Mortality and morbidity
The shorter the term of pregnancy, the greater the risks of mortality and morbidity for the baby primarily due to the related prematurity. Preterm-premature babies have an increased risk of death in the first year of life (infant mortality), with most of that occurring in the first month of life (neonatal mortality). Worldwide, prematurity accounts for 10% of neonatal mortality, or around 500,000 deaths per year.[63] In the U.S. where many infections and other causes of neonatal death have been markedly reduced, prematurity is the leading cause of neonatal mortality at 25%.[64] Prematurely born infants are also at greater risk for having subsequent serious chronic health problems as discussed below.
The earliest gestational age at which the infant has at least a 50% chance of survival is referred to as the limit of viability. As NICU care has improved over the last 40 years, viability has reduced to approximately 24 weeks,[65][66] although rare survivors have been documented as early as 21 weeks.[2] This date is controversial, as gestation in the case reported was measured from the known date of conception (by IVF) rather than, as usual, the date of the mother's last menstrual period, making gestation appear two weeks less than if calculated by the conventional method in this case.[67] As risk of brain damage and developmental delay is significant at that threshold even if the infant survives, there are ethical controversies over the aggressiveness of the care rendered to such infants. The limit of viability has also become a factor in the abortion debate[according to whom?].
Specific risks for the preterm neonate
Preterm infants usually show physical signs of prematurity in reverse proportion to the gestational age. As a result they are at risk for numerous medical problems affecting different organ systems.
Neurological problems include apnea of prematurity, hypoxic-ischemic encephalopathy (HIE), retinopathy of prematurity (ROP), developmental disability, transient hyperammonemia of the newborn, cerebral palsy and intraventricular hemorrhage, the latter affecting 25 percent of babies born preterm, usually before 32 weeks of pregnancy.[68] Mild brain bleeds usually leave no or few lasting complications, but severe bleeds often result in brain damage or even death.[68] Neurodevelopmental problems have been linked to lack of maternal thyroid hormones, at a time when their own thyroid is unable to meet postnatal needs.[69]
Children born preterm are more likely to have white matter brain abnormalities early on causing higher risks of cognitive dysfunction.[70] White matter connectivity between the frontal and posterior brain regions are critical in learning to identify patterns in language.[70] Preterm children are at a greater risk for having poor connectivity between these areas leading to learning disabilities.[71]
Cardiovascular complications may arise from the failure of the ductus arteriosus to close after birth: patent ductus arteriosus (PDA).
Respiratory problems are common, specifically the respiratory distress syndrome (RDS or IRDS) (previously called hyaline membrane disease). Another problem can be chronic lung disease (previously called bronchopulmonary dysplasia or BPD).
Gastrointestinal and metabolic issues can arise from hypoglycemia, feeding difficulties, rickets of prematurity, hypocalcemia, inguinal hernia, and necrotizing enterocolitis (NEC).
Hematologic complications include anemia of prematurity, thrombocytopenia, and hyperbilirubinemia (jaundice) that can lead to kernicterus.
Infection, including sepsis, pneumonia, and urinary tract infection [3]
A study of 241 on children born between 22 and 25 weeks who were currently at school age found that 46 percent had severe or moderate disabilities such as cerebral palsy, vision or hearing loss and learning problems. 34 percent were mildly disabled and 20 percent had no disabilities, while 12 percent had disabling cerebral palsy.[72][73]