The sensory system is a part of the nervous system that is responsible for processing sensory information. It is a group of subsystems used for detecting and understanding the world around us. Our sensory system includes:

• vision (visual perception)
• touch (tactile perception)
• hearing (auditory perception)
• taste (oral perception)
• smell (olfactory perception)

Our senses are the link from our physical world (our environment and our surroundings) to our inside world (our brain) where we interpret the information, creating our perception of the world and space around us.

Integration of the sensory system, whereby the sensory system is well organized and responding efficiently is an integral part of development.

In the womb, a baby is curled up, cozy, warm, in the dark, listening to their mother’s rhythmic heartbeat and muted sounds from the outside world. Meanwhile, the nervous system is developing at an astonishing speed, forming thousands of essential nerve cell connections. When a baby is born prematurely,  this process is interrupted. Their sensory organs are not fully developed and must continue to grow and mature in a foreign world outside the womb. For premature babies, this new foreign outside world may be overwhelming to them. Nothing sounds quite right, nothing feels quite right, and nothing smells quite right. The input from their physical world may seem stronger. The sounds of alarms, phones, and even voices may seem louder, the lights may seem brighter, and some smells  and even touch may be overbearing to them. What may seem like normal sound, light, smell, and touch to us, may be too much for their underdeveloped sensory system to handle. This "sensory overload" is a form of stress for a preemie.

Learning about how your baby's senses form can help you to understand how and what your baby may be feeling and what you can do, as a parent, to help your preemie grow and develop to their fullest potential, well beyond your NICU days.

The Eyes and Visual System

Sense of Sight In the womb, one of the first parts of the visual system to develop is the optic nerve. Starting at just 4 weeks gestation, your baby’s cells from the developing brain come together to form two optic nerves. At the same time, cells begin to develop into what will become the lens of the eye. At 8 weeks gestation, the retina- the layer of cells that perceive light- have begun to form.  By 16 weeks gestation, these cells have begun to pick up light. The blood vessels grow gradually toward the edges of the developing retina, supplying oxygen and nutrients.

Babies Born Very Early (Between 23 and 25 Weeks Gestation) may still have fused eyelids. When they do open, your preemie will be very sensitive to the light and will need protection from the brightness as much as possible. Controlling the physical environment is key. Using incubator covers and keeping lights dim or off will help your preemie adjust to their new visual world.

26 to 28 Weeks Gestation At 26 weeks gestation, your baby’s eyes are formed enough to sense light and can open their eyelids. Between 26- 28 weeks gestation, your baby’s eyes may just flicker and open slightly before shutting them again. Your preemie’s eyes are very vulnerable to bright and direct light because the retinas do not have a fully formed network of cells just yet. The pupillary reflex (ability to shut out access light) is absent in babies less than 30 weeks gestation and doesn’t completely form until 35 weeks gestation. The eyelids of a preemie between 26-28 weeks gestation are also very thin and give little protection. Your baby perceives light as blotches amongst the darkness. Controlling the physical environment continues to be key. Using incubator covers and keeping the light low and indirect is essential as the eyes continue to grow and develop.

29 to 32 Weeks Gestation The eyesight and retinas are still developing, but your baby can form images. The eyes can now blink and respond to the light and the darkness. At 30 weeks gestation, your baby may open their eyes in dim light and may be able to focus briefly on visual stimuli- such as your face.  Pictures and toys should not be placed in the direct visual space of a preemie between 29-32 weeks gestation.  New research does not support the use of black and white or red pictures for this gestational group.  Keep in mind it is normal for your baby to have rapid, uncoordinated eye movements as the eyes are still developing between 29-32 weeks gestation. It’s important to minimize light levels when possible.  Using incubator covers and diming the lights in the NICU at night will help with your preemie develop circadian rhythms.

33 to 36 Weeks Gestation The eyesight is still developing and they can form images, but keep in mind that too many patterns or colors may be over-stimulating to your baby.

 

The eyes are one of the last things to completely develop and will continue to mature even after 40 weeks gestation. At term gestation, babies can only focus about 8-12 inches from their face and can only see in black, white, and gray. As color vision begins to develop (10-12 weeks post term gestation), they will see red first and the full spectrum of colors by three months of age. By 6 months, post term gestation, a baby’s vision can be as good as 20/20.

 

Eye color:  At birth the pigment melanin, responsible for the color takes some time to build up. Because there is just a tiny bit of melanin present, the eyes appear blue. As your baby grows and the iris has built up more pigmentation, you will be able to tell what color your baby’s eyes actually are. (Typically between 6-9 months of age.)

The Ears and Auditory System

Babies Born Very Early (Between 23 and 25 Weeks Gestation) The auditory organs are fully formed at about 20 weeks gestation, but the inner and outer structures of the ear are not developed until 24 weeks gestation. Sounds are no longer muted by the tissues and fluid within the womb, so noises may be more intense for your preemie. Your baby is used to voices being filtered within the womb from the outside world. Sounds can be amplified within the walls of the incubator, which can be over stimulating and sometimes painful. It’s important not to be too loud and startle your preemie. Your baby recognizes your voice, speak softly and keep noise at a minimum at this gestational age.

