thumb|Low birth weight may be a result of [[preterm birth.]]
Low birth weight (LBW) is defined by the World Health Organization as a birth weight of an infant of or less, regardless of gestational age. LBW can be caused from a variety of things such as the infant simply being born at a low gestational weight (ie. preterm), environmental factors, or the health of the infant's mother while pregnant.
Birth weight may be classified as:
- High birth weight (macrosomia): greater than
- Normal weight (term delivery):
- Low birth weight: less than
- Very low birth weight (VLBW): less than
- Extremely low birth weight: less than
Causes
LBW is either caused by preterm birth (that is, a low gestational age at birth, commonly defined as younger than 37 weeks of gestation) or the infant being small for gestational age (that is, a slow prenatal growth rate), or a combination of both. Environmental risk factors include smoking, lead exposure, and other types of air pollution.
Preterm birth
Preterm birth could be spontaneous or medically-indicated in the form of prescribed cesarean sections to avoid complications from conditions such as pre-eclampsia.
Small for gestational age
Infants born small for gestational age may be constitutionally small, with no associated pathologic process. Others have intrauterine growth restriction (IUGR) due to any of various pathologic processes. Babies with chromosomal abnormalities or other congenital anomalies may manifest IUGR as part of their syndrome. Problems with the placenta can prevent it from providing adequate oxygen and nutrients to the fetus, resulting in growth restriction. Two significant placental pathologies include placental abruption and placental praevia. Infections during pregnancy that affect the fetus, such as rubella, cytomegalovirus, toxoplasmosis, and syphilis, may also affect the baby's weight.
Environmental factors
alt=a pregnant woman smoking |thumb|Smoking while pregnant doubles the chances an infant will be born with low birth weight.
Maternal tobacco smoking doubles risk of LBW for the infant. More recently, passive maternal smoking, commonly referred to as second hand smoke, has been examined for possible effects on birth weight, and has been shown to increase risk of LBW by 16%.
Air pollutants
The combustion products of solid fuel in developing countries can cause many adverse health issues in people. Because a majority of pregnant women in developing countries, where rate of LBW is high, are heavily exposed to indoor air pollution, increased relative risk translates into substantial population attributable risk of 21% of LBW.
Particulate matter, a component of ambient air pollution, is associated with increased risk of low birth weight. Because particulate matter is composed of extremely small particles, even nonvisible levels can be inhaled and present harm to the fetus. Particulate matter exposure can cause inflammation, oxidative stress, endocrine disruption, and impaired oxygen transport access to the placenta, all of which are mechanisms for heightening the risk of low birth weight. To reduce exposure to particulate matter, pregnant women can monitor the US Environmental Protection Agency's air quality index and take personal precautionary measures such as reducing outdoor activity on low quality days, avoiding high-traffic roads/intersections, and/or wearing personal protective equipment (i.e., facial mask of industrial design). Indoor exposure to particulate matter can also be reduced through adequate ventilation, as well as use of clean heating and cooking methods.
A correlation between maternal exposure to carbon monoxide (CO) and low birth weight has been reported that the effect on birth weight of increased ambient CO was as large as the effect of the mother smoking a pack of cigarettes per day during pregnancy.
It has been revealed that adverse reproductive effects (e.g., risk for LBW) were correlated with maternal exposure to CO emissions in Eastern Europe and North America.
Mercury is a known toxic heavy metal that can harm fetal growth and health, and there has been evidence showing that exposure to mercury (via consumption of large oily fish) during pregnancy may be related to higher risks of LBW in the offspring.
Other exposures
Elevated blood lead levels in pregnant women, even those well below the US Centers for Disease Control and Prevention's 10 ug/dL "level of concern", can cause miscarriage, premature birth, and LBW in the offspring. Exposure of pregnant women to airplane noise was found to be associated with low birth weight via adverse effects on fetal growth. Prevalence of low birth weight in Japan is associated with radiation doses from the Fukushima accidents of March 2011.
alt=a pregnant belly|thumb|There is a correlation between actions and stress of pregnant mothers and LBW.
