There are reams of paper written about normal pregnancy, parturition and early postnatal life. I will not try to replicate all that information but I will summarise those factors that are important when considering death of the newborn at or about the time of parturition, the perinatal period.
The fetus is in an aquatic environment. There is no air in the lungs only pulmonary fluid. Amniotic fluid (which contains keratin squames) is not breathed but it is swallowed and is therefore in the abomasum only. Meconium, containing bile pigment, desquamated epithelial cells and squames of keratin from amniotic fluid that was swallowed, is in the colon and rectum. Hooves have eponychia. The lamb is wet and 'slimy'. Loss at this stage is called an abortion or stillbirth depending on whether the lamb is independently viable (stillbirth) or not (abortion).
The fetus lives in an aquatic environment within the amniotic sac. This amniotic fluid provides a warm, protective and stable environment. The fetus drinks and swallows amniotic fluid which is then held in the stomach and gradually passes through the intestine. Stomach contents of the fetus therefore is a sample of amniotic fluid which is available for evaluation.The amniotic fluid normally contains keratin squames from the fetal epidermis. These keratin squames can be observed in the fluid in the stomach and also throughout the intestinal tract including the meconium. This is the basis of the simple test for a complete gastrointestinal tract – the presence of keratin squames within the rectum.
The umbilical cord is the connection between the fetus and the placenta, and therefore the gaseous exchange, nutrient and waste exchange with the mother. this umbilical cord contains two arteries, one vein and in the intra-amniotic component, the urachus. The arteries connect to the aorta of the fetus and a course each side of the bladder to the umbilicus. The vein with its nutrients and oxygen rich blood courses through the umbilicus up towards and into the liver and connects to the vena cava through the ductus venosis.
Development of the respiratory system
The development of the respiratory system and in particular the lungs progresses through five stages of maturation: embryonic, pseudo-glandular, canalicular, saccular and alveolar. The timing of these various stages is different between species. Humans are born in the saccular stage and maturation and differentiation to the alveolar stage occurs at 12 to 24 months of age. Lambs are born in the alveolar stage but development continues up to about three months of age with the gradual formation of Clara cells occurring in the first week and further development of ciliated epithelial cells and microvascularisation to about three months of age. While en utero, there is no air in the lungs but pulmonary fluid is produced and makes its way through the trachea into the pharynx where it is swallowed and joins amniotic fluid. While there is considerable debate about whether a fetus normally has "respiratory movement", it is generally believed that amniotic fluid does not enter the larynx or trachea. It makes no sense to me to have keratin squames within the lung where they can incite a severe inflammatory reaction.
Weight gain by the fetus is particularly prominent in the latter stages of parturition and each species and breed has an optimal fetal body weight that is often determined in the latter stages of development. It is influenced strongly by maternal nutrition. So-called over nutrition results in a larger than normal fetus, undernutrition results in a smaller than normal fetus.
The hooves or claws of foetuses have a gelatinous outer covering called an eponychium. This presumably prevents tearing and injury to the fetal membranes and the maternal tissues.
Fetuses in utero are maintained in a sleep like unconscious state, especially herbivores that are neurologically mature at birth. This unconscious state is maintained by certain substances and hormones as well as the warmth, buoyancy and cushioning of placenta and fluids.
Fetuses move in utero, and this movement is essential for normal development of muscle, joints and bone so that the fetus is mobile after birth. This is particularly important in those species where the fetus must essentially be immediately independently mobile after birth.
Many textbooks go into great detail about the stages of parturition, fetal movements and arrangements, maternal responses and relaxation of the genital tract, and contractions of smooth muscle of the myometrium. Correction of dystocia, the timing of birth and all of the physiological changes that occur are important during this period. Not only does the fetus have to pass out of the mother, but it has to change from being an aquatic animal into a terrestrial one. Fetuses are born outside of their membranes – passage of the fetal membranes is a separate event to the passage of the fetus. In some species the foetuses and membranes are all past at the one time and it is the mother (particularly of carnivores) who is responsible for breaking the fetal membranes and stimulating the fetus.
