There's no single rule that determines how long your premature baby will have to stay in the neonatal intensive care unit (NICU). Preemies have several milestones to meet before they are allowed to go home with their parents.

Often, a rule of the thumb is to send the baby away on the actual date that he was due to be born on but this varies. If the baby needs some more monitoring, the date of discharge may be delayed. 

Your preemie’s care, length of stay in the NICU, and chances of complications will depend on the category of preemie he or she is. Doctors classify preemies as late, moderate and very preterm babies.

Late preterm preemie (born at about 34 to 37 weeks gestation)

Babies born near term are less likely to have severe breathing problems this is because the lung maturing surfactant develops when the baby is inside, towards the last weeks), but may still have blood sugar problems as well as a slightly elevated risk of infection. They are more likely than full-term babies to have elevated jaundice levels requiring at least brief phototherapy. These preterm babies may also have some difficulty feeding, but the vast majority of near-term preemies have a short stay in the NICU with few complications.

Moderately preterm and Very preterm (born at about 28 to 34 weeks gestation)

Many babies born before 31 weeks will have breathing difficulties and will likely need to be placed on a respirator for a while. And since babies born this early didn’t get the immunity protection boost from mom during the last trimester, they are more prone to infections in general as well as hypoglycemia (low blood sugar) and hypothermia (they have a hard time staying warm). Moderate preemies usually won’t be able to start with breastfeeding or bottle-feedings right away, and they may also encounter feeding problems when they are ready for nipple feedings.

Extreme preemies (born before 28 weeks gestation)

These tiniest of babies are at highest risk of breathing difficulties because their lungs are so immature and not yet ready to function independently. Extreme preemies are also at highest risk of complications of prematurity, infections, hypoglycemia, and hypothermia. 

Premature babies aren’t categorised only by gestational age. A preterm baby’s health and course of treatment in the NICU also has a lot to do with size at birth usually the smaller the baby, the greater the chances for a longer hospital stay, and possibly for complications: Micro preemies are the smallest and youngest preemie babies born weighing less than 1 pound, 12 ounces (800 grams) or before 26 weeks gestation.

Remember: the main checks are around As and Bs

"As and Bs" stand for apnea and bradycardia, which are considered the main markers of prematurity. Apnea refers to periods in which a baby stops breathing for more than 20 seconds. The lack of breathing causes falling oxygen saturation levels, which in turn cause the baby's heart rate to lower (bradycardia). These responses are likely caused by the immaturity of the nervous system.

What will the doctor test for before discharging the baby?

Once the baby has achieved his weight milestones, some more screening tests will likely be required before you can take your baby home. These might include a hearing test (either the otoacoustic emission or the automated auditory brainstem response tests), testing for hyperbilirubinemia, and screening for heart disease.