Birth weight

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.
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:
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: