There is no single “best” formula for all premature babies, because the right choice depends on your baby’s gestational age, birth weight, and how well they’re growing. What is clear is that premature infants need formulas with significantly more calories, protein, and minerals than standard term formulas provide. The formulas designed for preterm babies fall into a few distinct categories, and understanding the differences will help you follow your baby’s feeding plan with confidence.
Why Standard Formula Isn’t Enough
Babies born early miss out on the final weeks or months of pregnancy, when the fastest growth and mineral accumulation happen. A fetus in the third trimester deposits roughly 90 to 120 milligrams of calcium and 60 to 75 milligrams of phosphorus per kilogram of body weight every day, building the skeleton that would normally be nearly complete at birth. Premature babies need to replicate that pace outside the womb, which standard infant formula simply cannot support.
Standard term formula delivers about 67 calories per 100 milliliters and no more than 1.7 grams of protein per 100 mL. Preterm infants, by contrast, need protein intakes that can reach 3.5 to 4.5 grams per kilogram per day once feedings are fully established. They also need higher concentrations of calcium, phosphorus, and long-chain fatty acids for bone strength and brain development. Feeding a premature baby regular formula would leave a measurable gap in nearly every nutrient that matters most during catch-up growth.
Categories of Premature Infant Formula
In-Hospital Preterm Formula
While your baby is in the NICU, the medical team typically uses a high-calorie preterm formula that provides around 80 calories per 100 mL, with protein above 2.0 grams per 100 mL. These formulas are calorie-dense, usually 24 calories per ounce, and packed with extra calcium, phosphorus, and vitamins. Compared to standard formula, this level of nutrition translates to roughly 500 grams more weight gain, 5 to 10 millimeters more length, and 5 millimeters more head circumference over the study periods measured. You won’t be choosing this formula yourself; it’s selected and often adjusted by the NICU nutrition team.
Post-Discharge (Transitional) Formula
Once your baby comes home, the most common recommendation is a post-discharge formula, sometimes called a transitional formula. These sit between hospital-grade preterm formula and regular formula: roughly 74 calories per 100 mL and protein above 1.7 grams per 100 mL. In the United States, the two widely available options are Enfamil EnfaCare and Similac NeoSure. Both are formulated at 22 calories per ounce out of the can, but they can be mixed to 24 or even 26 to 27 calories per ounce if your baby’s doctor recommends a higher concentration for faster weight gain.
These transitional formulas contain more calcium, phosphorus, and other bone-building minerals than standard formula, along with added long-chain fatty acids that support brain and eye development. The World Health Organization review of available evidence found that post-discharge formula improved growth outcomes compared to standard term formula, making them the preferred choice for most premature babies heading home.
Fortified Breast Milk
If you’re providing breast milk, that remains the preferred base for feeding a premature baby. However, breast milk alone doesn’t contain enough protein, energy, minerals, or micronutrients to meet the elevated needs of a preterm infant. Human milk fortifiers, which are added directly to expressed breast milk, close that gap. The NICU team will add fortifier to your pumped milk during the hospital stay, and some babies continue using it after discharge. This approach gives your baby the immune and digestive benefits of breast milk alongside the concentrated nutrition of a preterm formula.
How Long to Use Specialized Formula
A 2023 consensus statement from neonatologists at Children’s Hospital of Philadelphia recommends that high-risk babies born before 32 weeks or weighing under 1,500 grams at birth stay on their specialized feeding regimen until 52 weeks corrected gestational age, and potentially up to one year of age. “Corrected age” means counting from your baby’s original due date, not the actual birth date. So if your baby was born two months early, their corrected age at six months of life is only four months.
This also means premature babies should not switch to whole cow’s milk at their first birthday the way full-term babies often do. Your pediatrician will assess growth trends and let you know when it’s safe to transition, which may be later than you’d expect.
How to Tell the Formula Is Working
The clearest measure is steady weight gain. The target for premature infants is about 15 grams per kilogram of body weight per day, which mirrors the growth rate of a fetus during the third trimester. In practical terms, that works out to roughly 7 grams per day for every pound your baby weighs. A very small baby born at 24 weeks might gain as little as 5 grams daily, while a larger preemie at 33 or more weeks typically gains 20 to 30 grams a day.
Your baby’s medical team will track weight, length, and head circumference on a growth chart adjusted for prematurity. If growth is falling behind, the next step is often increasing caloric density. EnfaCare, for example, can be concentrated from 22 to 24 or 26 calories per ounce by adjusting the powder-to-water ratio. Never change the concentration on your own; even small errors can cause dehydration or electrolyte problems. Your pediatrician or a dietitian will give you exact mixing instructions.
Safety: Liquid vs. Powdered Formula
Premature babies face a higher risk of infection from a rare but dangerous bacterium called Cronobacter that can contaminate powdered formula. The FDA specifically identifies premature infants as a higher-risk group and recommends using liquid ready-to-feed formula whenever possible, because it’s manufactured to be sterile. Liquid concentrate is also sterile before mixing.
Powdered formula is not sterile, even in a sealed can. If you do use powdered formula at home, careful preparation matters: wash your hands thoroughly, sterilize bottles, use the exact water-to-powder ratio specified, and refrigerate any prepared formula you don’t use immediately. As your baby grows older and stronger, the risk from Cronobacter drops, but for the first couple of months after discharge, liquid ready-to-feed is the safer choice when it’s available and affordable.
Choosing Between Brands
Enfamil EnfaCare and Similac NeoSure are nutritionally similar. Both deliver 22 calories per ounce at standard dilution, both contain added calcium, phosphorus, and long-chain fatty acids, and both are designed for the post-discharge period. Some babies tolerate one better than the other based on the protein source or the specific fat blend, so a bit of trial and error is normal. Signs that a formula isn’t sitting well include persistent gas, excessive spitting up, or significant fussiness during and after feeds.
Store-brand post-discharge formulas also exist and are required to meet the same federal nutritional standards. They can be a reasonable option if cost is a concern, though availability varies. Whatever brand you use, confirm with your baby’s care team that the caloric density and nutrient profile match what was recommended at discharge. The “best” formula for your premature baby is ultimately the one that supports consistent, adequate growth while your baby tolerates it comfortably.