A premature baby’s size directly reflects the time spent developing in the womb. A birth is considered premature, or preterm, when it occurs before 37 completed weeks of gestation, compared to a full-term pregnancy of 39 to 40 weeks. The earlier a baby arrives, the smaller they will be, with less body fat and more underdeveloped organs. Size is a primary indicator of the specialized care needed, which is measured by gestational age at birth and specific weight classification.
Defining Prematurity by Gestational Age
The medical community classifies prematurity into four categories based on gestational age at birth, which is the most reliable predictor of size and developmental maturity.
Infants born between 34 and 36 weeks are considered Late Preterm and represent the largest group of premature births. A baby in this group is often close to the size of a full-term infant, typically weighing between 4 and 5.5 pounds (1,800 to 2,500 grams). These babies may still struggle with feeding, breathing, and maintaining their body temperature.
The next category is Moderately Preterm, including babies born between 32 and 34 weeks of gestation. These infants generally weigh between 3 and 5 pounds (1,400 to 2,300 grams). While they have a high survival rate, their organs are less mature than late preterm babies, and they usually require a stay in the Neonatal Intensive Care Unit (NICU).
Babies born between 28 and 32 weeks are classified as Very Preterm. At 28 weeks, a baby might weigh around 2 pounds, 6 ounces (1,000 to 1,100 grams). They are smaller, have less body fat, and have more fragile skin, necessitating comprehensive support for breathing and temperature regulation.
Extremely Preterm infants are those born before 28 weeks of gestation. These babies are the smallest, with some born as early as 24 weeks weighing just over 1 pound (500 to 750 grams).
Standard Weight Classifications
While gestational age dictates the baby’s maturity, formal weight classifications provide the quantitative measure of size used in the NICU for medical triage and resource allocation. The most common classification is Low Birth Weight (LBW), defined as any baby born weighing less than 5 pounds, 8 ounces (2,500 grams). This weight is the threshold for increased health risks and often necessitates specialized monitoring.
A more specific classification is Very Low Birth Weight (VLBW), which applies to infants weighing under 3 pounds, 5 ounces (1,500 grams). Most VLBW babies are born prematurely, and their lower weight indicates a greater probability of needing intensive medical support immediately after birth.
The smallest infants fall into the Extremely Low Birth Weight (ELBW) category, defined as weighing less than 2 pounds, 3 ounces (1,000 grams). These babies are the most medically fragile and almost always require prolonged, intensive care. Medical advancements have significantly improved the outcomes for these smallest infants, but their care remains highly complex due to their size and physiological immaturity.
How Size Varies Among Premature Infants
Not all babies born at the same gestational age will be the same size, as a range of factors influence growth in the womb. A baby’s actual birth weight is compared against the expected weight for their specific gestational age using growth charts to determine if they are Appropriate for Gestational Age (AGA), or if they are smaller than expected.
A baby who is significantly smaller than 90% of other babies born at the same gestational age is classified as Small for Gestational Age (SGA). This reduced size often results from Intrauterine Growth Restriction (IUGR), which means the fetus did not grow at the expected rate due to problems with the placenta, maternal health conditions, or other factors.
Issues such as maternal hypertension (high blood pressure), infection, or problems with the placenta’s ability to deliver nutrients and oxygen can restrict fetal growth. Multiple births, such as twins or triplets, also commonly result in smaller babies because the available resources and space must be shared. Being SGA carries additional risks compared to being AGA, even at the same gestational age.