Maternal Smoking And Other Factors In Low Birth Weight

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Written and compiled by Wanda Hamilton

The scientific evidence associating maternal smoking during pregnancy with low birth weight in neonates is substantial. However, even in this heavily researched area, many discrepancies exist among the studies. Some studies have found that babies born to smoking mothers aren't on average lower in birth weight than those born to nonsmokers when such confounders as socio-economic class, excessive use of alcohol and/or coffee during pregnancy, marital status, maternal age, maternal weight gain during pregnancy, maternal nutrition, maternal body mass, and other risk factors are taken into account.

Premature birth is far and away the greatest cause of low (less than 5.5 lbs) and very low (less than 3 lbs.) birthweight in babies, and it is also the single greatest cause of neonatal fatalities. Most leading scientists agree that maternal smoking is not associated with preterm birth of infants, very low birthweight, stillbirth, or neonatal fatality. Further, though the babies of smoking mothers may be smaller on average than those of non-smokers, they have a lower incidence of respiratory distress syndrome and a better survival rate.

"Ounce for ounce, [Dr. Allen Wilcox] said, 'the babies of smoking mothers had a higher survival rate.' As he explained this paradoxical finding, although smoking interferes with weight gain, it does not shorten pregnancy. Thus, among smoking women the smaller babies are more likely to be born full-term, but the smaller babies born to nonsmoking mothers are more likely to be born prematurely. So, he deduced, it is their prematurity, not their low birthweight, that explains the higher infant mortality rate among babies of low birth weight who are born to nonsmokers," ("High Infant Mortality in U.S. Is Linked to Premature Births," Jane E. Brody, New York Times, Mar 1, l995). Dr. Wilcox, a leading researcher in the field, is with the National Institute of Environmental Health Sciences.

Overall, the scientific evidence leaves little doubt that the single greatest risk factor for preterm birth and low and very low birthweight is low socio-economic class. Thus, in the U.S., which has both a higher rate of neonatal fatality and a higher rate of low and very low birthweight than nearly every other industrially developed country in the world, black women have by far greatest risk of having low and very low birthweight babies even though far fewer of them smoke than do white or Native American women (Source: The National Center for Health Statistics).

The U.S. rate of low and very low birthweight in babies has grown steadily since l980, despite a steady drop in the rate of maternal smoking during the same time period. In l980, the rate of low birthweight was 6.8%, rising to 7.0 by l989, to 7.2 by l993 and to 7.3 in l995. In l989 19.5% of mothers smoked during pregnancy, but by l993 the rate had dropped to 15.8%, and by l995 it had dropped to only 14%. (Source: The National Center for Health Statistics).

By contrast, in Sweden, where 24% of women smoke and 70% of them continue to smoke during pregnancy, the rate of low birthweight babies is only 4.5%.

Obviously something other than maternal smoking is causing more and more American babies to be born with low and very low birthweights. Not only is this a major health problem for our infants, but the social costs and drain on our medical resources are staggering.

"Adjusted odds ratios for significant risk factors for LBW are given for 9084 singleton births. . . .Risk of LBW is associated with maternal primiparity [first pregnancy], age 35 years or more in primiparous women, history of one or more spontaneous abortions, induced abortions or perinatal deaths, chronic illness, public health insurance and single marital status, and with fetal female sex and congenital anomalies." Maternal smoking was not listed as a risk factor in this study.

Herceg A, Simpson JM, Thompson JF, "Risk factors and outcomes associated with low birthweight delivery in the Australian Capital Territory 1989-90," National Centre for Epidemiology and Population Health, Australian National University, Journal of Paediatric Chld Health, Aug l994; 30(4):331-335.

"Black race, female sex, cigarette smoking, drug use, having a previous low birthweight infant, maternal hypertension and being short or thin or failing to gain weight each resulted in a birthweight decrease of 100 to 300 g [ca. 3 to 10 ounces]."

Goldenberg RL, Cliver SP, Neggers Y, Copper RL, DuBard MD, Davis RO, Hoffman HJ, "The relationship between maternal characteristics and fetal and neonatal anthropometric measurements in women delivering at term: a summary," Acta Obstet Gynecol Scand Suppl, l997; 165:8-13.

"Women who smoke during pregnancy have full-term babies which, on the average are 5 - 6 grams [a fraction of an ounce] smaller than full-term babies born to nonsmoking mothers"
Moore LC, University of Colorado, Health Sciences Center, "Maternal O2 Transport During Pregnancy at High Altitude," [CDC Bibliography on Smoking and Health for l984-85].

