussion on important aspects of auxological, developmental, neurologic and behavioral changes experienced by the growing human organism during prenatal phase of life {Feigelman 2008), some salient milestones of prenatal (embryonic and fetal) growth and development are summarized in Table 1.1.
Table 1.1 : Milestones of Prenatal Growth and Development
| Gestational Age (week) | EVENTS |
| 1 | Fertilization and implantation; beginning of embryonic period |
| 2 | Endoderm and ectoderm appear (Bilaminar embryo) |
| 3 | First missed menstrual period; mesoderm appears (trilaminar embryo); somites begin to form |
| 4 | Neural fold fuse; folding of embryo into human-like shape; arm and leg buds appear; crown rump length 4-5 mm |
| 5 | Lens placodes, primitive mouth, digital rays on hands appear. |
| 6 | Primitive nose, philturm, ··primary palate, crown rump length 21-23 mm |
| 7 | Eyelids ‘begin |
| 8 | Ovaries and testes distinguishable |
| 9 | Fetal period begins; crown rump length 5 cm,weight 9 gem) |
| 20 | Usual lower limit of viability, weight 460 gem); length 1’9cm |
| 25 | Weight 900 gem); length 25 cm |
| 28 | Eyes open; fetus turns head down; weight 1,OOOg(m) |
| 38 | Term |
With advancement of gestational age, as growth occurs the weight of the internal organs of the baby also ‘increases, which varies with maturity and birth weight of the infant. Mean weight of some internal organs (brain, heart, lungs, liver, spleen, thymus, kidneys and adrenals) obtained after autopsies of the fetuses exhibiting regular increase in values with advancement of gestational age . While recording weight ‘it is desirable to record weights of large organs to the nearest gram and that of smaller ones to the nearest 0.1 gem) by using accurate and appropriate weighing scales. Accuracy of weights of organs in neonatal autopsies.. is of great importance since some of the specific diagnosis like hyposlasia of an organ can be made only on the basis of weight (Singh, 20 I0).
FACTORS AFFECTING PRENATAL GROWTH
Every .fetus possesses an inherent growth potential and under normal circumstances it often grows into a healthy, normal and appropriate sized newborn baby. The rate of intrauterine growth depends on the:
- a) Genetic growth potential of the fetus which is determined at conception, and
- b) immediate intrauterine environment which is provided for the fetus and in turn depends on, (i) maternal organism who, through the circulation, supplies essential nutrients and oxygen to (ii) the placenta, itself, which exchanges nutrients and waste products between the fetal and maternal circulations. Collectively, these have been termed as growth support of the fetus (Gruenwald, 1974).
As long as intrinsic growth potential is allowed its full expression through an adequate intrauterine environment, the fetus will grow normally. Abnormal growth will occur if growth:
- a) potential is affected, as with anomalies arising at conception (chromosomal abnormalities) or during early intrauterine development (congenital infections and malformation),or
- b) support becomes a limiting factor, as for example, in the syndrome of placental insufficiency (Davies, 1981).
Fetal growth is chiefly influenced by fetal, placental, and maternal factors. In humans 40% of variation-in the birth weight is due to genetic factors while rest is due to environmental factors (Ghai et aI, 2009). Out of several bio-social, physical, endocrinal and nutritional determinants of birth weight, maternal influences in the literature have been documented as most important (Bhatia and Singh, 1988). However none of these aspects of intrauterine growth can be considered in isolation hence, for obvious reason an understanding of all the factors affecting prenatal growth becomes imperative.
- Genetic Potential: The somatic traits of the parents are usually transmitted to their offspring. Thus, tall parents often have tall children, while those born to short. statured parents possess shorter heights, and so on.
- Gender: The male fetuses weigh heavier than females (Babson et al., 1970), the average difference at term being 150-200 gem). This differential in weight growth due to gender of the fetus has been attributed to its genetic make up and to the effect of sex hormones (Ounsted and Ounsted, 1970) as concentration of the testosterone was found to be higher in male fetuses particularly during late first and early second trimester (Warne et al., 1977).
- Race: A compilation of mean birth weights of infants belonging to various ethnic groups by Roberts (1969) has shown that these can differ by as much as 700 gem) amongst ethnic groups. However, if environmental and other factors are controlled this difference may disappear (FAOIWHO Ad-hoc Expert-Committee, 1973).
- Fetal Hormones: Thyroxine and insulin play an important role in regulation of tissue accretion and its differentiation in the fetus. These two hormones are needed for normal growth and development. Glucocorticoids also play an important role, primarily towards the end of gestation. They influence the prepartum maturational events of organs such as liver, lungs and gastrointestinal tract in preparation for extrauterine survival. Growth hormone though present in fetus is not known to influence its growth.
