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Understanding healthy growth in the NICU

By Andi Markell, RD, LD

Medical Science Liaison

Babies in the womb develop rapidly during the third trimester. In these final few months, their internal organs, such as the lungs and the digestive system are maturing, and the surface of their brain is starting to go from smooth to grooved. The placenta is perfectly created to support this fast-paced growth by supplying the fetus with high amounts of protein, calcium, and phosphorus, to help build lean body mass and bones.

Preemie’s Nutritional Needs

When babies are born very early, the rapid growth and specialized nutrition that they would receive in the womb are interrupted. While these preemies are not able to receive nutrition directly into their gut, they still need appropriate nutrition so that their long-term growth is not compromised. As soon as possible, the medical team starts these babies on parenteral nutrition, a type of nutrition that is fed directly into the veins. Parenteral nutrition is not milk, but rather a special substance that includes carbohydrates, protein, and fat as well as vitamins and minerals. Within a few days of life, very small amounts of mother’s milk or donor human milk are introduced to the baby’s diet in order to slowly get the intestines ready to be fed. From that point, feeding volumes are slowly increased until parenteral nutrition can be stopped and all nutrition can consist of human milk. Along with human milk, preemies are given a fortifier to add crucial amounts of protein, calcium, phosphorus, and other vital nutrients to make up for the stepped-up nutrients they would have received in the womb. To maintain a 100% human milk diet – which is ideal for preemies – the fortifier must be made from human milk.

Measuring a Preemie’s Growth

In the neonatal intensive care unit (NICU), weight, length, and head circumference are measured to ensure that your baby is growing appropriately.  One of aims of your preemie’s medical team is to achieve a growth pattern similar to that of a fetus of the same post-conceptual age as your preemie.1

This is important because growth problems in early infancy can have long-lasting negative health effects, such as increased risk of obesity and type 2 diabetes.2

Weight Goal: 15-20 grams per kilogram of body weight per day3

It’s normal for preemies to lose weight right after birth as they leave the fluid-filled uterus. Once they are supported on the right amount of nutrition, weight is measured and should increase daily. But it’s important to note that weight may be influenced by factors such as surgery or medication.

Length Goal:  >0.9 centimeters per week3

In the neonatal intensive care unit, length is measured once a week. The most accurate way to measure length is using a length board, or a hard piece of plastic that has a ruler built into it. Length is less likely to be influenced by medical conditions or interventions than weight is.

Head Circumference Goal:  >0.9 centimeters per week3

Head circumference is also measured once a week, using a tape measure.  Often head circumference is linked to neurodevelopmental outcomes, but it can also be influenced by what is happening medically. For babies born early, the first few months of life are a crucial period of development and the right nutrition is key to maintaining healthy growth. A large focus in the NICU is growing babies the way they would grow in utero – proportional babies with emphasis on weight, length, and head circumference in combination.  The health care team works together to make sure that each baby’s individual nutritional needs are met giving them the best possible nutritional start to life.


  1. Neonatal weight gain and nutrition. Accessed October 21, 2018.
  2. Nutrition: recommendations and benefits. Accessed November 11, 2018.
  3. Groh-Wargo S, Thompson M, Hovasi Cox J, eds. AND Pocket Guide to Neonatal Nutrition. 2nd ed. Chicago, Illinois: Academy of Nutrition and Dietetics