Research

Maternal Smoking, Premature Birth And Neonatal Death

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Written and compiled by Wanda Hamilton

The United States has one of the highest neonatal death rates in the industrialized world. Though we spend more per capita on health than any other country, our newborn fatality rate is much higher than in other developed western nations.

Congential abnormalities are responsible for the greatest percentage of stillbirths and neonatal deaths in the U.S. However, according to medical experts, our rates of congenital abnormalities are not excessive compared to the rates in other countries. Instead, it is our higher rate of premature birth, which is responsible for the excess in neonatal deaths. Preterm birth is estimated to occur in 10% of all births in the U.S. and is associated with 83% of perinatal deaths not caused by congenital anomalies (Wheeler, D G, "Preterm birth prevention," Journal of Nurse Midwifery, Mar l994; 39(2 Suppl): 66S-80S).

One thing is clear: Maternal smoking during pregnancy is not the cause of the high rate of preterm births and neonatal fatalities in the United States.

While our rate of maternal smoking during pregnancy has dropped steadily to a low of 14% in l995, our rate of preterm birth has risen by 17% since l981, with an estimated incidence of 11% of all births in l995. Further, our rate of maternal smoking is much lower than in most other developed nations. In Sweden, for example, though 24% of females smoke and 70% of those continue to smoke during pregnancy, the neonatal death rate is only 5.4 per thousand, compared with the U.S. rate of 8.8 per thousand.

Moreover, maternal smoking during pregnancy is not associated with either congenital defects nor preterm births. In fact, the preterm and low birthweight infants of maternal smokers have a better survival rate than those born to nonsmokers. This phenomenon is primarily due to the lower incidence of infant respiratory distress syndrome [RDS] among the babies of smokers. RDS is a frequent cause of death for premature babies.

Precisely what causes our high rate of premature births is not known, though placental infection, poverty and, of course, congenital malformations are the factors most frequently cited in the medical literature.

Very premature birth (prior to 30 weeks gestation) results not only in our excessive rates of neonatal mortality and morbidity but is also linked to increased health problems throughout childhood It is well established, for example, that premature birth is associated with increased incidence of allergies and asthma and with problems in cognitive and physical development.

Increasingly researchers believe that our public health dollars would be better spent on prenatal campaigns to prevent excessive rates of preterm birth, rather than on the prevention of low birth weight and maternal smoking, in order to lower our tragically high rate of neonatal deaths.

NOTES

Study quotes are from author abstracts and most may be found on PubMed. "In l986, there were 12 nations with lower infant death rates [than the U.S.]; now there are 28."

Parade Magazine(Sunday supplement) 4/13/97.

". . . the United States still lags behind most industrialized nations in infant mortality, and no change in the rate of low birthweight has occurred in recent decades. Multiple lines of evidence support a role for infection as an etiologic factor in preterm labor."

"Preterm birth complicates 8 - 10% of all pregnancies in the United States and is the leading cause of infant morbidity and mortality. Neonatal morbidity and mortality is concentrated among very low-birthweight and extremely premature infants, particularly those delivered prior to 30 weeks gestational age."

Andrews WW, Goldenberg RL, Hauth JC, "Preterm labor: emerging role of genital tract infections," Dept. of Obstetrics and Gynecology, University of Alabama, Infect Agents Dis, Dec l995;4(4):196-211.

"Higher mortality among US infants is in fact due entirely to a small excess of preterm deliveries. The lighter weights of US newborns at term appear not to affect perinatal survival. "The prevention of excess mortality among US infants depends on the prevention of preterm births, not on changes in mean birth weight."

Wilcox A, Skjaerven R, Buekens P, Kiely J, National Institute of Environmental Health Sciences, "Birth weight and perinatal mortality. A comparison of the United States and Norway," JAMA, Mar l995;273(9):709-711.

"Preterm births account for a substantial portion of infant mortality, the major difference in mortality between African-American and white births in the United States, and the key reason that US infant mortality exceeds that of other developed nations."

Hogue CJ, Hargraves MA, "Preterm birth in the African-American community," Semin Perinatol, Aug l995;19(4):255-262.

"Preterm birth has been identified by the National Commission to Prevent Infant Mortality (l988) as the primary cause of the increased infant mortality rate in the United States."

Graf RA, Perez-Woods R, "Trends in preterm labor," J Perinatol, Mar l992;12(1):51-58.

"Tobacco use during pregnancy has declined steadily since l989 [when statistics on maternal smoking during pregnancy began being collected by the federal government]. In l995, 14 percent of pregnant women smoked." ". . . there was no change for l995 in the rate of preterm births (less than 37 weeks completed gestation). In l995, 11.0 percent of births were preterm; this proportion has risen 17 percent since l981."

National Center for Health Statistics. "Report of Final Natality Statistics, l995," by Stephanie J. Ventura, Joyce A. Martin, et al, Monthly Vital Statistics Report, Vol 45, No. 11, Supplement. The report is based on 100 percent of the births registered in all States and the District of Columbia and is reported to the National Center for Health Statistics through the National Vital Statistics Cooperative Program.

"Sweden had the lowest reported infant mortality rate (IMR) among countries of the world from l920 until l980. Since l981, Sweden, Japan and Finland have shared this number one ranking on a somewhat rotating basis."

Wallace HM, Ericsson A, Bolander AM, Vienonen M, "Infant mortality in sweden and Findland: implications for the United States," J PerinatologyMar l990;10(1):3-11.

"About 30% of the Swedish pregnant population smoke in early pregnancy. Despite that practically all pregnant women in Sweden today are aware of the hazards of smoking, about 70% of the smokers continue to smoke during pregnancy."

