Outcomes of babies born at 34, 35, and 36 weeks (Late Preterm Babies)

Late preterm baby

Late preterm infants constitute a unique group of babies who are born prematurely. Even though they are born quite close to their due date, they do not perform as well as term babies. Late preterm infants (LPIs) may develop some medical problems after birth requiring hospitalization for more than just a few more days. After discharge home, they are at risk of a more frequent need for readmission. Finally, during their childhood, they suffer more often than term babies from various medical conditions and developmental problems.

In my article, I will talk about the definition of late preterm babies and explain how they differ from newborns born at term.

Definition of Late Preterm Babies

We assign the term Late Preterm Infants or Babies to any infant born at a gestational age between 34 weeks 0 days and 36 weeks and 6 days. In other words, one can also say that those are the babies who were born prematurely and are not younger than 34 weeks. Finally, we can also describe them as premature babies older than 33 weeks and six days (Reference).

Please remember that any baby born at or after 37 weeks will be considered full term.

The other terms circulating in use pertaining to Late Preterm Babies are: “near term,” “marginally preterm,” “minimally preterm,” or “mildly preterm.”

In my opinion, “Late Preterm Babies or Infants” is the most accurate term because it emphasizes that these babies are born early. Still, they will probably perform better than more severely premature babies.

How common is it to deliver Late Preterm Baby?

In 2016, we had in the USA 3.9 million births. 7.09% (276 510) were late premature infants. It is also worth knowing that late premature babies constitute over 70% of all premature births. To compare, during that same year, 25% of babies were born at 37-38 weeks and 58% at 39-40 weeks of gestational age.

Physical measurements of late preterm babies

The first question from parents I am almost always asked is, “how much does my baby weigh?”.
If your baby has not been born yet and you wonder about the average weight and length for a baby depending on their gestational age at birth, you will find that information below in tables 1 and 2. Table 1 depicts those numbers for girls and Table 2 for boys. It should be no surprise that these two measurements are slightly different between them. Boys tend to be a little heavier and taller at birth than girls (Reference).

Table 1: Body Measurements in Girls

Weeks at birthWeight in kgWeight in lbHeight in cmHeight in inches
342.154 lb 11.8 oz4417.3 inch
352.355 lb 2.8 oz4517.7 inch
362.605 lb 11.7 oz4618.1 inch
372.856 lb 4.5 oz47.518.7 inch
403.407 lb 7.9 oz50.519.9 inch

Table 2: Body Measurements in Boys

Weeks at birthWeight in kgWeight in lbHeight in cmHeight in inch
342.254 lb 15.3 oz45 cm17.7 inch
352.505 lb 8.1 oz46 cm18.1 inch
362.725 lb 15.9 oz47 cm18.5 inch
372.956 lb 8 oz48 cm18.9 inch
403.557 lb 13.2 oz51 cm20 inch

What medical problems may occur after birth in Late Preterm Babies?

Respiratory problems are not very common, but they will be most scary for the families because sometimes we have to place a baby on a ventilator to treat that ailment. 

On the other hand, temperature problems and feeding difficulties are most common but are not that difficult to treat; you just need time. The latter two are mild issues in doctors’ view, but for families may be very pesky due to their duration (usually 1-3 weeks) and inability to take the baby home until these problems get resolved.

Respiratory problems

Organs of the respiratory system grow and mature during fetal life but continue that maturation process for many years after birth. As a result, the lungs and airways of late preterm infants show both anatomical and functional immaturity. Lungs have a lower surface area for gas exchange, decreased ability to absorb amniotic and lung fluid after birth, and decreased amount of surfactant (surfactant is a substance in the lungs that prevents them from collapsing at the end of exhalation). 

Three respiratory conditions occur more often among late preterm babies than full-term newborns. 

1. Respiratory distress syndrome (RDS) is a respiratory disease due to deficiency or malfunction of surfactant in the baby’s lungs (my article on RDS). 

2. Transient Tachypnea of the Newborn (TTN), also called “wet lungs,” – Is a respiratory condition due to an increased amount of fluid within the lungs caused by an inefficient absorption process.

RDS and TTN tend to have similar symptoms, although RDS is usually more severe and may require more aggressive treatments. Signs of respiratory problems are present right after birth or occur within 4-6 hours. Babies may present with increased respiratory rate, increased work of breathing, cyanotic or bluish skin color, and decreased oxygen levels.

Treatment options include observation without any other intervention for very mild cases and more aggressive measures for babies with significant respiratory symptoms. For example, we can provide oxygen to breathe, place a nasal cannula in their nose (flow of air with oxygen), and put them on a CPAP machine to provide extra pressure to keep their lungs open. Finally, the sickest babies will be placed on a breathing machine (also called a ventilator). 

