The first 48 hours
Two indicators - the duration of pregnancy and weight at birth - are used to define how premature a baby is, paediatrician Barbara Lam Cheung-cheung says.
Infants born at under 32 weeks (full term is 37 weeks) and weighing less than 1,500 grams are considered severely premature. Those under 28 weeks and less than 1,000 grams are considered more acutely premature.
In either case, it means a marathon struggle to overcome critical health problems because the vital organs are far from developed. Even if the struggle succeeds, the damage may still leave the child with severe disabilities. So doctors and parents of extremely premature babies face difficult choices about where to draw the line on treatment.
Dr Ellis Hon Kam-lun, a professor of paediatrics at Chinese University, says most babies born after less than 24 weeks of gestation don't survive.
'The first 48 hours are the most critical,' he says. 'We will administer treatment to boost body functions and treat problems that may arise.'
It is legal to end a pregnancy within the first 24 weeks, so some view decisions to stop treatment as a form of abortion, but doctors say this is not the case.
'It's not that we give up on those born at 23 weeks and six days and exhaust all means to save those at 24 weeks,' Lam says. 'We make decisions based on the baby's condition. We don't want to shift the burden to parents who do not have medical expertise.
'But after more than 20 years working in the neonatal intensive care unit, I have seen all kinds of parents.
'Some give birth after 26 weeks of gestation and, fearing that the baby will develop problems later in life, ask us to view it as an abortion.
'That's not acceptable, as the baby is not given a chance in life. If there's a high likelihood that the baby will develop disabilities after treatment, we will accept non-treatment. Babies weighing 300 or 400 grams at birth have a low survival rate.
'Most parents I have encountered cling to the babies even though they have developed all kinds of serious complications. They want us to do everything possible to save them.'