A Week-by-Week Guide to Premature Babies

23 to 24 Weeks

Premature babies born between 23 to 24 weeks gestation are called micro-preemies. They weigh just over a pound and measure about 8 inches long from their head to their bottoms. Here are some other things to note:

• Some 30% of babies born at 22 weeks who receive medical care live to see life outside the NICU. If they are born just two weeks later, at 24 weeks, babies who are medically treated have more than a 50% chance of survival.
• 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.
• Most of the body's systems are underdeveloped at 23 to 24 weeks gestation. The lower airways are only beginning to develop, which is why many 23-weekers and 24-weekers need respiratory support for long periods.
• Babies at this age have fully formed hearing systems. While that means they can hear your voice, loud noises can overwhelm their underdeveloped nervous system.

25 to 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. By 26 weeks gestation:

• Premature babies' lungs are starting to develop alveoli, the air sacs that allow gas exchange. While they are still too young to breathe without help, this is a major milestone.
• The startle reflex is developing. A baby born at this time will startle at loud noises—a normal reaction of their nervous system.
• Their footprints and fingerprints are also developing.

27 to 28 Weeks

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

By 28 weeks, premature babies weigh about 2 1/2 pounds and are about 16 inches long from head to toe. Other developmental milestones to know are:

• Premature babies born after 27 weeks can blink and no longer have fused eyelids.
• The retinas are still developing, which puts babies at risk for retinopathy of prematurity (ROP). Their eyes can form images at this stage.
• Premature babies are also starting to develop more coordinated sleep/wake cycles and will have periods of REM sleep.

29 to 30 Weeks

By 29 to 30 weeks, a growing baby has matured a great deal. A premature baby born between 29 and 30 weeks will still require a NICU stay, but their vital organs are much more developed than those of babies born earlier.

At 29 to 30 weeks, premature babies weigh about 3 pounds and are about 17 inches long. You may also notice:

• While they're still very small, 29-weekers and 30-weekers have more fat stored under their skin, making them look more like full-term babies.
• They're also starting to shed their lanugo (the fine hair that covers a preemie's body).
• Their eyes can now blink, but bright lights and loud noises might still make them uncomfortable.
• The brain is starting to look grooved and wrinkled.
• They are also mature enough to begin controlling body temperature.
• They feel secure and cozy with swaddling and nesting.
• The baby's stomach and intestines are maturing and getting ready to digest milk.
• They still cannot nipple feed, so will be fed through a tube. But they can begin sucking on a pacifier to help develop these muscles. 

Kangaroo care becomes appropriate for babies starting around 28 weeks. Holding your baby skin-to-skin will not only help you bond, it has been shown to lower babies' pain response.

31 to 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, but there are a few differences to note.

• 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 good body temperature without the help of an incubator.
• They use all five senses to learn about their environments but still may get overstimulated by bright lights and loud noises. That being said, at this age, your baby will likely enjoy seeing your face up close.

Preemies must be several milestones before most NICUs will discharge them. They must be able to:

• Take milk by breast or bottle without needing tube feedings
• Maintain a healthy body temperature without the need for warming devices
• Gain weight steadily

33 to 34 Weeks

Premature babies born between 33 and 34 weeks are called "moderately pre-term 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.

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 biological boys, the testicles are descending into the scrotum.

Although they are getting bigger, 33- and 34-weekers are still immature and may need a stay in the NICU.

The respiratory system doesn't finish developing until the last weeks of pregnancy. Antibodies are only starting to pass from parent to baby—so preemies' immune health is still a bit compromised.

35 to 36 Weeks

Premature babies born at 35 to 36 weeks are called "late pre-term infants." These babies are about 20 inches long and usually weigh between 5 1/2 and 6 pounds.

These babies have reached their full height, are gaining weight rapidly, and have fingernails that come to their fingertips and fully formed footprints. They often look just like full-term babies.

Their lungs won't be completely developed for another couple of weeks, and they may need a bit more time to learn how to breastfeed or bottle-feed. Continuing to protect their sleep and time in the NICU until they are ready to go home is important.