Doctors estimate that late preterm newborns have a 4 to 9 times higher risk of respiratory problems after birth than full-term infants. The most aggressive therapy – ventilator, is needed on average in over 6% of babies born at 34 weeks, 3.6% born at 35 weeks, and 2.3% born at 36 weeks. So again, you can see that babies born earlier are getting more sick and need more extensive treatments. 

3. Apnea – A situation in which a baby “forgets” to breathe for short periods of time. It occurs in newborns due to their brain neurological immaturity and airway obstruction (my article on apnea). 

Apnea may be a “benign” condition due to the baby’s immaturity. However, it may also be a sign of other more serious medical conditions such as infection, bleeding in the brain, and many others. Therefore, doctors may decide to conduct an additional diagnostic workup if apnea occurs.

Once we decide that apnea is only due to the baby’s immaturity, we must observe the baby in the hospital until it resolves. During that time, we keep such patients on a cardiorespiratory monitor. Sometimes, we give them medication called Caffeine (Caffeine stimulates their breathing), and occasionally we have to place them on a nasal cannula or CPAP device. The latter two forms of respiratory support to treat apnea are rarely needed in late preterm babies. 

Temperature regulation problems

Approximately 10% of late preterm newborns have body temperature regulation problems. They get cold, and we need to intervene to keep their body temperature within normal ranges. The average temperature for babies should be similar to the one in adults. 

There are multiple reasons why the babies get cold, and obviously, smaller and more immature babies tend to have that problem more often:

  • the lesser amount of brown fat tissue that is an energy source after birth
  • lower amount of fat tissue under the skin, which normally acts like insulation or a warm coat for babies
  • hormonal immaturity
  • thinner skin
  • an increased ratio of body surface area to body weight which makes baby lose their inner heat more easily

What can we do to prevent or treat low body temperature in babies (the medical term for that is “hypothermia”)?

  1. We should raise the temperature in delivery rooms and operating rooms where babies are born. 
  2. Also, we should warn mothers who like to have cooler temperatures in their rooms after delivery because they feel hot that this may not be such a good idea for their babies. 
  3. Encourage mothers to increase the amount of skin-to-skin contact with their babies, provided their babies are not sick and require treatment with hospitalization in a separate monitored nursery. 
  4. Delay bath and shorten the bath duration; do it several hours after birth or even the following day, once the infant’s body temperature is stable.
  5. If the techniques mentioned above do not work or can not be instituted due to other medical problems, place the baby under a radiant warmer (table-bed with a heat lamp above it) or in an isolette (a plastic box with heated air in it). 

Low glucose (sugar) levels (hypoglycemia)

Glucose has a critical role in our bodies. It is an energy source for our brain, heart, and kidneys. Complicated regulatory mechanisms exist to keep glucose levels within the normal range (stored glycogen, fat, hormones, and enzymes). Not surprisingly, late preterm babies are at higher risk of developing hypoglycemia (the medical term for low sugar or glucose levels in the blood). 

Hypoglycemia is three times more common in late preterm babies than full-term newborns. About 15% of late preterm infants get affected. 

Common clinical symptoms in babies with hypoglycemia include: 

  • Jitteriness
  • Poor feedings
  • Lethargy
  • Poor tone
  • Irritability
  • Apnea
  • Pallor
  • Lower body temperature (hypothermia)
  • Seizures

After the birth of your late preterm baby, nurses will be actively monitoring the baby’s sugar levels for at least 12-24 hrs to ensure that your baby is safe. It will involve a skin prick to get a drop of blood from your baby’s heel. Then, we place it on the laboratory paper and insert it into a bedside machine. The whole procedure takes no more than 2 minutes to get results. 

If blood glucose levels are low, an intervention will be needed. If the situation is not that serious, we may encourage you to breastfeed more often, we can provide glucose gel to the baby to buy us time until your breast milk comes in, or we can supplement feedings with formula. 

Suppose the baby cannot take milk by mouth due to feeding problems or another medical condition. In that case, there are two options: insertion of the feeding tube or starting your baby on intravenous fluids containing glucose. The latter intervention (IV fluids) might still be necessary if all previous attempts failed to keep glucose levels within a safe range. 

After intravenous fluids are started, it usually will take several days until we can stop them. Baby will have to demonstrate first ability to keep proper glucose levels on tube or oral feedings; it is always a process to establish that. 

Difficulties with feedings

Feeding problems are the most commonly cited reason for a delay in the discharge home of late preterm infants. The more premature baby is, the more issues with their feeding. Data show that over 30% of late preterm babies have feeding problems compared to 5%-7% of full-term newborns. 

