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.
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.
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, 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 G”rowth 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 p.ccur. 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 DattaBanik’ (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 4th and 5th parity, 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.