Fetal stimulation arouses them from their unconscious state. Strong intrapartum tactile stimulation and hypoxia and a cold terrestrial environment induces arousal. Continued pressure, particularly around the chest after birth, will maintain their somnolent state.
Once it is born, the fetus must begin to breath air. Hypoxia stimulates respiratory movements and breathing normally begins after birth and with stimulation of the fetus. Maternal nudging, licking and other stimulations cause the fetus to gasp for air. The first breath is the hardest as the lungs must be expanded and the surface tension of the pulmonary fluid must be overcome. Eventually and in a very short time, the normal pulmonary tissue is inflated with air and normal gaseous transfer can occur.
Reactive oxygen species
When a fetus begins to breathe, it is exposed to an oxygen rich environment. Tthe foetus intrautero is in a hypoxic environment of PO2 20–25 mmHg oxygen tension. Extrauterine normoxic environment is 100 mmHg PO2.
Alveolar distension activates endothelial nitric oxide synthetase allowing generation of nitric oxide. There is concurrent generation of reactive oxygen species. In human babies diseases such as bronchopulmonary dysplasia (BPD)/chronic lung disease (CLD), periventricular leukomalacia (PVL), neonatal encephalopathy, retinopathy of prematurity, neonatal renal dysfunctions and necrotizing enterocolitis (NEC) are attributed to excessive reactive oxygen species. In domestic mammals, cardiovascular and pulmonary dysfunctions, acidosis, gastrointestinal anatomical and functional alterations, reduction in intestinal absorption and failure of passive transfer of immunoglobulins are attributed to hypoxic/hyperoxic damage.
Either during the process of parturition or soon afterward, the umbilical cord breaks. The cord is pulled and stretchs and the ends constrict into a "V"or tapered shape. Haemostasis occurs quickly. It is common for the mother to then consume the fetal membranes. Within hours of birth the remnants of the umbilical cord dry and become mummified. It can take several days for it to disappear completely.
The newborn begins to move almost immediately and those of herbivorous species attempts to stand. During this initial phase the eponychia tear off the hooves. The exception is when there is soft bedding material.
The newborn, after accommodating for all of the vascular physiological changes and changing respiration to a lung and air based system, must find the mammary gland to consume colostrum and to acquire nutrients. This milk enters the glandular stomach and the gastrocolic reflex begins to stimulate passage of the meconium. Milk passes into the duodenum and jejunum where it is absorbed. during the first hours after birth colostral antibodies are absorbed through the enterocytes and enters the blood stream of the fetus. Lipids are absorbed and lipid rich chyle is visible in the lymphatics.
Newborns must thermoregulate as the terrestrial environment is often at a much lower temperature than normal body temperature. Brown fat acts as a ready source of energy if there is any delay in milk uptake or if there is increased energy requirement. In extreme conditions of cold and wet, this supply can last only several hours, otherwise it is present for many days. The length of time it is available is also dependent on the quantity of brown fat that was produced prior to parturition.
Normal newborns are vigourous after birth and move around quickly. Mothers are very protective of their offspring and are aware of their presence.
Diesch TJ1, Mellor DJ. (2013) Birth transitions: pathophysiology, the onset of consciousness and possible implications for neonatal maladjustment syndrome in the foal. Equine Vet J 2013; 45: 656-660.
Martineau HM, Dagleish MP, Cousens C, Underwood C, Forbes V, Palmarini M, Griffiths DJ ( 2012) Cellular differentiation and proliferation in the ovine lung during gestation and early postnatal development. J Comp Path 2012, 1-13Mutinati M1, Pantaleo M, Roncetti M, Piccinno M, Rizzo A, Sciorsci RL. Oxidative stress in neonatology: a review. Reprod Domest Anim. 2014; 49: 7-16