These issues are present due to various factors:

  • Decreased suck strength and efficacy 
  • Increased sleepiness
  • Difficulties with feeding and breathing coordination
  • Difficulties with suck and swallow reflexes coordination
  • Weakened lower esophageal muscle tone and its immaturity leading to gastroesophageal reflex (excessive spitting or vomiting)
  • Excessive attempts to breastfeed when a mother does not have milk yet, and the baby is getting quickly exhausted

The good news regarding feeding difficulties is that all neurologically healthy babies without any other underlying conditions will eventually achieve full oral feedings. However, it is often a very frustrating problem for the families to deal with. 

Parents look at their baby and see a healthy-looking baby of good size who does not eat well and can not go home. This situation is often very upsetting to them. They worry that there is something wrong with their baby. Sometimes they think that doctors imagine things that do not exist, and they have unnecessary concerns about discharging their baby home. 

Let me give you data regarding the average time for late preterm babies to achieve adequate oral skills to alleviate these worries. The median time to full oral feedings for babies born at 34, 35, and 36 weeks is 12, 3, and 2 days (Reference).

Dealing with feeding difficulties involves supporting the mother’s desire to breastfeed, using IV fluids and gavage (tube) feedings where necessary. If a baby does not gain weight appropriately, we increase the number of calories in breast milk or formula by using fortifiers or richer calories formula. All these steps may upset parents desiring to breastfeed only; however, delaying them does not serve their baby well. Weak and tired out babies from excessive breastfeeding sessions that do not result in consumed calories make babies more sleepy, lose weight and even get dehydrated and more jaundiced. 

Early and prolonged newborn jaundice

Many newborns, including full-term babies, develop jaundice that may last for 1-2 weeks and sometimes requires treatment. If you have never heard the term jaundice, I encourage you to read my article, giving you general knowledge on that topic.

Few important points about jaundice in newborns to know:

  1. A Chemical called bilirubin is a culprit, and we all produce it in our bodies.
  2. Newborn babies produce more bilirubin per body mass compared with adults. Due to their physiologic immaturity, babies have difficulty excreting it from their bodies even if they were born at term. Consequently, levels of bilirubin rise, and their skin turns yellow.
  3. The most common sign of jaundice is yellow skin, but we should not rely on that to assess it; measuring bilirubin levels in the blood is an essential part of the evaluation.
  4. For many babies, jaundice is benign and goes away on its own
  5. We treat jaundice when the bilirubin level rises to potentially unsafe levels. Treatment aims to avoid brain damage from toxic bilirubin levels.
  6. The most crucial treatment is phototherapy, but sometimes we have to use other treatments.
  7. Equally important for premature babies with jaundice is proper hydration with adequate feedings.

Physiologic immaturity found in late preterm babies makes them particularly prone to developing jaundice early and having jaundice for longer. 

Immature enzymes in the liver and relatively poor hydration resulting from feeding difficulties are probably the two most important factors contributing to the fact that jaundice and jaundice requiring treatment are more frequent among late preterm babies than in term newborns. 

We have data that babies born at 35-36 weeks are 13 times more likely to be readmitted to the hospital for treatment of significant jaundice than babies born after 40 weeks of gestation. 

To keep your baby safe, it is good for you to have a conversation with a pediatrician about an evaluation for jaundice before taking your baby home. Also, ask about appropriate follow-up for jaundice after discharge home.

If you are at home with your baby with significant jaundice not recently evaluated by a medical professional, you should treat it as an emergency. You need to call your doctor immediately or go to the emergency room. 

Jaundice is a benign and easily treatable condition for most babies. Still, if it is recognized too late, it can be dangerous, harmful, and even deadly for them (including term babies). 

More frequent infections and more use of antibiotics

We treat late preterm babies with antibiotics more often than full-term babies. There are two reasons for that. 

Firstly, due to the immaturity of their immune system, preterm babies are more prone to infections. The second reason is difficulty determining which late preterm baby has an infection and which one does not. 

We do not have a good diagnostic tool that would allow us to diagnose infection quickly. The most reliable test, called culture, takes many hours to yield results. Sometimes culture returns positive results in 10 hours, but sometimes it will be 48 hours. In the meantime, it is prudent to put the baby on antibiotics whenever we suspect symptoms of possible infection. 

Unfortunately, the infection does not have a unique set of symptoms that would allow us to diagnose it. Medical problems such as low body temperature, low glucose levels, breathing problems, or feeding difficulties can occur in late preterm babies with and without infection. To keep babies safe, we treat with antibiotics many more babies than need it. 

If your baby is receiving antibiotics for the reasons described above, usually we stop antibiotics within 48 hours, provided the baby is doing well clinically, and blood cultures are negative. 

What are mortality rates for late preterm infants?

