When a baby is born weighing significantly less than average, they are often referred to as a “small baby.” But what exactly constitutes a small baby? There are a few key factors that doctors look at.
Birth Weight
One of the main criteria for classifying a small baby is birth weight. The average newborn weighs between 5 pounds 8 ounces and 8 pounds 13 ounces. Babies weighing less than 5 pounds 8 ounces at birth are generally considered small or small for gestational age (SGA).
There are different categories of low birth weight:
- Low birth weight – less than 5 pounds 8 ounces
- Very low birth weight – less than 3 pounds 5 ounces
- Extremely low birth weight – less than 2 pounds 3 ounces
The lower the birth weight, the higher the risk for health problems and complications. However, some small babies born at a healthy weight may simply have a smaller body size due to genetics.
Common Causes of Low Birth Weight
There are a variety of reasons why a baby may be born small, including:
- Preterm birth – Being born too early before reaching full growth
- Intrauterine growth restriction (IUGR) – Poor growth of the baby while in the womb
- Multiple pregnancy – Twins or triplets often have lower birth weights
- Genetics – Some babies are just smaller but healthy
- Infections – Certain maternal infections can restrict fetal growth
- Placental problems – Issues with the placenta impairing nourishment to the baby
- Substance abuse – Use of alcohol, drugs, or tobacco during pregnancy
- Maternal health issues – Conditions like high blood pressure or diabetes
Identifying the cause can help guide treatment to best support the health of a small newborn.
Gestational Age
Gestational age, referring to the number of weeks the baby has been developing in utero, is another key factor. Babies born earlier in pregnancy have less time to grow and gain weight.
A full-term pregnancy is 40 weeks. Babies born before 37 weeks are considered preterm.
The earlier a baby is born, the lower the expected birth weight:
| Gestational Age | Category | Average Weight |
|---|---|---|
| Less than 28 weeks | Extremely preterm | 2 pounds 3 ounces |
| 28-31 weeks | Very preterm | 3 pounds – 4 pounds 4 ounces |
| 32-33 weeks | Moderately preterm | 4 pounds 5 ounces – 5 pounds 3 ounces |
| 34-36 weeks | Late preterm | 5 pounds 4 ounces – 6 pounds 9 ounces |
Babies born between 34-36 weeks are often referred to as “late preterm” babies. They may be small simply because they were born a few weeks early before reaching their full growth potential.
Is My Preterm Baby Small for Gestational Age?
Some preterm babies may also be small for their gestational age in addition to being born early. This means their growth has also been restricted in the womb.
Doctors use growth charts to plot a preterm baby’s weight against the expected norm for the number of gestational weeks. This helps identify babies who are not only small due to prematurity but also due to inadequate growth.
Length and Head Circumference
While weight is the primary measurement used, length and head circumference at birth are also important growth parameters. Just like weight, length and head size have average expected measurements based on gestational age. Small body length and head circumference can also indicate stunted fetal growth.
Here are the average length and head circumference measurements by weeks gestation:
| Gestational Age | Length | Head Circumference |
|---|---|---|
| 26 weeks | 12.6 inches | 9.4 inches |
| 30 weeks | 15.7 inches | 10.6 inches |
| 34 weeks | 17.2 inches | 11.4 inches |
| 37 weeks (full term) | 18.9 inches | 13.0 inches |
Measurements that plot significantly below the averages on growth charts may indicate growth restriction in addition to prematurity.
Monitoring Growth After Birth
After birth, small babies are closely monitored to ensure they are continuing to grow adequately. Doctors track measurements such as weight, length and head circumference and assess developmental milestones.
Slow growth after birth or failure to reach certain milestones could signal an underlying condition that needs to be addressed.
Health Risks
Small babies, especially those born very prematurely or with very low birth weights, are at higher risk of certain health problems compared to full-term infants. Some of the risks include:
- Respiratory Distress Syndrome (RDS) – difficulty breathing due to underdeveloped lungs
- Patent Ductus Arteriosus (PDA) – a heart defect leaving an open vessel that doesn’t properly close
- Low oxygen levels
- Difficulty regulating body temperature
- Jaundice
- Anemia
- Low blood sugar
- Infection
- Feeding difficulties
- Neurodevelopmental disorders like cerebral palsy
The severity of risks depends on factors like just how small the baby is, presence of other medical issues, and quality of neonatal intensive care available. But in general, the lower the birth weight, the higher the risks.
Long-Term Outcomes
Some studies show small babies are more likely to experience:
- Delayed cognitive and motor development
- Learning disabilities
- Behavioral disorders
- Chronic health problems
- Vision and hearing impairment
However, with excellent neonatal care and proper monitoring and interventions after discharge, many small babies go on to have typical development and minimal long-term effects.
Special Care for Small Babies
Small, premature infants require specialized medical care after birth provided in a neonatal intensive care unit (NICU). Some interventions may include:
- Incubators and warmers to regulate body temperature
- Monitors to track breathing, heart rate, and oxygen levels
- Ventilators to aid breathing
- IV fluids and nutrition
- Treatment for any infections or other medical issues
Parents of NICU babies should receive education on proper care techniques like safe holding positions, feeding, and developmental activities to support growth and bonding.
After discharge, regular follow-up is essential to monitor for any lingering or new issues requiring intervention.
Interventions to Aid Development
Early intervention services that may benefit small babies include:
- Physical therapy – For muscle tone, strength, and coordination
- Occupational therapy – To help with movement, feeding, and self-care skills
- Speech therapy – To aid communication development
- Counseling – For emotional, behavioral, or processing issues
- Education services – For cognitive, learning, or intellectual disabilities
- Nutritional supplements – To support healthy growth
Starting interventions as soon as possible optimizes outcomes for small infants.
Conclusion
Small babies are defined as those born weighing significantly less than the average newborn. Very low birth weights, especially those linked to extreme prematurity, raise health risks.
But modern neonatal intensive care has improved outcomes for many small babies. With comprehensive medical care, dedicated parents, and early intervention services, many go on to have typical development.
Careful monitoring and follow-up are essential to quickly identify and address any special needs or delays to ensure the best possible prognosis for small infants.