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What causes low birth weights? 8 possible conditions

What Is Infant Low Birth Weight?

Infant low birth weight (LBW) is when babies weigh less than five pounds, eight ounces at birth. LBW often occurs in babies who are born prematurely, before 37 weeks of gestation. It is also common in multiple birth situations. According to the Lucile Packard Children’s Hospital at Stanford, about 7 percent of all babies born in the U.S. each year have LBW. The number is increasing, potentially due to the fact that multiple births are increasing. Developing countries have much higher incidences of infants with LBW.

The average birth weight in the U.S. is around seven pounds (LPCH). Babies born with LBW appear smaller than normal newborns. They are usually thin, have minimal body fat, and have disproportionately large heads.

What Causes Infant Low Birth Weight?

Low birth weight is primarily caused by premature birth. Since babies grow a lot in the later stages of pregnancy, many babies born before the 37th week are small, or have LBW.

LBW may also be caused by:

  • problems with the placenta, or intrauterine growth restriction (IUGR)
  • complications with the pregnancy
  • not enough weight gain by the mother
  • birth defects

Poor maternal nutrition, incomplete prenatal care, or drug or alcohol abuse by the mother can also cause LBW.

In the U.S., babies born to African-American mothers or very young mothers (under 15 years old) are more at risk (Child Health USA). Babies who are part of multiple births are also more likely to be born with LBW.

What Complications Does Infant Low Birth Weight Cause?

Babies born with low birth weight have a higher risk of developmental difficulties, health complications, and death than babies born at a normal weight. These babies are often weaker than babies with normal birth weight.

LBW babies often have trouble eating, gaining weight, staying warm, and warding off illness and infection. Some common health complications of LBW infants include respiratory problems, underdeveloped organs (such as lungs), eye or ear complications, digestive problems, neurological problems, and sudden infant death syndrome (SIDS). The lower the birth weight, the greater the risk of complications.

How Is Infant Low Birth Weight Diagnosed?

Low birth weight is determined by weighing the baby at birth. If the baby weighs less than five pounds, eight ounces, he or she will be diagnosed with LBS. Very low birth weight (VLBW) is the diagnosis for babies weighing less than three pounds, five ounces. 

Doctors monitor the approximate size and weight of the baby throughout prenatal care. This helps to identify a potential LBW situation early on. Ultrasound technology measures the baby while in utero.

How Is Infant Low Birth Weight Treated?

Treatment for low birth weight depends on the unique situation of the baby and mother. Babies born with LBW often need to stay in the hospital until they gain enough weight. Babies with other complications, such as underdeveloped lungs or intestinal problems, will stay in the hospital until their complications have improved through medical care. LBW infants are often cared for in the neonatal intensive care unit (NICU), using special temperature-controlled beds and feeding techniques.

The World Health Organization advises that low birth weight infants should be fed their mother’s milk whenever possible. This aids growth and weight gain (WHO). If the mother’s milk isn’t available, human donor milk is best. Formula is considered a last resort for nutrition.

What Is the Outlook For Infant Low Birth Weight?

Infants born with low birth weight but no other complications often grow up normally. In some cases, developmental delays, minor mental disabilities, or health problems will persist throughout life, with varying degrees of severity.

Prognosis for LBW infants with other complications depends on what health challenges they face. Advances in medicine have made it more likely that babies with low birth weight and related complications will survive. The lower the birth weight, the higher the chance of mortality.


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See a list of possible causes in order from the most common to the least.


Nutritional Deficiencies (Malnutrition)

A nutritional deficiency occurs when the body doesn't absorb the necessary amount of a nutrient. Deficiencies can lead to a variety of health problems.

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Stress And Anxiety

Stress can be triggered by an event that makes you feel frustrated or nervous, such as moving, death, starting a new job, and having a baby. Long-term stress can produce both physical and psychological symptoms.

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Fetal Alcohol Syndrome

Fetal alcohol syndrome (FAS) occurs when a baby is born with varied mental and physical defects due to the mother consuming alcohol during pregnancy.

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Fibroids are abnormal growths that develop in or on a woman's uterus. Heavy bleeding, pain in the pelvis or lower back, cramping, and bloating could indicate fibroids.

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Birth Defects

According to the Centers for Disease Control and Prevention, one out of every 33 babies in the United States is born with some type of abnormality that developed in utero (CDC). Birth defects can be minor or severe, an...

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Congenital Toxoplasmosis

Congenital toxoplasmosis is a disease that occurs in fetuses infected with Toxoplasma gondii (a protozoan parasite). It can cause miscarriage or stillbirth. It can also cause serious and progressive visual, hearing...

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Premature Infant

Birth is considered premature, or preterm, when it occurs before 37 weeks of gestation (development time in the womb). Normal, full-term gestation is 37 to 42 weeks. Those final weeks in the womb are crucial for health...

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Fanconi Anemia

Fanconi anemia or FA is a genetic disorder that results in bone marrow failure. It is a recessive gene disorder. This means that in order for you to develop FA, your parents will both have to have the defective FA gene...

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This feature is for informational purposes only and should not be used to diagnose.
Please consult a healthcare professional if you have health concerns.