The causes of low birth weight babies are quickly being uncovered. We will focus on what be believe to be the most important causes: the genetic background of the mother and the baby, the age of the mother, her nutrition level, her access to prenatal care, and whether or not she smokes. These factors are considered in order of their amount of internal or biological causes (i.e. genetics is entirely biological and unchangeable whereas smoking is entirely environmental). These primary determining factors are the focus of much attention both nationally and internationally. Each one by itself can cause low birth weight, but they almost always happen together in predictable ways (see SECONDARY FACTORS).

• Genetics
• Age
• Nutrition
• Prenatal care
• Smoking

Detailed national natality data are used to investigate social, demographic, and genetic effects on birth weight differentials. It suggests that recognition of racial differences in risk factors and exposure-relationships may be valuable in specifying interventions for intrauterine growth retardation among different racial groups. Analysis of birth weight differences among infants with white, black, and mixed-black-white parents indicates that a portion of the observed weight differentials appear to be due to biological factors. Infants with a black mother and father have the lowest mean birth weights, while infants born to two white parents have the highest weights. Newborns with mixed-race parents have intermediate birth weight distributions. Multivariate analysis suggests that the effects of parental race on birth weight are not the result of maternal/obstetric differences among parents of the same or mixed race. Similar studies by the Northwestern University Medical School on African, West Indian, and European white infants indicate that the black race is a risk factor for very low birth weight in the United Kingdom as well. Thus, these are widespread genetic differences.

Unidentified factors closely related to maternal race underlie the black-white disparity in infant birth weight. Paternal and consequent infant race does not affect the birth weight distribution of those born to white mothers and black fathers. Group differences were more strongly related to the mother's race than to the father's race, and the trends were related to the mother's race. Maternal factors appear to be more important for abnormal outcomes in birth weight.

Johns Hopkins University Department of Maternal and Child Health performed a study that investigated racial differences in gestational age-specific birth weight in a sample of 21,288 Chinese, 11,882 Japanese and 65,818 white livebirths. The mean birth-weights of the Chinese and Japanese as compared to that of white infants were 4-5% lower than preterm births, and 5-6% lower among term births, after adjustment was made for gestational age, demographic variables, use of antenatal care and infant gender. The racial differences in gestational age-specific birth weight should be considered in both clinical evaluation of newborns and in epidemiological studies. Significant interaction was found between race and such maternal variables as education, marital status, birthplace, and month during which antenatal care began.

The Division of Nutrition at the Center for Disease Control has completed several studies of race-specific variation in birth weight. Results show that when controlled for sociodemographic background, there is great variance in birth weight between Chinese infants and white infants. More white infants had a larger birth weight while Chinese infants have a higher incidence of low birth weight. Similar incidence of low birth weight with different birth weight distribution was found among infants born in the United States to two Chinese parents, to one Chinese parent and one white parent, and to two white parents. The possibility of race-specific influences on birth weight distribution is suggested by these findings. (see THE AFRICAN AMERICAN PUZZLE)

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II. Age

The age of the mother is also a factor in birth weight. When a woman is between the ages of 18 and 35, she is in the prime of her childbearing years and is more likely to conceive a healthy child. The incidence of low birthweight is higher among mothers under the age of 18 or over the age of 35. For these two age groups, especially pregnant women under the age of 15 or over the age of 40, their uterus does not sustain pregnancy as well as during the prime childbearing years.

These complications arise because the human organism is just not organized for women to bear children. The onset of puberty is the primary determinant for the possibility of pregnancy. After this event, it takes the female body a few cycles to begin to produce an environment conducive to pregnancy. Young mothers, sometimes as young as 13, have a much greater frequency of low birth weight babies. The opposing process to puberty is, of course, menopause. This usually happens around the ages of 45-55. Menopause changes the hormone levels required for maintaining the uterine environment. For these reasons, older women have a higher chance of giving birth to low birth weight babies.

Factors other than biological health come into play when we are talking about the age of mothers. These relationships are developed in the secondary factors section of this site.

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An adequate supply of nutrients is essential for the normal progression of healthy fetal growth and development. Studies on low birthweight and prematurity, as well as neural tube defects and other non-genetic congenital abnormalities, have shown that nutritional status is extremely important from around the time of conception and throughout pregnancy (Doyle 1).