"High altitude acts independently from other factors to reduce birthweight and accounts for Colorado's high rate of low birthweight."

Jensen GM, Moore LG, "The effect of high altitude and other risk factors on birthweight: independent or interactive effects?" Am J Public Health, Jun l997; 87(6):1003-1007.

"Increasing maternal age at first childbirth is an independent risk factor for low birth weight and preterm delivery of white infants in the United States."

Aldous MB, Edmonson MB, "Maternal age at first childbirth and risk of low birthweight and preterm delivery in Washington State," Journal of the American Medical Association, Dec 1 l993; 270(21):2574-2577.

"Mothers with unhealthy gums are at higher risk of giving birth to infants with low-birth weight, says a study by the School of

Dentistry of the University of Alabama at Birmingham." (Reported in the Miami Herald, 4/5/96, p. 2F).

Pregnant women receiving care from doctors who aren't board certified in obstetrics or who deliver fewer than 25 babies a year are also at higher risk for delivering low birthweight babies, according to a study by Harvard University researchers who analyzed more than 80,000 births. (Reported in the Miami Herald, 8/27/95).

Poor nutrition. "Other factors reported to compromise infant outcome such as ethnic background, income status, age and smoking did not significantly affect infant birth weight."

Bruce L, Tchabo JG, "Nutrition intervention program in a prenatal clinic," Obstetrics and Gynecology, Sep l989, 74(3 Pt. 1):310-12.

"These data suggest that smokers in all social classes have a poorer quality of diet."

Haste F.M, Brooke OG, anderson HR, et al, "Nutrient intakes during pregnancy:observations on the influence of smoking and social class," American Journal of Clinical Nutrition, Jan l990; 51(1):29-36. [CDC bib, l990]

"A significant reduction in birth weight was found to be associated with an average caffeine intake of more than or equal to 71 mg per day, after adjustment for gestational age, infant sex, parity, and maternal height and weight, but only in infants born to nonsmoking mothers."

Vlajinac HD, Petrovic RR, Marinkovic JM, Sipetic SB, and Adanja BJ, "Effect of Caffeine Intake During Pregnancy on Birth Weight," American Journal of Epidemiology, l997; 145:335-8.

"A larger number of low birthweight children were observed in nonsmoking women of the working class than among women smokers of the middle class."

da Silva AA, Gomes UA, Bettiol H, Dal Bo CM, Mucillo G, Barbieri MA, "Correlation between maternal age, social class and smoking, and birth weight," Rev Saude Publica, Jun l992; 26(3):150-154.

"The lowest birth weights were found among babies born to unskilled and unemployed women even after adjustment for smoking habits, prepregnancy height and weight and a number of other potential confounders. . . .The study shows that socioeconomic status is especially a risk factor for low birth weight."

Olsen J, Frische G, "Low birth weight, stillbirth and congenital malformations. Social differences." Ugeskr Laeger, Sep 19 l994; 156(38):5519-5523.

"White infants were heavier and born later than black infants. The white women in this sample smoked more cigarettes, moved more frequently, and had worse psychosocial scores. The black women had lower incomes, were less likely to be married and had more hypertension, anemia and diabetes."

Goldenberg RL, Cliver SP, Mulvihill FX, Hickey CA, Hoffman HJ, Klerman LV, Johnson MJ, "Medical, psychosocial, and behavioral risk factors do not explain the increased risk for low birth weight among black women," American Journal of Obstetrics and Gynecology, Nov l996;175(5):1317-1324.

". . . the rate of LBW [low birth weight] is twice as high and the rate of VLBW [very low birth weight] is three times as high for black infants compared to white infants."

Luke B, Williams C, Minogue J, Keith L, "The changing pattern of infant mortality in the US: the role of prenatal factors and their obstetrical implications," International Journal of Gynaecology and Obstetrics, Mar l993; 40(3):199-212.

"The deficits of weight at birth in children born to mothers who smoked during pregnancy are overcome by 6 months of age. These deficits are probably not permanent when smoking habit during pregnancy is not associated with other unfavourable variables (such as lower socioeconomic class)."

Conter V, Cortinovis I, Rogari P, Riva L, "Weight growth in infants born to mothers who smoked during pregnancy," British Medical Journal, Mar 25 l995; 310(6982):768-771.

"Lower socioeconomic status (SES) is probably the most powerful single contributor to premature morbidity and mortality, not only in the United States but worldwide."

Redford B. Williams, MD, "Lower Socioeconomic Status and Increased Mortality," Editorial, Journal of the American Medical Association, June 3 l998; 279(21): 1745-6.

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