26-28 Weeks Gestation Babies between 26-28 weeks gestation are still developing their sense of hearing and can be sensitive to loud over stimulating environmental noise. Your preemie can recognize your voice but may still not respond fully to it. Signs of overstimulation will be shown in the only language the preemie knows- by sneezing, hiccupping, or even dropping the heart rate. As you spend more time with your preemie, you will get to know their specific stress signals and be able to respond appropriately to give your preemie the very best environment for growth and development.

29- 32 Weeks Gestation Although babies between 29 and 32 weeks can hear your voice, loud noises are often too much for their underdeveloped neurological system and they will startle easily. Continue to

speak softly to your baby as your voice is recognizable to your preemie and is a form of comfort. Between 30-33 weeks gestation, your baby is beginning to become more in tune with the surrounding and environment. Your preemie will show you outward signs of stress or stabilization in the only language they know, through their behavior. When your baby becomes overwhelmed or has had too much activity they may hiccup, sneeze, or even cry- these are physical signs of overstimulation. As you spend more time with your preemie, you will get to know which signs are stress signals and which are stabilization cues.

33 weeks Gestation to Term Gestation  Your baby’s hearing is fully developed, but they are still sensitive to strong harsh noises. Your baby may show a dislike to loud noises by frowning, startling, or even crying. Your baby can pick out your voice and prefers it over other voices.

 

What’s that Sound? 

Loud noises in the NICU can be detrimental to the growing preemie.  These sounds can interfere with sleep-which is essential or growth, and deplete energy- which wastes calories that are also essential for growth.

The decibel (dB) is a unit that is used to express the ration of power, level, and intensity of sound. The following examples are used to help you understand basic noise levels based on scenario and how it translates to your baby’s hearing.

A lawn mower - 100 dB A busy restaurant - 70-80 dB A conversation between two people- 60-65dB Library- 30-35 dB Breathing- 10dB

In the NICU Closing incubator doors (100 dB) or placing objects on top of incubator- 111 dB A noisy nurse’s station at shift change- 70-80 dB A conversation between the nursing staff and parents- 60-65dB The inside of an incubator with all equipment off- 53dB A mom speaking softly to her newborn in a quiet room- 30-35dB Kangaroo Care in quiet environment listening to heartbeat- 10-25dB Recent studies show that the average sound levels in a NICU range between 70 to 80 dB, but the American Academy of Pediatrics recommends a maximum safe noise level of 45 to 50 dB  The hearing threshold (the intensity at which one perceives sound) at 27 to 29 weeks of gestation is approximately 40 dB and by term gestation (40 weeks) decreases 20dB, equal to that of an adult.

In utero, the baby is protected from high-frequency sounds and hears low-frequency sounds- such as its mother’s heartbeat. The infant born prematurely does not have protection from these loud noises in the neonatal intensive care. In neonates, noise exposure >60 dB has been associated with consistent sleep disturbance and have a negative effect on the baby’s vital signs, oxygen levels, and ultimately growth and development. Repeated exposure to loud noise in the NICU puts premature newborns at risk for hearing loss and future developmental delays.

 

What can I do to help protect my baby from the noisy NICU environment?

• Avoid tapping on the incubator walls or placing objects on the top of the incubator.
• Shut the incubator doors and portholes gently.
• Avoid loud conversation around your baby’s incubator.
• Speak softly to your baby in a calm even toned voice.
• Use a dark incubator cover- this helps to dampen the noise as well as provide a dark environment for your baby to sleep and grow.

Is my premature baby at an increased risk for hearing problems? Premature babies are more likely than term babies to develop hearing impairment, but the risk is fairly low. The increased risk stems from the immature auditory system at the time your baby is born.  Preemies are highly sensitive to noise. Babies born prematurely also experience some medical conditions and treatments that may lead to hearing problems. When your baby is getting closer to discharge, they will receive a hearing screen. It’s important to talk to your baby’s healthcare team about your infant’s individual risk and possible need for follow-up.

 

The Nose and Sense of Smell

Babies Born Very Early- 23 to 25 Weeks Gestation Your baby’s sense of smell has been developing since 22 weeks gestation; however it is not completely formed until around 29 weeks gestation.  In utero, amniotic fluid moves through the nostrils as the baby begins to practice breathe within the womb. Babies born between 23-25 weeks gestation may not respond to noxious odors, but it’s important to protect  them from strong smells such as perfumes and colognes, cigarette smoke on clothing and on hands, as well as the smell of alcohol- from wipes or even the hand sanitizer commonly used in the NICU.  As a parent, you can help educate family and friends about the importance of keeping these strong scents away from your preemie.

