Effects of Maternal Cigarette Smoking on Birth Weight and Preterm Birth -- Ohio, 1989

In 1989, most states began using revised birth certificates that provide more detailed information about maternal behaviors during pregnancy and complications of pregnancy. The availability of information on cigarette smoking by mothers in Ohio permitted the Ohio Department of Health (ODH) to examine the proportion of low birth weight (LBW), very low birth weight (VLBW), and preterm births that were attributable to maternal cigarette smoking.

The ODH study included live infants born to Ohio resident mothers in Ohio hospitals from January 1 through June 30, 1989. The analysis was restricted to singleton infants of white (n=62,732) and black (n=11,407) mothers. Gestational age was imputed in the 12% of certificates for which a direct estimate from the date of the last menstrual period was not possible; calculations were based on both birth weight and months of completed gestation (1). An infant was classified as having LBW if the birth weight was less than 2500 g ( less than 5 lbs 8 oz), having VLBW if the birth weight was less than 1500 g ( less than 3 lbs 4 oz), and being born preterm if the gestational age was less than 37 weeks. The Ohio birth certificate includes these items: "Tobacco use during pregnancy" and "Average number of cigarettes per day."

Multiple logistic regression was used to control for factors that affect the risk for LBW and preterm delivery, including mother's educational attainment (a measure of socioeconomic status), age, race, prepregnancy weight, and weight gain and alcohol consumption during pregnancy; child's birth order; the month prenatal care began; and previous terminations of pregnancy.

Odds ratios (ORs) were estimated for LBW, VLBW, and preterm birth in relation to in utero exposure to maternal cigarette smoking; these ORs represent measures of the risk for these outcomes in women who smoked compared with nonsmoking women. These findings permitted estimation of the population-attributable risk percentage (PAR%) (i.e., the proportion of all LBW, VLBW, and preterm birth attributable to maternal smoking). The PAR% was approximated as (p X(OR-1)) X 100(p X (OR-1) + 1), where p is the proportion of women in the total population who smoke and OR is estimated in the multivariate model.

Overall, 23% of Ohio mothers were reported to have smoked during pregnancy; this prevalence did not vary by race. Among smokers, white women were more likely than black women (8.8% and 4.7%, respectively) to smoke more than one pack of cigarettes per day during pregnancy. The overall rate of LBW was 5.7%: for whites it was 4.8%; for blacks, 12.1% (Table 1). Overall rates of VLBW and preterm birth were approximately 2-3 times higher for blacks than for whites. Among whites, all three outcomes were more prevalent among younger women; among black women, variation by age group was limited.

Infants born to smokers were more than twice as likely to have LBW as were infants born to nonsmokers (Table 2). In addition, among women who smoked, risk for LBW increased by level of exposure: adjusted ORs were 1.8, 2.2, and 2.4 for light (less than one half pack per day), moderate (one half pack to one pack per day), and heavy smokers (more than one pack per day), respectively. Consumption of even less than 10 cigarettes per day appeared to double the risk for LBW. For both blacks and whites, the risk was directly proportionate to levels of smoking.

Maternal cigarette smoking also increased the risk for VLBW and preterm birth (Table 3). However, these risks were similar for light and heavy smokers.

An estimated 20% of all LBW in the total Ohio population (i.e., smokers and nonsmokers) in the 6-month period was attributable to maternal smoking (Table 3). Similarly, more than 8% of all VLBW and more than 6% of all preterm deliveries were attributable to smoking. For each of the three outcomes, adjusted ORs and PAR% were slightly lower for blacks than for whites. Reported by: RS Hopkins, MD, LE Tyler, MS, BK Mortensen, PhD, Div of Epidemiology and Toxicology, Bur of Preventive Medicine, Ohio Dept of Health. Pregnancy and Infant Health Br, Div of Reproductive Health, Center for Chronic Disease Prevention and Health Promotion; National Center for Health Statistics, CDC.

Editorial Note

Editorial Note: Smoking by mothers is an important preventable cause of adverse pregnancy outcome (2). In Ohio, the deleterious effects of cigarette smoking by mothers during pregnancy on the rates of LBW, VLBW, and preterm birth were substantial, even when adjusted for other risk factors identified from the birth certificates. The effect of smoking on fetal growth may be partially mediated through lower maternal weight gain. The adjustment for maternal weight gain in this multivariate model may have underestimated the ORs for LBW and VLBW and thus the PAR%. Conversely, the effects reported here could also partially reflect the impact of other factors (e.g., illegal drug use) that were not reported on the birth certificate but that are more common among smokers than nonsmokers (3). Under these circumstances, the PAR% may have been slightly overestimated.

This study relied on data collected during the first 6 months of use of the revised Ohio birth certificate; the reliability of the smoking-related and other data may be expected to improve over time as reporting of new information becomes routine. Nonetheless, the findings in Ohio are similar to those in other studies, some of which used different data sources (2,4-7).

Birth certificates are a useful surveillance tool for identifying subgroups of women who are likely to smoke during pregnancy. These subgroups can then be targeted for special prevention or cessation efforts. Birth certificate data can also be used to evaluate the success of a state's antismoking programs. In 1989, only seven states did not collect information about maternal smoking habits that was comparable to that collected in Ohio on birth certificates.

Smoking during pregnancy increases infant morbidity and mortality through effects on birth weight and preterm birth (5,6). In Ohio and other states, successful efforts to reduce or eliminate smoking during pregnancy could substantially reduce rates of LBW, VLBW, and preterm birth and, in turn, reduce infant morbidity and mortality and the cost of health care in the state (8).

References

1. Taffel S, Johnson D, Heuser R, NCHS. A method of imputing length of gestation on birth certificates. Hyattsville, Maryland: US Department of Health and Human Services, Public Health Service, CDC, 1982; DHHS publication no. (PHS)82-1367. (Vital and health statistics; series 2, no. 93).

2. Office on Smoking and Health. The health consequences of smoking for women: a report of the Surgeon General. Rockville, Maryland: US Department of Health and Human Services, Public Health Service, 1980.

3. Zuckerman B, Frank DA, Hingson R, et al. Effects of maternal marijuana and cocaine use on fetal growth. N Engl J Med 1989;320:762-8.

4. Kleinman JC, Madans JH. The effects of maternal smoking, physical stature, and educational attainment on the incidence of low birth weight. Am J Epidemiol 1985;121:843-55.

5. Kleinman JC, Pierre MB, Madans JH, Land GH, Schramm WF. The effects of maternal smoking on fetal and infant mortality. Am J Epidemiol 1988;127:274-82.

6. Malloy MH, Kleinman JC, Land GH, Schramm WF. The association of maternal smoking with age and cause of infant death. Am J Epidemiol 1988;128:46-55.

7. Alameda County Low Birth Weight Study Group. Cigarette smoking and the risk of low birth weight: a comparison in black and white women. Epidemiology 1990;1:201-5.

8. Oster G, Delea TE, Colditz GA. Maternal smoking during pregnancy and expenditures on neonatal health care. Am J Prev Med 1988;4:216-9.

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