Cnattingius S, "Smoking during pregnancy. Pregnancy risks and socio-demographic characteristics among pregnant smokers," International J Technol Assess Health Care, l992; 8 Suppl 1:91-95.

"Ounce for ounce, he [Dr. Wilcox of the National Institute of Environmental Health Sciences] said, 'the babies of smoking mothers had a higher survival rate.' This phenomenon is explained by the fact that while the babies of women who smoke during pregnancy weigh less on average than the babies of mothers who do not smoke, smoking does not shorten pregnancy or cause premature delivery."

"High Infant Mortality in U.S. is Linked to Premature Births," Jane E. Brody, The New York Times, 3/1/95.

"Gestation length was not associated with cigarette smoking in this sample."

Schell LM, Hodges DC, Am J Phys AnthropolDec l985;68(4);549-554.

"Maternal smoking did not have an adverse effect on the incidence of preterm labor except in twin pregnancies, for which early delivery was more likely."

Roberts WE, Morrison JC, Hamer C, Wiser WL, "The incidence of preterm labor and specific risk factors," Obstetrics and Gynecology, Jul l990; 76(1 Suppl):85S-89S.

". . . there was no indication of any SA [spontaneous abortion] effect resulting from active smoking. Presumably, active smoking women would have also been exposed to passive smoking; but such women showed no indication of an increase in risk."

Mantel N. "Re: Tobacco smoke exposure and pregnancy outcome among working women: a prospective study at prenatal care centers in Orebro County, Sweden," American Journal of Epidemiology, Apr 1, l992; 135(7):837-8.

"Psychosocial stressors and limited duration of schooling appeared to influence preterm delivery." But not maternal smoking.

Nordentoft M, Lou HC, Hansen D, et al, "Intrauterine growth retardation and premature delivery: the influence of maternal smoking and psychosocial factors," American Journal of Public Health, Mar l996; 86(3);347-354.

"Among women with an intake of less than 400mg of caffeine per day no difference in the risk of preterm birth between smokers and nonsmokers was found."

Wisborg K, Henriksen TB, Hedegaard M, Secher NJ, British J of Obstet Gynaecol, Aug l996; 103(8):800-805.

"Preterm labor was associated independently with young maternal age, low pre-pregnant weight, low weekly weight gain, nulliparity, previous preterm births, histories of two or more induced abortions, spontaneous abortions, or stillbirths, uterine exposure to diethylstilbestrol (DES), incompetent cervix, uterine anomaly, and pyelonephritis." It was not associated with maternal smoking in this study of 23 factors, including maternal smoking, on preterm and low birthweight babies.

Lang JM, Lieberman E, Cohen A. "A comparison of risk factors for preterm labor and term small-for-gestational-age birth," Dept. of Epidemiology and Biostatistics, Boston University School of Public Health, Epidemiology, Jul l996; 7(4):369-376.

"The paucity of associations is consistent with the conclusion of other investigators that most of the causes of preterm delivery are unknown." This cohort study of 1,825 enlisted service women who delivered from l987 through l990 at four U.S. Army medical centers found no association between maternal smoking and preterm delivery.

Adams MM, Sarno AP, Harlass FE, et al, Division of Reproductive Health, Centers for Disease Control, "Risk factors for preterm deliery in a healthy cohort," Epidemiology, Sep l995; 6(5):525-532.

"The greatest fraction of the incidence of prematurity among low-risk pregnancies was due to unknown factors associated with carrying a first live birth, regardless of preterm delivery mechanism. . . .Other than nulliparity, male sex of the fetus accounted for the greatest fraction of spontaneous labor-induced prematurity incidence, a maternal age greater than 30 years or a positive urine culture accounted for the greatest fraction of PROM-induced prematurity incidence. All other risk factors for either preterm labor or PROM [premature rupture of the membranes] accounted for less than 5% of the incidence."

Harlow BL, Frigoletto FD, Cramer DW, et al, Obstetrics and Gynecology Epidemiology Center, Brigham and Women's Hospital Harvard Medical School, "Determinants of preterm delivery in low-risk pregnancies. The RADIUS Study Group," J Clinical Epidemiology, Apr l996; 49(4):441-448.

"More than half of the increase in preterm births was caused by acute chorioamnionitis. Other factors that made major contributions to the overall mortality increase were rises in the frequencies of older gravidas (ages 35-50 years), gravidas who had diabetes mellitus, children who had major congenital malformations, and dizygous twins." Study showed that perinatal mortality rates progressively increased from thin to obese subjects. 56,857 pregnancies which produced offspring were analyzed.

Naeye RL, "Maternal body weight and pregnancy outcome," American J of Clinical Nutrition, Aug l990; 52(2):273-279.

". . . the respiratory distress syndrome had a lower frequency at very preterm gestational age in the neonates of smokers than of nonsmokers. The greater the number of cigarettes smoked, the lower were the frequency and the case fatality rate of the neonatal respiratory distress syndrome." More than 56,000 pregnancies which produced offspring were analyzed. Many other studies support this data.

Naeye, RL, Disorders of the Placenta, Fetus, and Neonate, CV Mosby, New York: l992, p.84.

"The wider social differences in US infant mortality are a consequence of less consistent and thorough attempts at social equity and universal health care. US Black infant mortality continues to be twice that of Whites. . . ."

Hogue CJ, Hargraves MA, "Class, race and infant mortality in the United States," American Journal of Public Health, Jan l993; 83(1):9-12.

"Current public health policies in the United States emphasize the prevention of low birthweight. The present analysis suggests that the prevention of early delivery would benefit babies of all birth weights."

Wilcox AJ, Skjaerven R, "Birth weight and perinatal mortality: the effect of gestational age," Epidemiology Branch, National Institute of Environmental Health Sciences, Am J Public Health, Mar l992; 82(3):378-382.


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