The transition from foetal to neonatal life is accompanied by physiological, anatomical and biochemical changes. All systems of the body undergo transformation including respiratory, digestive, cardiovascular, renal, hematologic, hepatic, neurologic, musculoskeletal and integumentary. This period is as stressful for the child as it is for the mother although most newborns are able to successfully make this transition.
In most cases, labour occurs spontaneously once the foetus reached optimal maturation. This transition from foetal to neonatal life is accompanied by physiological, anatomical and biochemical changes some of which take place in the days leading up to labour. While these adaptations enable a healthy transition the period of birth is as stressful for the foetus as it is for the mother. Every major system within the neonate's body – lungs, heart, nervous system, gastrointestinal tract, kidney, and liver (see Table 331_03-01) undergoes transformational changes. With birth, the infant's systems are now responsible for gas exchange, coordination of enteral intake of food, digestion and absorption of nutrients, waste excretion via urine and stool, and temperature homeostasis (Graves & Haley, 2013).
Table 331_03-01 Transitional events and observable physical parameters in the neonate adapted from van Woudenberg et al., 2012.
At birth, clamping of the umbilical cord terminates blood flow through the ductus venosus. The clamping changes blood pressure and oxygen saturation within the neonatal circulatory system. These pressure changes in the circulatory system result in the closing of the foramen ovale. The neonatal lungs expand with the first breath and begin to take in oxygen. The expansion of the lungs increases the blood flow to the lungs and reverses the direction of the flow through the ductus arteriosus. The ductus arteriosus closes within the first 24 hours after birth (van Woudenberg et al., 2012, Graves & Haley, 2013).
The nervous system is the first to develop and the last to mature. The foetal nervous system matures progressively until at birth the neonate is able to suck, swallow and manage autonomic functions e.g. breathing, maintaining temperate and heart rate. At birth, the foetus is suddenly exposed to a multitude of stimuli and responds with many erratic movements and excessive jitteriness. Most neonates go through "the first period of reactivity" for about 15 ‑ 30 minutes after birth. This is the time when many neonates are very active, the heart rate is variable and high because of crying and present with rapid irregular respirations with mild grunting. Neonates may smack their lips, grimace, move the head side to side or open and close their eyes. Newborn infants may also suddenly become quiet after a period of sudden cries and may sleep for up to four hours. They also display a "second period of reactivity" once awake and become responsive with rapid respirations for a short period of time; their skin colour may also change and flush. Most infants stabilise after the second period of reactivity because by then recovery from the birth process is complete (van Woudenberg et al., 2012).
The gastrointestinal system takes over the process of digestion, absorption and excretion from the placenta. Peristalsis begins as soon as the infant swallows air and the bowels inflate. After birth, the initiation of enteral feeding stimulates development of the gastrointestinal system. Hence, breastfeeding should ideally start within the first hour after birth (WHO, 2009). When close to the breast, the neonate's rooting, suckling, and swallowing reflexes are activated. After identifying the nipple, the infant needs to find ways for a coordinated latch, suckle, swallow, and breathe and make efforts to draw milk from the breast. Colostrum, the first milk produced by the mother after delivery, is a rich source of immune reagents, growth factors and bioactive compounds. This milk stage is easily digested and facilitates passage of meconium. Colostrum also stimulates the maturation of the gastrointestinal tract by signalling the secretion of gastrointestinal hormones and saliva. Therefore, feeding colostrum not only provides immunity but is also plays an important role in preparing the gut for more nutrient-rich feeds (WHO, 2009). Most healthy term neonates will pass meconium within 12 ‑ 24 hours after birth and almost all will pass it within the first 48 hours (van Woudenberg et al., 2012).
Kidneys take over the function of excretion from the placenta and maintenance of fluid and electrolyte balance. Most healthy term infants will void urine within 24 hours after birth. This diuresis causes a notable decrease of extracellular fluid from 40% at birth to 30%, and accounts for much of the weight loss occurring within the first five days after birth (Graves & Haley, 2013).
The neonatal liver starts to break down glycogen and fat to provide energy. After delivery, catecholamines released during labour facilitate the use of glycogen stores for energy and growth. While birth triggers some liver enzymes others need more stimuli for their activation. This includes the bilirubin conjugating enzyme that only gets activated by increased levels of bilirubin in the blood after birth. This causes jaundice of the newborn. Bilirubin concentrations continue to increase for a few days after birth and start to decrease once the liver begins to conjugate bile more efficiently (van Woudenberg et al., 2012).
A healthy term newborn has approximately 30 g of brown adipose tissue. Brown adipose tissue facilitates rapid metabolism, heat production and outward transfer of heat. The foetal sympathetic nervous system, activated by labour, is further stimulated at birth by thermal receptors in the hypothalamus and cold receptors in the skin and spinal cord. The sympathetic nervous system then releases norepinephrine from the brown adipose tissue to produce heat via non-shivering thermogenesis. Preterm infants have lesser amounts of brown adipose tissue, which increases their risk of hypothermia (Graves & Haley, 2013).
Graves BW, Haley MM. Newborn transition. Journal of midwifery & women’s health 2013; 58(6):662–70. at: https://pubmed.ncbi.nlm.nih.gov/24103003/
van Woudenberg CD, Wills CA, Rubarth LB. Newborn transition to extrauterine life. Neonatal network : NN 2012; 31(5):317–22. at: https://connect.springerpub.com/content/sgrnn/31/5/317
WHO. Infant and Young Child Feeding. Model Chapter for textbooks for medical students and allied health professionals. Geneva, Switzerland: World Health Organization (WHO) press; 2009. at: https://www.who.int/maternal_child_adolescent/documents/9789241597494/en/back