In 30-35%, the cause is not known.

Some causes arise with the baby. This is most likely true if the baby is abnormal with one or more severe problems in the development of organs.

Infection of the fetus weeks to months before birth, called congenital infection.

Abnormalities in the blood vessels of the mother and/or placenta or diseases which limit the amount of oxygen and nutrients that get to the baby. These include mothers with:

Very small parents often have small babies

Drugs, heavy smoking, moderate to heavy drinking and very poor nutrition

Twins, triplets and other multiples

These babies have little reserve of energy and oxygen. They may be more stressed with labor and delivery. If they do not tolerate labor well, a caesarean section (delivery by surgery) may be needed.

They may have their first stool, called meconium, before birth. If stool is taken into the lungs with the first breath, it can cause pneumonia, called meconium aspiration.

They may have more trouble with delivery and need more help in breathing in the delivery room.

Low blood sugar. They lack the stored energy reserves such as fat and sugar to help keep their blood sugar normal.

High number of red blood cells. Red blood cells carry oxygen. These babies made more blood cells before birth to carry more oxygen. If the red blood cell count is very high, called polycythemia, it may make the blood too thick to easily flow through the smallest blood vessels.

High bilirubin This is called jaundice. It comes from the normal breakdown of red blood cells.

Lung problems. These are most likely if the infant is premature or if the baby has passed stool (meconium) and inhaled it before birth causing pneumonia in the lung. Pneumothorax and pneumomediastinum can also occur.

Primary pulmonary hypertension or persistent fetal circulation. When a baby is in the uterus, most of the blood by-passes the lungs because the mother and placenta control the oxygen and carbon dioxide for the fetus. At delivery the baby must increase the amount of blood flowing to the lungs. If this does not happen normally, the baby has persistence of the fetal circulation.

Keeping warm. They don't have stores of fat and sugar to use to keep themselves warm.

Increased risk of infection after birth.

If the baby is delivered early, s/he can have all the common problems of preemies.

If a baby is low in weight but has a normal length and head size, the baby will usually catch up in growth over the next few months or years.

If the baby is small because both parents are small, the child will continue to be small like the parents.

If the baby is also short in length and has a small head size, there may be some catch up, but usually growth will remain less than normal and the child will be smaller than expected for the family.

If a baby is low in weight but has a normal length and head size, the baby usually develops normally.

If the baby is small because both parents are small, the child usually develops normally

If the baby has small head size and length at birth (<10%) in addition to low weight, there may be problems in development. Minor abnormalities are common. These may appear slowly, be difficult to detect, or may not be obvious until preschool or grade school. They can include:

Major problems are less common. Major problems in development include:

Infants at highest risk for major problems are:

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