Maternal Health
The health of the mother during pregnancy may act as a sole or confounding factor in the cause of LBW. Maternal health factors that may influence LBW include but are not limited to their nutritional status, smoking habits, their weight prior to becoming pregnant and the weight they gain while pregnant, as well as inadequate prenatal care such as failure to take supplements and attend routine appointments with an OBGYN. Specifically in regard to maternal weight, the less total gestational weight gain, or weight gained by the mother during the pregnancy, the higher the risk for pre term birth and low birth weight.
Parity
There have been multiple studies regarding maternal parity and LBW. Parity is the number of births or pregnancies carried to viability a woman has had. Women who are nulliparous, or who never delivered a fetus at viable gestational age, are at higher risk for having a LBW child as compared to primiparous or multiparous women. The risk of low birth weight can be reduced with treatment of the periodontal disease. This therapy is safe during pregnancy and reduces the inflammatory burden, thus decreasing risk for preterm birth and low birth weight.
Maternal Discrimination in the United States
In the United States, being African American is one of the major predictive factors in adverse birth outcomes like LBW and PTB. As compared to non-Hispanic white women, African American mother face over a 6% higher rate of LBW infants. While not statistically causal in nature, there is a consistent positive relationship between racial discrimination faced by the mother and LBW children. Warmed incubators in the NICU aid in thermoregulation for LBW infants.
Fluid and electrolyte balance
Frequent clinical monitoring of volume status and checking of serum electrolytes (up to three times daily) is appropriate to prevent dehydration, fluid overload, and electrolyte imbalance. VLBW newborns have an increased body surface to weight ratio, increasing risk for insensible fluid losses and dehydration. Humidified incubators and skin emollients can lessen insensible fluid loss in VLBW newborns. Donor human milk and maternal expressed breast milk are both associated with this benefit. One drawback of human milk is the imprecision in its calorie content. The fat content in human milk varies greatly among women; therefore, the energy content of human milk cannot be known as precisely as formula. ELBW newborns may require as much as 4 g/kg/day of protein.
Prognosis
Perinatal outcomes
thumb|Low birth weight infants may require respiratory support such as [[intubation and mechanical ventilation due to lung immaturity.]]LBW is closely associated with fetal and perinatal mortality and morbidity, inhibited growth and cognitive development, and chronic diseases later in life. At the population level, the proportion of babies with a LBW is an indicator of a multifaceted public-health problem that includes long-term maternal malnutrition, ill health, hard work and poor health care in pregnancy. On an individual basis, LBW is an important predictor of newborn health and survival and is associated with higher risk of infant and childhood mortality.
Low birth weight constitutes as sixty to eighty percent of the infant mortality rate in developing countries. Infant mortality due to low birth weight is usually directly causal, stemming from other medical complications such as preterm birth, PPROM, poor maternal nutritional status, lack of prenatal care, maternal sickness during pregnancy, and an unhygienic home environment.
Long-term outcomes
Hyponatremia in the newborn period is associated with neurodevelopmental conditions such as spastic cerebral palsy and sensorineural hearing loss. Rapid correction of hyponatremia (faster than 0.4 mEq/L/hour) perinatally is also associated with neurodevelopmental adverse effects. Additionally, low birth weight has associations with cardiovascular diseases later in life, especially in cases of large increases in weight during childhood. For men, higher birth weight has strong associations with increased risk of cancer deaths. These findings are continued to be investigated, but remain a driving force for future scientific investigations. Aggregate prevalence of LBW in United Nations-designated Least Developed Countries is 13%.
In the United States, the Centers for Disease Control and Prevention (CDC) reports 313,752 LBW infants in 2018, for a prevalence of 8.28%. This is increased from an estimated 6.1% prevalence in 2011 by the Agency for Healthcare Research and Quality (AHRQ). The CDC reported prevalence of VLBW at 1.38% in 2018, similar to the 2011 AHRQ estimate.