A Weekly Look at Premature Babies and Complications

Premature babies are any babies born before the 37th week of a woman's pregnancy. However, if you've ever spent time inside a NICU, you know that preemies who are born at different stages of pregnancy are very different from one another.

All premature babies are small, require complex medical care, and may face serious complications both in the NICU and at home. A baby born 3 to 4 months early, though, will face far different complications from a baby born 1 to 2 months early.

Let's take a closer look at how premature babies differ week by week.

23-24 Weeks

Health care worker leaning over incubator and holding tiny hand of a premature baby with gloved hands, low angle view.
Eddie Lawrence/Dorling Kindersley/Getty Images

Over half of premature babies born between 23 and 24 weeks of pregnancy will survive delivery and live to see life outside of the NICU. Babies born before 23 weeks may survive – the youngest preemie ever to survive was Amillia Taylor, born at only 21 weeks and 6 days gestation. But, 23 to 24 weeks is often considered the age of viability for premature babies.

Premature babies born at 23 to 24 weeks are called micro preemies. They weigh just over a pound and measure about 8 inches long from their head to their bottoms. Babies born at this time will be covered by fine hair called lanugo, to keep them warm, as they have not yet developed brown fat. Their skin is also very thin and delicate. Although their eyes will most likely be fused shut, they'll have fully developed eyelashes and brows. They will even have tiny fingernails!

That being said, most of the body's systems are underdeveloped at 23 to 24 weeks gestation. The lower airways are only beginning to develop, so 23-weekers and 24-weekers will need respiratory support for long periods of time.

You may be surprised to learn that a baby at this age has a fully formed hearing system, so he or she can hear your voice, although loud noises may be overstimulating and overwhelming to their underdeveloped nervous system.

25-26 Weeks

By 25 to 26 weeks, premature babies weigh about 1 1/2 to 2 pounds and are about 9 inches long when measured from head to bottom. Babies born at this time are called micro-preemies, and they face long NICU stays and have many health issues related to prematurity.

By 26 weeks gestation, premature babies' lungs are starting to develop alveoli, the air sacs that allow gas exchange. Although they are still too young to breathe without help, this is a major milestone. Other developmental milestones for 25 and 26 weekers include the development of the startle reflex—a baby born at this time will startle at loud noises, and this is a normal reaction of their nervous system. Footprints and fingerprints are also developing.

27-28 Weeks

By 27 weeks, premature babies are no longer considered micro preemies. Now called "very premature babies," these babies have a greater than 95 percent rate of survival past birth and NICU discharge. However, 27 and 28 weekers still require a lot of medical care and can be expected to stay in the NICU for long periods of time.

By 28 weeks, premature babies weigh about 2 1/2 pounds and are about 16 inches long from head to toe. Rapid eye development is occurring, and premature babies born after 27 weeks can blink and no longer have fused eyelids. Their retinas are still developing (making them still at risk for retinopathy of prematurity, or ROP), but their eyes can form images.

By 27 and 28 weeks, premature babies are also starting to develop more coordinated sleep/wake cycles. They are starting to have periods of REM sleep, and parents watching their babies sleep may wonder what they're dreaming about.

29-30 Weeks

By 29 to 30 weeks, a growing baby has matured a lot. Premature babies born between 29 and 30 weeks will still require long NICU stays, but their vital organs are much more developed than those of babies born earlier.

By 29 to 30 weeks, premature babies weigh about 3 pounds and are about 17 inches long. Although they're still very small, 29 weekers and 30 weekers have more fat stored under their skin, so they look more like "real" babies. They are starting to shed their lanugo, the fine hair that covers a preemie's body. Also, their eyes can now blink, but bright lights and loud noises are usually still uncomfortable.

In addition to all of this outside maturity, the brain goes through a period of rapid growth as well. The brains of 29- and 30-week premature babies are starting to look grooved and wrinkled, and are mature enough to begin controlling body temperature.

At this age, the premature baby feels secure and cozy with swaddling and nesting. Also, at this time, their stomach and intestines are maturing and getting ready to digest milk. They are not yet ready to nipple feed but can begin sucking on a pacifier to help develop their eating muscles. In addition to using the pacifier, kangaroo care while being fed will help your baby thrive, and help you and your baby bond.

31-32 Weeks

By 31 to 32 weeks, premature babies weigh between 3 1/2 and 4 pounds and are between 18 and 19 inches long. That's almost as long as a baby born at term. Premature babies born at 31 and 32 weeks are called moderately preterm babies. Although they're still immature at birth and will require several weeks of NICU care, most 31 and 32 weekers quickly catch up to their peers and have few long-term effects of prematurity.

Between 31 and 32 weeks, babies gain a lot of body fat. Premature babies born at this age are starting to look plump and may be able to maintain a good body temperature without the help of an incubator.

They use all 5 senses to learn about their environments but still may get overstimulated by bright lights and loud noises. Overstimulation by their environment may be expressed through a hiccup, sneeze, or cry. That being said, at this age, your baby will likely enjoy seeing your face up close.

Parents usually want to know when their babies can come home from the NICU. Even though at this age, your baby looks like a tinier version of a full-term baby, your baby still needs special care, especially as their immune health continues to mature, and they develop their suck for feeding. While your baby will be awake more, their sleep needs to be protected, so they can continue to thrive and grow.

Before discharge, there are several milestones that premature babies must reach: they need to be able to eat, breathe and stay warm without any help from NICU staff or equipment.

Preemies born at 31 and 32 weeks may be able to do one or two of these things at birth, but it will take time to reach all three milestones.

33-34 Weeks

Premature babies born between 33 and 34 weeks are called moderately preterm babies. Weighing between 4 and 5 pounds at birth and measuring almost 20 inches long, these babies are getting much closer to the size of a baby born at term. Although they are getting bigger, 33 and 34 weekers are still immature and may need to stay in the NICU for several weeks.

Premature babies are almost fully developed by 33 and 34 weeks. Their bones are fully formed, their fingernails come to the ends of their fingertips, and in boys, the testicles are descending into the scrotum. However, the respiratory system doesn't finish developing until the last weeks of pregnancy, and antibodies are only starting to pass from mom to baby—so their immune health is still a bit compromised.

At 33 and 34 weeks, most premature babies will have fairly short NICU stays with only a few complications. They may need help breathing for a short time, but learning to eat may take the longest. The suck-swallow-breathe reflex is not well coordinated, and these babies may not be strong enough to take in enough nourishment to grow and gain weight.

During this time, it's also important to watch out for signs of overstimulation from the environment like hiccuping, sneezing, crying, or arching away. Protecting your baby's time for sleep is very important during this time.

35-36 Weeks

Premature babies born at 35 to 36 weeks are called late preterm infants. These babies are about 20 inches long and usually weigh between 5 1/2 and 6 pounds. Although 35 and 36 weekers look just like full-term babies, they are still premature and may face some problems of prematurity.

By the last weeks of pregnancy, most babies have turned to a head-down position. They have reached their full height, are gaining weight rapidly, have fingernails that come to the tips of their fingers, and have fully formed footprints.

Although they look like full-term babies, 35 and 36 weekers are premature babies. Their lungs won't be completely developed for another couple of weeks, and they may not have enough fat to stay warm or enough strength to breast or bottle feed effectively.

Continuing to protect their sleep and time in the NICU until they are ready to go home is important.

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