The duration of a normal pregnancy is around 38 to 42 weeks of gestation, and full-term infants weigh between 2,500 and 4,000 g at birth.⁸ The definition of a preterm baby is one born alive at less than 37 weeks,⁸ with ‘very preterm’ infants born at less than 32 weeks of pregnancy and ‘extremely preterm’ infants born at or before 28 weeks.⁹ Since gestational age is only accurate if based on an early first trimester scan, birth weight is also used to classify preterm babies into low birth weight (< 2,500 g; generally equivalent to all preterm babies), very low birth weight (< 1,500 g; generally equivalent to < 31 weeks’ gestation) and extremely low birth weight (< 1,000 g; generally equivalent to < 27 weeks’ gestation).⁹ "However, some more mature babies may be of low birth weight for their gestational age, so these correlations between birth weight and gestational age are not always accurate," said Prof. Marlow.

The factors that affect the premature baby after birth can be classified into two major groups: injuries (particularly to the brain, resulting in problems such as cerebral palsy) and developmental issues (Figure 1).¹⁰ “Being born early means that the baby’s development tends to slow down, with a range of adverse outcomes. In the lung, infection and the use of mechanical ventilation may lead to a loss of lung function. But being born early also leads to an arrest in the development of the alveoli and a reduced number of alveoli at full term, which exacerbates these injuries. Similar changes can be seen in vascular function, sensory function, and in other organ systems,” said Prof. Marlow. Premature infants are prone to a wide range of complications, including pneumonia, infections, necrotising enterocolitis, ROP, and many others.¹¹

Advances in obstetric practice; newborn thermal and nutritional care; management of antibiotics; infections and respiratory complications; and improvements in neonatal intensive care over the past century have led to a reduction in neonatal mortality rates in the United States, England, and Wales from 40 deaths per 1,000 live births in 1900 to fewer than 5 in 2010.¹² Survival rates for extremely preterm babies have steadily and substantially increased, even over the past 20 years (Figure 2).^13-15 However, while the proportion of babies surviving has increased, the rate of disability in the overall population has remained relatively unchanged (Figure 3).¹⁶ “We have now gone from trying to save more lives to looking at the quality of those lives that we’re saving,” said Prof. Marlow. “We also have to think about our care with respect to what happens after the neonatal period as the child grows up, because we know that this has a strong relationship to the perinatal period.” A direct relationship has been noted between gestational age at birth and the risk of special educational needs later in childhood.¹⁷ Other outcomes of prematurity that can continue to impact individuals through to adulthood include neurocognitive issues (e.g. educational and behavioral problems), respiratory problems (e.g. asthma), and ocular problems (e.g. strabismus, low visual acuity, and myopia).^5,9

ROP is an important morbidity associated with prematurity, but its prevention is challenging. Strategies include the use of antenatal steroids to reduce the rate of respiratory disease and the need for oxygen, as well as ensuring that oxygen saturation targets are maintained.¹⁸ However, these approaches are not necessarily simple. The Neonatal Oxygenation Prospective Meta-analysis (NeOProM) collaboration, which investigated the use of different oxygen saturation targets in extremely preterm infants, reported that lower oxygen saturation targets were associated with a higher risk of death and necrotizing enterocolitis, but a lower risk of ROP.¹⁹ “We’ve known since the late 1940s about the danger of giving excess amounts of oxygen to premature babies, but this must be balanced against the risk of giving insufficient oxygen and causing excess mortality. For the neonatologist, managing oxygen is a very fine line between reducing untreatable outcomes and causing ROP,” said Prof. Marlow.

It is therefore very important that strong protocols are in place to monitor and minimize the use of oxygen, as well as those for identifying babies requiring screening. United Kingdom guidelines recommend screening for babies of birthweight ≤ 1,500 g or < 32 weeks of gestation at birth, with screening at 2-week intervals from 30 to 31 weeks postmenstrual age or 4 to 5 weeks of age, to be increased in frequency based on the presence of signs of ROP.²⁰ “All of this demands close liaison between the neonatologist and the ophthalmic team. Although the baby remains in the neonatal unit, the responsibility for screening and carrying out that screening sits with the ophthalmologist,” said Prof. Marlow.