Birth weight is the body weight of a baby at its birth. The average birth weight in babies of European heritage is 3.5 kilograms (7.7 lb), though the range of normal is between 2.5 kilograms (5.5 lb) and 4.5 kilograms (9.9 lb) (all but 5% of newborns will fall into this range). Babies of south Asian and Chinese heritage weigh about 240 grams (0.53 lb) less.
There have been numerous studies that have attempted, with varying degrees of success, to show links between birth weight and later-life conditions, including diabetes, obesity, tobacco smoking and intelligence. Low birth weight is associated with neonatal infection and infant mortality.
There are basically two distinct determinants for birth weight:
- The duration of gestation prior to birth, that is, the gestational age at which the child is born
- The prenatal growth rate, generally measured in relation to what weight is expected for any gestational age.
The incidence of birth weight being outside what is normal is influenced by the parents in numerous ways.
The health of the mother, particularly during the pregnancy. Intercurrent diseases in pregnancy are sometimes associated with decreased birth weight. For example, Celiac disease confers an odds ratio of low birth weight of approximately 1.8. Stressful events have been demonstrated to produce significant effects on birth weight. Those mothers who have stressful events during pregnancy, especially during the first and second trimester, are at higher risk to deliver low-birth weight babies.  Researchers furthered this study and found that maternal stressful events that occur prior to conception have a negative impact on birth weight as well, and can result in a higher risk for preterm and lower birth weight babies..   Women who experienced abuse (physical, sexual, or emotional) during pregnancy are also at increased risk of delivering a low-birth weight baby.  For example, in a study completed by Witt et. al, those women who experienced a stressful event (ie. dealth of close family member, infertility issues, separation from partner) prior to conception had 38 % more of a chance to have a very low birth weight baby compared to those who had not experienced a stressful life event. These findings can pose evidence for future prevention efforts for low birthweight babies. One way to decrease rates of low birth weight and premature delivery is to focus on the health of women prior to conception through reproductive education, screening and counseling regarding mental health issues and stress, and access to primary care.
Environmental and Social Factors
Environmental factors, including exposure of the mother to secondhand smoke Economic status of the parents gives inconsistent study findings according to a review on 2010, and remains speculative as a determinant. Other factors, like multiple births, where each baby is likely to be outside the AGA (appropriate for gestational age), one more so than the other.
- A low birth weight can be caused either by a preterm birth (low gestational age at birth) or of the infant being small for gestational age (slow prenatal growth rate), or a combination of both.
- A very large birth weight is usually caused by the infant having been large for gestational age.
Influence on adult life
A baby born small or large for gestational age (either of the two extremes) is thought to have an increased risk of obesity in later life, but it was also shown that this relationship is fully explained by maternal weight.
Growth hormone (GH) therapy at a certain dose induced catch-up of lean body mass (LBM). However percentage body fat decreased in the GH-treated subjects. Bone mineral density SDS measured by DEXA increased significantly in the GH-treated group compared to the untreated subjects, though there is much debate over whether or not SGA (small for gestational age) is significantly adverse to children to warrant inducing catch-up.
Some studies have shown a direct link between an increased birth weight and an increased intelligence quotient. Increased birth weight is also linked to greater risk of developing autism.
Poor neonatal care
Recent evidence suggests that the effects of low birth weight are constant across developmental years, suggesting that poor neonatal care has long term impacts.
- Barker's hypothesis
- MOMO syndrome
- Low birth weight paradox
- Prenatal nutrition and birth weight
- Infant Mortality
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