26 to 28 Weeks Gestation The sense of smell is still being developed in the preemie between 26-28 weeks gestation. Preemies at this age will begin to recognize and respond to the smell of their parents as they do skin-to-skin care. It’s important to remember that your preemie is still sensitive to strong environmental scents such as perfumes and colognes, cigarette smoke, and disinfectant or cleaning products. Your baby may find comfort in the smell of your breast milk.  Ask your healthcare provider if you can place a small amount of milk on a piece of gauze during gavage feedings and place it in the incubator for your baby to smell.

29 to 32 Weeks Gestation The sense of smell is fully formed in the premature baby at 29 and 32 weeks gestation. The rooting reflex should be present and your preemie may begin to associate the smell of milk with feeding times. Smell cues will alert the preemie when feeding time is approaching and will help stimulate digestion and contribute to the satisfaction and the importance of feeding becoming an enjoyable experience for your baby. Between 29 and 32 weeks gestation, your baby will continue to thrive and grow with the sweet smell of your skin when doing kangaroo care.

33 Weeks to Term Gestation Babies show a preference for certain tastes and smells and are drawn to the odor of breast milk. Newborns also are drawn to the smell of their own mother and this helps them learn how to suck and then to feed. These smells can have a calming effect on your baby and may help soothe the baby during painful or uncomfortable NICU procedures. Kangaroo Care continues to be very important for you and your baby.  Please speak with your baby’s healthcare team to make a plan on how this can be incorporated into your baby’s care.

 

The Mouth and Sense of Taste

Babies Born Very Early (23-25 Weeks Gestation) Premature babies at this gestation have a developed sense of taste. When in utero the baby begins to swallow amniotic fluid around 12 weeks gestation and can begin to taste its sweet, salty, sour components at around 15 to 16 weeks gestation. The taste buds are well developed at about 21 weeks gestation and a baby can discern many flavor using their senses of smell and taste. ( Preemies at this age will not be getting any nutrition by mouth because of their underdeveloped digestive and neurologic system.(Suck, swallow breathe doesn’t begin to develop until about 33 weeks gestation.) However, babies at this gestation can have mouth care done with breast milk. Breast milk has natural antibodies that can be absorbed through the oral mucosa and it will also help to keep the mouth and lips moist and healthy.

26-28 Weeks Gestation The taste buds of a preemie between 26- 28 weeks are well developed. However, they may not be getting fed anything by mouth just yet. (Suck, swallow, breathe is not fully developed until about 33 weeks gestation.) At 28 weeks, most preemies enjoy sucking on a pacifier or on their finger to relax and comfort themselves. The muscles may not be strong enough to hold the pacifier in for long periods of time, but it’s important to have this connection during tube feedings. Practice will help develop the muscles that will later be used to hit this important milestone. Putting a small amount of milk on the end of the pacifier will help your preemie recognize the connection between feeding times and hunger cues and they will be able to taste and smell the sweetness of the milk.

29-33 Weeks Gestation Babies between 30-33 weeks gestation are approaching a major milestone- feedings by mouth may begin. Your premature baby at 33 weeks may be able to nipple feed, but they may need practice and patience as they remember to coordinate sucking, swallowing, and breathing. Pacifiers continue to be an important practice tool for preemies during this stage.

34 Weeks to Term Gestation Sensitivity to sweet and bitter taste is present and babies can begin to distinguish between the two by about 35 weeks gestation. Your infant’s sweet taste buds are the most predominant making breast milk the perfect diet for your baby. By term, a baby has about 30,000 taste buds. They are not just on the tongue but also on the sides, back, and roof of the mouth. By adulthood the number of taste buds has decreased to about 10,000.

 

Oral Aversion and Feeding Difficulties

Premature babies who were born before 26 weeks gestation, who have been on a ventilator or respiratory support for a long period of time and have chronic lung disease, or who have had a medically complicated journey, may have a harder time progressing to feeding by mouth. Medical equipment and supplies such as ventilator tubes, suctioning, tape and tubes on the face may create a negative oral experience for some babies. When preemies have nothing but unpleasant experiences around their mouth, they may grow to dislike having anything in or near their mouths. This is referred to as oral aversion.

How can I help my baby avoid oral aversion and help to create a positive experience?

Offer your baby a pacifier. Pacifiers can provide a positive experience for babies, even when they are intubated. A pacifier will also help your baby associate sucking with feeding.

Let your baby taste your milk. Place a small drop on your baby’s pacifier or provide mouth care using small amounts of breast milk. Ask your baby’s healthcare team to guide you through this task during routine hands on care.

Sucrose drops (sometimes called “Sweet-ease”) are often used in the NICU to help decrease pain responses during uncomfortable procedures.  Ask your baby’s healthcare team about the use of sugar to help your baby cope with some of the more unpleasant and painful NICU experiences.

 

 

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