Birth weight is the weight of a baby at birth. The average birth weight in babies of Europeans is ; the range of normal weights is from   . On average, babies of south Asian and Chinese people weigh about . The birth weight of a baby is important.  Babies with a low birth weight are 100 times more likely to die compared to babies with a normal one. The rates of babies with low birth weight have changed: In 1970, 7.9% had a low birth weight, in 1980, this dropped to 6.8%, in 2006, it slightly increased to 8.3% in 2006; and the current level is at 8.2%, for the year 2016. The prevalence of low birth weight has trended slightly upward from 2012 to present day.

Studies have tried to link birth weight with other conditions that occur in later life, such as diabetes, obesity, tobacco smoking, and intelligence. Low birth weight is associated neonatal infection and infant mortality.

Causes 

 Babies born early usually have a low bith weight. This is also the case, for babies who are small; very often, both of these factors can be found together. 
 Other causes of a low bith weight are that the mother  had health problems,  there were some genetic factors, or problems in the placenta.
 Babies who are large usually have a high birth weight. Babies who are large for their age also have a higher risk of dying as infants.

Things that influence birth weight

Genetics 
To some extent, birth weight is heritable: If their parents had a low birth weight the babies may also have one. (This is the same for a high birth weight).  The heritability of birth weight ranges from 25-40 %. There is a complex relationship between a baby's genes and the maternal environment that the child is developing in. Foetal genes influence how the fetus grows in the womb, and the genes of the mother influence how the environment affects the growing fetus.

Health of the mother 
The health of the mother during pregnancy can affect birth weight. The fact that the mother is sick during the pregnancy, or that she has a chronic condition can affect birth weight. Usually, this will lead to a lower bith weight. For example, celiac disease confers an odds ratio of low birth weight of approximately 1.8. Certain drugs, such as those used to treat high blood pressure or epilepsy can also have this effect.  Other factors that affect birth weight are that if the mother is going to give birth to twins, which is called multiple births in English. Another factor that affects birth weight is the age of the mother: Girls 15 or younger, or women over 35 have a higher risk of having a child with a low birth weight.

Stress 
Stressful events have significant effects on birth weight. Mothers who are exposed to stressful events during their pregnancy have a higher risk to give birth to babies with a low birth weight. This is especially true if the events happen during the first or the second trimester. Reserchers also found that stressful events that happen before conception can also have a negative effect. The same is true for women who were exposed to situations of abuse.

Environment 
The environment the woman lives in can also determine the weight of the baby. One of these factors is  secondhand smoke 

Environmental factors, including exposure of the mother to secondhand smoke can be a factor in determining the birth weight of child. In 2014, 13% of children exposed to smoke were born with low birth weight compared with 7.5% of those children born to nonsmokers.  Children born to mothers who smoked or were exposed to secondhand smoke are more likely to develop health problems earlier in life such as neurodevelopmental delays. When mothers actively smoke during pregnancy, their child is at a higher risk of being born with a low birth weight. Smoking can also be a stress management tool used by expecting mothers. There is some support for lower socioeconomic status of the parents being a determinant of low birth weight, but there is conflicting evidence, as socioeconomic status is tied to many other factors.

Neonatal care 
Most babies admitted to the intensive care unit are born before 37 weeks of pregnancy or have low birth weight which is less than 5.5 pounds. They could also have a medical condition that requires special care. In the United States nearly half a million babies are born preterm. Because of this, many of these babies also have low birth weights. There are four levels of care in the neonatal care units. Intensive Care, High Dependency Care, Low Dependency, and Transitional Care are the four levels:

 Intensive Care: For babies with serious problems. This includes babies born three months early and have extremely low birth weight.
 High Dependency Care: For babies with less serious problems, but who still may not to be looked after or babies that are recovering from a critical illness.
 Low Dependency Care: For babies that do not need a continuous supervision.
 Transitional Care: For babies that still need medical treatment, but are well enough to be called for at their mother’s bedside.

Influence on the first few years of life 

Children born with an very low birth weight can have significant problems during the first few years of life. They may have trouble gaining weight, obtaining adequate nutrition, and supporting a strong immune system. They also have higher risks of dying, behavior problems, and mental deficiencies. Low birth weight babies are more likely to develop the following conditions after birth compared to normal birth weight babies:

 Breathing problems (infant respiratory distress syndrome)
 Bleeding in the brain (intraventricular hemorrhage)
 Patent ductus arteriosus (PDA)
 Necrotizing enterocolitis
 Retinopathy of prematurity
 Jaundice
 Infections

See also 
 Infant mortality

References

Further reading

Other websites 

 
 Fetal Growth Restriction at eMedicine
Pregnancy and childbirth
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