Along with extra calorie requirements, pregnant women need to eat a wide variety of foods, with a focus on vitamin and mineral intake. Women who received nutritional supplements or sufficient nutrients through diet gave birth to babies who were heavier and had lower rates of congenital malformations (Bonifield 448). One study showed that women who received intensive nutritional counseling with each prenatal visit gave birth to babies weighing 100 grams more than a group of women who received only one short nutrition class in the beginning of their prenatal care (Bonifield 448).

Several factors that can help identify nutritional risk in a pregnancy include: adolescence, anemia, abnormal pre-pregnancy weight, multiple gestation, medical illness or medication that interferes with absorption of vitamins and minerals, cigarette smoking, alcohol abuse, and low socioeconomic status (Kolasa & Weismiller 205). By screening pregnant women for these factors, nutritional risks can be identified early on and dealt with in an effective manner.

One of the most predominant causes of low birthweight is the mother's access to prenatal care. The chances of having a low-birthweight baby are substantially higher for women who do not receive prenatal care. Studies have found that even after adjusting for other differences like socioeconomic status and maternal age, infants born to mothers who received no prenatal care weighed considerably less, on average, than those whose mothers received prenatal care (Henderson 28).

Prenatal care programs provide nutritional counseling, careful monitoring of maternal weight gain, screening for genetic or behavioral risk factors, and emotional support for pregnant women. Prenatal care programs with a focus on preventing premature delivery have been shown to lower the incidence of low birth weight among women of all ages. For example, early prenatal care in the first trimester (the first three months) among white teenagers showed a 27 percent reduction in low-weight births (Henderson 21).

Although prenatal care cannot control for socioeconomic status and environmental differences that result in poor birth outcome, it has proven its worth in identifying the factors that affect birth outcome, such as cigarette smoking, alcohol consumption, drug use, and poor diet. Once these confounding factors have been identified, they can be reduced or eliminated through careful prenatal counseling (Henderson 30).

Different ethnic groups show varying degrees of prenatal care utilization, with 76 percent of all women seeking prenatal care within the first trimester, and only 61 percent black and Hispanic women seeking prenatal care during the same time period. Underutilization of prenatal care is often attributed to poor socioeconomic statuses like inability to pay for prenatal care, a lack of knowledge in the importance of prenatal care, and inadequate location and availability of prenatal care providers.

Despite the clear benefits of prenatal care, the United States is one of the few industrialized countries that does not yet provide universal healthcare for pregnant women (Bonifield 443). Unfortunately, women in low socioeconomic statuses who need prenatal care the most, are the least likely to get it (Berger 137).

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V. Smoking

Tobacco smoking is associated with adverse pregnancy outcomes because smoking during pregnancy harms both the mother and her baby. Prenatal smoking is thought to account for an estimated 20 to 30 percent of cases of low birth weight (<2500g), and also increases the risk of shortened gestation, respiratory distress syndrome, and sudden infant death syndrome. Babies born with lower-than-average birth weight are more likely to get infections and have other health problems.

Cigarette smoking is the principal cause of low birth weight in developed countries. Intrauterine growth retardation is the most strongly documented adverse effect of smoking during pregnancy. Smokers inhale nicotine and carbon monoxide, which reach the baby through the placenta and prevent the fetus from getting nutrients and oxygen needed to grow. Retarded fetal growth in the offspring of smokers may be attributed to many factors, including the vasoconstriction properties of nicotine, elevated fetal carboxyhemoglobin levels, fetal tissue hypoxia, reduced delivery of nutritional elements, and elevation of heart rate and blood pressure. Even after controlling for alcohol use, socioeconomic status, maternal height, maternal weight and years of education, smoking has been implicated in long term effects such as poor cognitive performance on achievement tests and decreased physical growth.

Second hand smoke also adds a risk to pregnancy. Exposure to environmental tobacco smoke (passive smoking) may have a modest adverse effect on birth weight. Breast milk often contains whatever is in the mother's body. If the woman smokes, the baby ingests the nicotine in her breast milk.

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