- Fetal Growth Factors: In addition to fetal hormones, several growth factors have been identified in the fetal tissues. These are synthesized locally and often act by autocrine and paracrine mechanisms. Besides, influencing other aspects of fetal growth their prime action is on cell division. These factors have both growth promoting (e.g. Insulin like growth factor (lGF)-I and IGF-II which include epidermal growth factor (EGF), transforming growth factor (TGF-a) etc.) or inhibitory factors” . like TGF-/3, mullerian inhibitory substance, inhibinlactivin family of proteins.
- Placental Factors: Fetal growth depends on the structural and functional integrity and capability of the placenta. The weight of the placenta increases with advancement of gestational age to cater to increased needs of the fetus growing, in utero. It has been observed that at term, in most species fetal weight correlates well with placental weight at term and for efficient functioning of the placenta several positive transformations in its structure occur. These positive changes increase transportation of nutrients across the placenta, to fulfill needs of growing fetus.
- Maternal Factors: Besides, mother’s own growth during her fetal and post-natal period, nutritional status and nutrient intake at the time of conception as well as during pregnancy, have important life long health related implications for her children. Teenage or advanced age, high parity and anemia have been known to adversely affect intrauterine growth. The use-of drugs and alcohol by mother, particularly during pregnancy also retard fetal growth. Obstetric complications such as pregnancy induced hypertension, pre-eclampsia and multiple pregnancies’ also result into fetal growth restriction. Both pre-exisiting chronic systemic disorders (chronic renal failure, congestive heart failure) and acquired infections (rubella, syphilis, hepatitis B, HIV etc.) also influence fetal growth.
- Multiple Gestation(s): The pattern of intrauterine growth in twins is similar to that of singleton babies until early part of the third trimester thereafter, the rate of growth of twin’s decreases (Bhargava et aI., 1983) in magnitude.
- Maternal Age: The prevalence of small for gestational age (SGA) and low birth weight babies was found to-be more in mothers below 20 years of age (Ghosh et al., 1977; Khatua et al., 1979). Though some researchers have noticed that the mean birth weight increases with advancing maternal age, yet Datta Banik’ (1978) have reported that after the age of 30 years this relationship is reversed. Ghosh et al. (1977) showed that the mean birth weight increased up to 34 years of maternal age and thereafter, it seemed to get stabilized.
- Parity: The birth weight of both male and female babies tends to increase with increasing maternal parity (O’Sullivan et aI., 1965; Thomson et aI., 1968). In general, mean birth weight increases and number of low birth weight babies decreases till 4thand 5thparity, after that a reversal of trend was observed (Kloosterman et aI., 1970; Ghosh et aI., 1977; Thomson et aI., 1968; Billewicz and Thomson, 1974;). Dougherty and Jones (1982) observed that the babies born to multiparous mothers weighed 104 g more than those to primiparous mothers. The lower birth weight in babies born to primiparous women have been attributed to their poor nutrition (O’sullivan et aI., 1965) and greater metabolic needs (Khatua et aI., 1979). .
- Physical Activity’ and Rest during Pregnancy: Several researchers have opined that heavy work done by mothers during’ the last trimester of pregnancy may be harmful to fetus (Tafari et aI., 1980; Illsley et aI., 1953; Naeye and Peters, 1982; Mamello and Laumon, 1984) as it diminishes uteroplacental perfusion, resulting in reduction in both blood volume, nutrients and oxygen supply to growing fetus (Morris et aI., 1956). The loss of water and electrolytes associated with hard work in hot climate also has deleterious effects (Hytten, 1948) on fetal growth. A 7.5 fold decrease in rate of low birth weight girls was noticed when the duration of maternal rest was more than 21 days (Manshande, 1987). While studying effect of caloric intake Tafari et aI., (1~80) reported that the, mothers of full term infants who had caloric intake less than 70 per cent of recommended standards, had a mean birth weight of 3060±355g(m) as ‘compared to 3270±368g(m) of the newborns of less physically active mothers having similar low caloric intake.
- Smoking and Tobacco Chewing: Maternal smoking is known to have detrimental effect on growth of fetus. Hardy and Mellits (1972) have shown that when more than 10 cigarettes per day are consumed by the mother, fetal growth gets retarded. Both tobacco smoking (Miller and Hassanein, 1973; Davies et aI., 1976) and tobacco chewing (Krishna, 1978; Verma et aI., 1948) during pregnancy have been reported to influence intrauterine growth adversely.