Most people probably understand that premature babies will have a higher risk of death due to immaturity. The same medical factors and calamities that lead to early delivery may also negatively influence the baby’s well-being. 

In 2013 mortality rate for babies born at 39-40 weeks of gestation was 1.85 per 1000 live births. The same mortality rate for newborns born at 34-36 weeks was 7.2 (Reference).

Neonatal mortality rates will vary a lot depending on the reason for the baby’s early delivery. For example, that rate will be much lower for a population of babies born due to isolated preterm labor in a mother. On the other hand, the mortality rate will be much higher if the reason for premature delivery is an obstetric indication (for example, sudden and severe vaginal bleeding). 

How long late preterm infants may need to stay in the hospital after birth?

The second most common question I hear from parents after they already had asked about birth weight is when their little one can go home. It is essential to them if they know that it will not happen in 1-2 days which would be typical for a normal healthy, full-term baby. 

As I described above, many late preterm babies develop various health issues that need time to resolve. Consequently, those babies need to stay in the nursery for a few more days and sometimes even a few weeks. 

Fortunately, a published study provides us with the average length of stay for different gestational ages at birth. According to the authors, babies born at 34 weeks will need to stay in the hospital for an average of 12.6 days. Those numbers for babies born at 35 and 36 weeks of gestational age are 6.1 and 3.8 days consecutively (Reference). However, you need to understand that statistics provides us only with averages. It means that a value of interest may be much higher or lower in real life.

What is the risk for readmission to the hospital during the neonatal period among late preterm babies?

It has been reported that babies categorized as late preterm infants have the highest readmission rate to hospitals during the neonatal period (first month of life). The readmission rate for newborns born at less than 34 weeks is 3%, for babies 34-36 weeks is 4.4%, and finely for full-term newborns born at more than 37 weeks is 2% (Reference).

It is hard to say why that is. Is it due to cavalier behavior among doctors and parents who think that those babies are almost full-term and therefore expect them to behave similarly? Or is it because these babies “fool us” by doing well for a day or two and then run out of energy.

We have some data to point us in the right direction in our search for answers. Studies show that the increased rate of readmission correlates with the following factors:

  1. The short duration of hospitalization after birth
  2. First-time mother
  3. Breastfeeding rather than formula feeding
  4. Asian and Pacific Islander ethnicity

Additionally, the most commonly cited reasons for readmission of late preterm babies are jaundice, feeding problems, irregular breathing (apnea), and infections.

What are long-term outcomes for late premature babies?

The brain of a baby born at 34 weeks of gestational age weighs only about 65% of that of a full-term infant. During the period between 34 and 40 weeks, cortical parts of our brains grow by 50% and the cerebellum by 25%. In addition, neurons grow during that time in numbers and increase the number of connections (synapses) among themselves. 

These developmental factors place late preterm babies at risk of having less favorable long-term neurodevelopment outcomes. 

Literature tells us that when compared to full-term newborns, late preterm infants have a higher incidence of the following outcomes: 

  • poor performance on tests of early school readiness
  • poor educational achievement at the age of 5 years
  • poor school performance at the age of 7
  • neuro-developmental delay at the age of 18
  • cerebral palsy
  • schizophrenia

We should note that even though there is a difference in the occurrence of those developmental outcomes mentioned above, their frequencies in absolute numbers are not that great. The vast majority of late preterm babies will grow to succeed in their professional, social, and personal lives.


Late premature babies are the largest group of all premature babies. Even though they are born “just” a few weeks earlier than their due date, they still may suffer from the consequences of being a “preemie baby.” Granted, they mostly avoid severe complications of prematurity such as ROP, NEC, BPD, and IVH (you will find my article about these conditions here). However, they still may need to undergo treatments in the nursery, and even after discharge, they are not entirely out of trouble. Fortunately, the good news is that long-term consequences are not as frequent in absolute numbers and very significant clinically as they could seem from comparisons to full-term babies. Let’s remember that even among full-term and healthy newborn babies, we will later find children with lower mental capacity, cerebral palsy, seizures, and learning problems. The truth is that being born at term also does not guarantee a “perfect” medical outcome.

If you are interested to learn more about premature babies, particularly the ones born earlier than 32 weeks of gestational age, I encourage you to read my book.


This article is only for general information purposes. It should not be viewed as any medical advice. There is always a chance that information here may be inaccurate. It would be best if you always discussed all health-related matters with your doctor before making any decisions that may affect yours or your family members’ health.


Dr.Wisniewski is a board-certified pediatrician and neonatologist with over 20 years of clinical experience in the USA. He authored the book: "Babies Born Early - A guide for Parents of Babies Born Before 32 Weeks" Dr.Wisniewski loves educating parents on various health conditions affecting their newborn babies and children.

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