Pregnant women are already at an increased risk of cardiovascular events, including acute MI,14
and venous thromboembolism,16
compared with women who are not pregnant. In previous case-control studies we performed using the same database and comparing women who experienced cardiovascular events during pregnancy with those who did not, we identified smoking as a risk factor.14,15,17
In this study we found that pregnant smokers had a >4-fold increased risk for acute MI, a 2-fold increased risk for PE, and a 1.3-fold increased risk for DVT compared with pregnant nonsmokers. The additional risk of cardiovascular events conferred by smoking is over and above the risk conferred by pregnancy.
We found multiple other effects on maternal health. We found a higher incidence of pulmonary complications, such as asthma, bronchitis, and pneumonia. Nonpregnant smokers are more likely to experience gastrointestinal ulcers,18
and this was true of the pregnant smokers in this study, with an OR of 3.7 (95% CI 2.6, 5.5). Smokers were slightly more likely to have anemia and HIV infection, possibly reflecting the lower socioeconomic status of the pregnant smokers. There was a higher incidence of ectopic pregnancies among smokers. Other studies have also found a 1.4–4-fold increased incidence of ectopic pregnancies among smokers,19,20
with heavier smokers having the highest risk.19
We found a 5-fold increased risk, perhaps because the smokers who were identified were heavier smokers. Theories on how smoking causes ectopic pregnancy include fallopian tube ciliary damage causing decreased ovum transit times and inflammation interfering with fertilization.21
Like other studies in the literature, this study found that smokers have a higher incidence of poor pregnancy outcomes, including PPROM,20,22
and intrauterine fetal demise.27,28
There are several limitations to this study. First, the data were collected from a database of all hospital discharges. Therefore, there is no verification of actual smoking status by biochemical markers and no supplemental data to confirm pulmonary or cardiovascular complications. Data are limited to events and comorbidities identified during hospital admissions in pregnancy or at the time of childbirth. Comorbidities may have been underestimated. Studies that have used linked birth and death certificates have found that a more complete assessment of maternal mortality includes maternal deaths up to 1 year postdelivery. Obtaining data on these late maternal deaths was not possible using the deidentified data in the NIS database.
Second, there is a possibility that bias could be introduced if smoking is a known risk factor for certain conditions. Smoking could be coded more often, exaggerating the effects. For example, more physicians might label a patient a smoker if the patient were diagnosed with a PE. However, we did find a negative association with preeclampsia and a positive association with ectopic pregnancy, conditions for which a bias in coding for smoking would not be anticipated.
Lastly, the smoking rate in our dataset is around 4%, and we know that the smoking rate in pregnant women nationwide is around 11%.2
The rate of smoking appears to be underreported in this database. Pregnant women do underreport smoking, perhaps because of the social undesirability of smoking in pregnancy.29
As most of the hospital admissions were at the time of childbirth, it is possible that women quit smoking during the pregnancy. It is also possible that physicians are not identifying behavioral modifiers, such as smoking, when admitting patients or patients are not identifying themselves as smokers when they are admitted.
The discrepancy between smoking rates in this sample and in national samples is curious. The national samples used birth certificate records to ascertain smoking status and included women who were giving birth at hospitals. Linking of birth certificate data with NIS data might have provided a more complete enumeration of women who reported smoking during pregnancy and, therefore, provided a more complete picture of the effects of smoking on maternal health. Again, however, the data in the NIS are deidentified, which eliminates the possibility of linking the data to records from other sources. Because the smoking rate in this database is much lower than anticipated, there are likely to be smokers in the control group, and the effects are likely diluted. Therefore, the association between smoking and poor maternal outcomes could be higher than we are reporting. On the other hand, because cardiovascular events are rare in pregnant women, no one medical center would have enough numbers to establish an incidence and identify risk factors, which is possible with a large database, such as the NIS.
There are several implications from the findings from this study. Our study stands in contrast to a tendency to focus primarily on how maternal disease affects neonatal outcome while failing to notice maternal health as an end in itself. Although the American College of Obstetricians and Gynecologists (ACOG) Committee Opinion on smoking in pregnancy reviews the detrimental consequences of maternal smoking on pregnancy outcomes (ectopic pregnancy, IUGR, placenta previa, placental abruption, PPROM, low birth weight, perinatal mortality, sudden infant death syndrome [SIDS], and increased risks to children's health), there is no mention of the increased risk of cardiovascular or pulmonary complications.30
The oversight may be attributed to a paucity of data regarding maternal outcomes associated with smoking, but the lack of data itself highlights the need for data on maternal as well as fetal outcomes of pregnancy in the conduct of future clinical and epidemiological research during pregnancy.
Our findings also have several implications for clinical care. First, practice guidelines should be revised so that clinicians will recognize that smokers are at risk for cardiovascular and pulmonary complications during pregnancy. Second, our data suggest that counseling about smoking cessation should be a priority both preconceptionally and during pregnancy. As physicians, this should include educating women about the increased risks associated with smoking while pregnant—not only the fetal and neonatal complications but also the short-term and long-term consequences to maternal health. Because smokers are more likely to have significant comorbidities, adequate information about maternal and fetal risks of smoking may be critical not only to decisions about smoking cessation but also to decisions about when and whether to pursue pregnancy. Data from smoking cessation studies have shown that patient education and empowerment have a higher success rate than drug therapy alone.
In the end, maternal health cannot be separated from fetal health. A stroke or an MI during pregnancy will have immediate consequences for maternal and fetal health, and it will most certainly also have a long-term impact on the health of the woman, her children, and her family. Often, the best way to ensure the health of the fetus is to ensure the health of the pregnant woman. Identifying and preventing maternal complications of smoking during pregnancy are important components of improving the health of both women and the fetuses they carry.