Anatomy of the Neonates

The Anatomy of Full Term Neonate
Gray's Anatomy 39th edition. Elsevier. 2008

Figure 1 Timetable of development of the body systems. The development of individual systems can be seen progressing from left to right. Embryonic stages, weeks of development and embryo length are shown. Embryonic stages are associated with external and internal morphological features rather than embryonic length. To identify the systems and organs at risk at any time of development, follow a vertical progression from top to bottom. (Click Image to Enlarge)Figure 2 The two timescales used to depict human development. Embryonic development, in the upper scale, is counted from fertilization (or from ovulation, i.e. in postovulatory days; see O'Rahilly & Muller 1987). Throughout this book, times given for development are based on this scale. The clinical estimation of pregnancy is counted from the last menstrual period and is shown on the lower scale; throughout this book, fetal ages relating to neonatal anatomy and growth will have been derived from the lower scale. Note that there is a 2 week discrepancy between these scales. The perinatal period is very long, because it includes all preterm deliveries.

Immediately after parturition the fetus, once it has been exposed to the environment external to the maternal uterus, becomes a neonate. In Western societies, technological advances have enabled successful management of preterm infants, many at ages that were considered non-viable a decade or two previously. Now, the study of neonatology very much overlaps the later stages of fetal development. Preterm infants, although obviously past organogenetic processes, are still engaged in maturational processes with local interactions and pattern formation driving development at local and body-system levels. The sudden release of such fetuses into a gaseous environment, of variable temperature, with full gravity and a range of microorganisms promotes the rapid maturation of some systems and the compensatory growth, in terms of effect of gravity or enteral feeding or exposure to microorganisms, of others. To understand this multitude of mechanisms operating within a newly delivered fetus, as much information as possible concerning normal embryological and fetal development is required.

Details of the relative positions of the viscera and the skeleton in a full term neonate are shown in Figs 3A, B, C; 4. The newborn infant is not a miniature adult, and extremely preterm infants are not the same as full-term infants. Thus, just as there are immense differences in the relations of some structures between the full-term neonate, child and adult, so there are also major differences between the 20 week gestation fetus and the 40 week fetus, just before birth. The study of fetal anatomy at 20, 25, 30 and 35 weeks is vital for the investigative and life-saving procedures carried out on preterm infants today.

Figure 11.4 Topographical representation of the anatomy of a full-term neonate. The surface markings of all organs are shown, with some coloured and others only in outline. The female genital tract is shown on the right of the body in C, with the male tract on the left.Figure 4. The extent of the ossified skeleton in the full term neonate. Note the derivation of the parts of the skeleton: the skull is derived from paraxial mesenchyme and neural crest mesenchyme; the axial skeleton, vertebrae and ribs are derived from paraxial mesenchyme; the skeletal elements in the limbs are derived from the somatopleuric mesenchyme, which forms the limb buds.

Details of the relative positions of the viscera and the skeleton in a full term neonate are shown in Figs 3 and 4. The newborn infant is not a miniature adult, and extremely preterm infants are not the same as full-term infants. Thus, just as there are immense differences in the relations of some structures between the full-term neonate, child and adult, so there are also major differences between the 20 week gestation fetus and the 40 week fetus, just before birth. The study of fetal anatomy at 20, 25, 30 and 35 weeks is vital for the investigative and life-saving procedures carried out on preterm infants today.

Neonatal measurements and period of time in utero
The 10th to 90th centile ranges for length of full-term neonates are c.48 cm to c.53 cm Length of the newborn is measured from crown to heel. In utero, length has been estimated either from crown-rump length, i.e. the greatest distance between the vertex of the skull and the ischial tuberosities, with the fetus in the natural curved position, or from the greatest length exclusive of the lower limbs. Greatest length is independent of fixed points and thus much simpler to measure. It is generally taken to be the sitting height in postnatal life. This measurement is recommended by O'Rahilly and Muller (2000) as the standard in ultrasound examination. The 10th to 90th centile ranges for weight of the full-term infant at parturition ranges are c.2700 g to c.3800 g , the average being 3400 g; 75-80% of this weight is body water and a further 15-28% is composed of adipose tissue. After birth, there is a general decrease in the total body water, but a relative increase in intracellular fluid. Normally, the newborn loses c.10% of the birth weight by 3-4 days postnatally, because of loss of excess extracellular fluid and meconium. By 1 year, total body water makes up 60% of the body weight. Two populations of neonates are at particular risk, namely those who are preterm, and those who are small-for-dates, some of whom have suffered 'intrauterine growth restriction'.
Low birth weight has been defined as less than 2500 g, very low birth weight as less than 1500 g, and extremely low birth weight as less than 1000 g. Infants may weigh less than 2500 g but not be premature by gestational age. Measurement of the range of weights fetuses may attain before birth has led to the production of weight charts, which allow babies to be described according to how appropriate their birth weight is for their gestational age, e.g. small for gestational age, appropriate for gestational age and large for gestational age. Small for gestational age infants, also termed 'small-for-dates', are often the outcome of intrauterine growth retardation. The causes of growth restriction are many and various and beyond the scope of this text.
For both premature and growth-retarded infants, an assessment of gestational age, which correlates closely with the stage of maturity, is desirable. Gestational age at birth is predicted by its proximity to the estimated date of delivery and the results of ultrasonographic examinations during pregnancy. It is currently assessed in the neonate by evaluation of a number of external physical and neuromuscular signs. Scoring of these signs results in a cumulative score of maturity that is usually within ± 2 weeks of the true age of the infant. The scoring scheme has been devised and improved over many years. For an account of methods of assessing gestational age in neonates, consult Gandy (1992).