Cigarette smoking during pregnancy in relation to placenta previa Michelle A. Williams, SM: Robert Mittendorf, MD, MPH:' Ellice Lieberman, MD, DrPH,"' d Richard R. Monson, MD, SeD: Stephen C. Schoenbaum, MD, MPH,r and David R. Genest, MDe C

Boston, Massachusetts To evaluate the relationship between cigarette smoking and the occurrence of placenta previa, we used interview and medical record data to conduct a case-control analysis of 69 placenta previa cases and 12,351 controls. The unadjusted relative risk estimate of placenta previa for women reported to have "ever smoked" during pregnancy relative to nonsmoking mothers was 1.9 (95% confidence interval, 1.2 to 3.0). The risk rose after adjusting for potential confounders (odds ratio, 2.6; 95% confidence interval, 1.3 to 5.5). In contrast to a previous report, the duration of smoking was not an independent risk factor for placenta previa. These results suggest that cigarette smoking during pregnancy is a determinant of placenta previa. Carbon monoxide hypoxemia, which is one possible mechanism for this association, may result in compensatory placental hypertrophy. Placentas with increased surface areas are more likely to cover the cervical os, causing placenta previa. (AM J OSSTET GVNECOL 1991 ;165:28-32.)

Key words: Placenta previa, pregnancy, smoking, risk factor Although placenta previa is uncommon, it is a significant cause of maternal and perinatal morbidity and mortality. Placenta previa has been reported to occur in 3.3 to 9.9 per 1000 deliveries.1,2 However, its cause is poorly understood. Risk of placenta previa has been found to increase with increasing parity, maternal age, previous induced and spontaneous abortions, previous cesarean section, a history of dilatation and curettage, and short maternal stature; it also occurs more frequently with singleton male pregnancies and twin pregnancies of both sexes and when gestation takes place at high altitude. I . Io Two prior studies found a positive association between placenta previa and maternal cigarette smoking during pregnancy.5.11 Meyer et al. 5 observed a 25% and 92% increase in the prevalence of placenta previa for smokers of less than one pack and smokers of one or more packs of cigarettes per day, respectively. More From the Departments of Epidemiologya and Maternal and Child Health,' Harvard School of Public Health; St. Margaret's Hospital for Women, Tufts University School of Medicine'; the Departments of Obstetrics and Gynecologyd and Pathology,' Brigham and Women's Hospital, Harvard Medical School; and the Harvard Community Health Plan/ Supported by a grant from the March of Dimes Birth Defects Foundation and by an Occupational and Environmental Health Center grant (2P 30 E S00002) from the National Institute ofEnvironmental Health Sciences. Ms. Williams was supported by National Institute of Environmental Health Sciences National Research Service Award (51'32 ES07069) through the Harvard School of Public Health. Received for publication October 19, 1990; revised December 27, 1990; accepted December 28, 1990. Reprint requests: Michelle A. Williams, SM, Epidemiology, Harvard School of Public Health, Boston, MA 02115. 611 127689

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recently, Kramer et al. 11 also found an association between smoking and placenta previa (odds ratio, 1.9; 95% confidence interval, 1.5 to 2.8). Naeye I2 reported that the duration of maternal cigarette smoking was associated with placenta previa. He found that the frequency of placenta previa was more strongly associated with the number of years mothers had smoked than with their smoking habits during pregnancy. In this study we evaluated the association between cigarette smoking and placenta previa in a large cross-sectional data base.

Material and methods The Delivery Interview Program, conducted at the Boston Hospital for Women from August 1977 to March 1980, was designed to study the occurrence of adverse outcomes of pregnancy.l3 Information was obtained by personal interviews and medical records review. During the study period 14,458 mothers (84.4% of all deliveries) were asked to participate in the study. Some mothers were not asked to participate because there were too few interviewers (14.1 %) or because their physicians would not grant permission for participation in the study (1.4%). When too few interviewers were available, patients were selected randomly. Of the mothers with singleton deliveries, 90.0% of those approached (12,825 women) were interviewed. The reasons for not having been interviewed were early discharge (5.5%), refusal to be interviewed (3.0%), the presence of a language barrier (1.6%), and medical conditions precluding an interview (0.1 %). The interview provided information on demographic character-

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Cigarette smoking and placenta previa 29

Table I. Labor and delivery characteristics of women with placenta previa (cases) and control subjects Placenta previa cases Variable

(N = 69)

Control subjects (N = 12,351)

Cesarean section (%) Birth weight (gm)* Weeks' gestation* Neonatal deaths/lOOO deliveries (deaths in first 7 days of life) Perinatal deaths/lOOO deliveriest

82.6 2645 ± 798 36.1 ± 4.2 14.5

21.3 3316 ± 675 39.7 ± 2.5 3.7

14.5

9.1

*Mean ± SD. tStilibirths and neonatal deaths.

istics, previous contraceptive practice, medical and reproductive histories, cigarette smoking habits, and exposure to other possible risk factors. Information regarding the duration of smoking; smoking status at the time of conception; and the average number of cigarettes smoked per day during the first, second, and third trimesters, respectively, was obtained from mothers who had ever smoked. Information was collected from the medical records regardless of whether an interview had occurred. Information regarding pregnancy outcome and condition of the newborn was abstracted from the mothers' and infants' medical charts. Completed interviews and medical records information were available for 12,718 women who were delivered of singleton infants. For this analysis, cases consisted of nondiabetic women with singleton deliveries whose medical records indicated a physician's diagnosis of placenta previa (n = 69). Detailed information regarding gestational age at diagnosis and type of placenta previa (marginal, partial, or total) were not available. The controls were the 12,351 nondiabetic women with singleton deliveries that were not complicated by placenta previa. This study was restricted to nondiabetic women because only 2.3% of the study population were diabetic. Women with a diagnosis of abruptio placentae (independent of placenta previa) indicated on their medical records (n = 135) were excluded from this analysis. Abruptio placentae, another cause of third-trimester bleeding, has been shown to be associated with maternal cigarette smoking during pregnancy.5 Analyses including these women yielded results similar to those reported here. Women were considered to be cigarette smokers during pregnancy if they had reported ever having smoked during the first or second trimester. Three levels of cigarette smoking during pregnancy were considered: nonsmokers, smokers of 1 to 9 cigarettes per day, and smokers of 2:: 10 cigarettes per day. Three levels of the duration of cigarette smoking were also considered: never smokers, smokers for 1 to 5 years, and smokers for 2::6 years. To estimate the risk of placenta previa associated with

Table II. Distribution of placenta previa cases and control subjects by selected risk factors and cigarette smoking status

Variable

Age >34 yr* Black Medicaid recipient Marital status single Parity* 0-1 2-4 ~5

Spontaneous abortion* 0 1

~2

Prior induced abortion Prior cesarean section * Prior uterine surgery DES in utero* Maternal height

Cigarette smoking during pregnancy in relation to placenta previa.

To evaluate the relationship between cigarette smoking and the occurrence of placenta previa, we used interview and medical record data to conduct a c...
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