A full-term pregnancy lasts between 39 and 40 weeks, while a premature, or preterm, birth occurs before the 37th week of pregnancy. This difference in gestational time leads to one of the most noticeable distinctions between these newborns: their physical size. The variation in weight, length, and development at birth sets the stage for different paths of growth and care.
Defining Full-Term Infant Size
A full-term infant serves as the baseline for newborn size. According to the World Health Organization (WHO), the average birth weight for a full-term male is approximately 7 pounds 6 ounces (3.3 kg), while for a female, it is around 7 pounds 2 ounces (3.2 kg). The normal range for babies born between 37 and 40 weeks is between 5 pounds 8 ounces and 8 pounds 13 ounces (2.5 to 4 kg).
Beyond weight, other measurements provide a fuller picture of a newborn’s size. The average head circumference for a full-term newborn is about 13.5 inches (34.2 cm), a measurement that reflects brain development. Length is another standard metric, with most full-term infants measuring between 19 and 22 inches long. It is important to recognize that these figures represent averages, and healthy variations are common.
Understanding Premature Infant Size
An infant’s size is directly linked to how early they are born, and prematurity is often broken down into classifications. Late preterm infants, born between 34 and 36 weeks, are the largest of premature babies. They may weigh between 4.5 and 6 pounds and can appear similar to small full-term newborns.
As gestation decreases, so does birth weight. Moderately preterm infants, born between 32 and 34 weeks, often weigh between 3 and 5 pounds. Very preterm infants, born before 32 weeks, typically weigh less than 3 pounds 5 ounces (1.5 kg). The smallest and most fragile are the extremely preterm infants, born at or before 25 weeks, who can weigh less than 2 pounds 3 ounces (1 kg), and often have a proportionally larger head and a lack of body fat.
The Last Trimester Growth Spurt
The reason for the significant size difference lies in the growth that occurs during the final trimester of pregnancy. This period is marked by a rapid growth spurt. A fetus gains a significant portion of its birth weight in the last two months of gestation. During these final weeks, the fetus accumulates layers of body fat, which adds to its weight and helps regulate body temperature after birth.
Premature babies miss out on some or all of this developmental window. An infant born at 32 weeks, for example, has missed two months of this growth period. This interruption is the direct cause of their lower birth weight and smaller measurements compared to their full-term counterparts. The earlier the birth, the more of this growth phase is skipped, resulting in a smaller infant at delivery.
Tracking Growth and Catching Up
To accurately assess development, healthcare providers use a concept known as ‘corrected age’ or ‘adjusted age.’ This is calculated by subtracting the number of weeks the baby was born early from their chronological age. Using this adjusted age allows for a more appropriate comparison of a preemie’s progress against developmental milestones.
Specialized growth charts, like the Fenton preterm growth chart, are used to monitor these infants. Many premature infants experience a period of accelerated development known as ‘catch-up growth,’ where their rate of growth surpasses that of their full-term peers. This phase can continue for months or even years. While the timeline varies, many premature babies catch up in height and weight by the time they are around two years old.