Copyright (c) 2012 Morgan D
Preterm birth is defined as delivery of a baby before 37 completed weeks of gestation. Preterm birth complicates approximately 12 percent of all pregnancies in the United States and is a leading cause of infant morbidity and mortality (March of Dimes, 2003). Among preterm births, a majority of infants (84 percent) are born between 32 and 36 weeks, 10 percent are born between 28 and 31 weeks, and about 6 percent are born at less than 28 weeks of gestation (March of Dimes, 2004). Babies delivered preterm have an increased risk of dying in utero or during infancy and also have a high risk of suffering from various life-threatening complications. Some of the important risk factors associated with preterm birth are multi-fetal pregnancies, cervical abnormalities, medical history, smoking, alcohol use, low socio-economic status and race/ethnicity (March of Dimes, 2003).
Preterm birth accounts for a major fraction of perinatal mortality and is an important determinant of neonatal and infant morbidity, including neuro-developmental handicaps, chronic respiratory problems, infections and ophthalmologic problems. Maternal smoking during pregnancy is a major risk factor for preterm birth. Efforts to reduce the adverse outcomes associated with preterm birth include both interventions to prevent preterm birth or prolong gestation intended to reduce prematurity-associated morbidity and mortality. Some of the chief findings from this literature review are –
Smoking during pregnancy is causally associated with preterm birth. Increase in the number of cigarettes smoked is associated with an increased risk of preterm birth. This is commonly referred to as a dose-response effect. Cessation of smoking in pregnancy supports an increased reduction of preterm birth.
Some of the key studies that help establish an association between smoking cessation and preterm birth are outlined below. The systematic review by Lumley, Oliver, Chamberlain and Oakley (2004) shows that if women quit smoking during pregnancy, the risk of preterm birth will be reduced by 16 percent. Although the study by Cnattingius (2004) did not discuss the association between smoking cessation and preterm birth, it does however provide convincing clinical and epidemiological data on smoking during pregnancy as a risk factor for adverse pregnancy outcomes including preterm birth. The systematic review by Shah and Bracken (2000) shows a dose-response effect associated with maternal smoking and preterm birth. Heavy smokers are at increased risk of preterm birth compared to moderate or light smokers. The other important studies that report concrete percentile values are the study conducted by Arch and Williams (1994) which shows that discontinuing smoking during the first trimester reduced the rate of preterm birth by 26 %. The study by Harlow et al.( 1996) also shows that women who smoked during the first trimester of pregnancy had an increased risk of preterm birth.
The results from the literature review suggest that there is a strong association between maternal smoking and increased risk for preterm birth. The review also answers the original PICO question by providing strong evidence of an association between smoking cessation and decreased risk of preterm birth. The significant reductions in preterm birth and low birth-weight in the intervention arm of smoking cessation trials confirm that smoking cessation can reverse the adverse effects of smoking on perinatal outcomes. However, it is not clear if discontinuing smoking before or during the course of pregnancy lowers the risk for preterm birth. Some key findings from the literature review are listed below-
If women quit smoking during pregnancy then the risk of preterm birth will be reduced by 16 percent (Lumley et al, 2004). women who smoked during the first trimester of pregnancy had an increased risk of preterm birth (Harlow et al,1996) Discontinuing smoking during the first trimester can reduce the rate of preterm birth by 26 % and low birth-weight infants by 18 % (Arch & William, 1994).
Although one study (Arch & William, 1994) concludes that discontinuing smoking during the first trimester can reduce the rate of preterm birth significantly, in order to ascertain if this is true larger and better research studies need to be designed to ascertain if smoking cessation before or during the course of pregnancy leads to lower risks of preterm birth. Although a dose-response pattern between number of cigarettes smoked and preterm birth is provided in the study conducted by Shah & Bracken (2000), whether such an association persists in individual studies remains inconclusive. The review of pertinent literature revealed several limitations in preterm birth research, including errors in determining gestational age, publication bias, heterogeneity of exposure criteria, with cigarette smoking ascertained at different points in pregnancy and smoking categorization differing from one study to the next and multiple factors that can cause preterm birth, inability of studies to detect clinically important outcomes, contamination of controls, redundancy in classification between various categories of preterm labor, absence of precise labor/delivery data and selection of a cohort that under represents parous women with previously poor pregnancy outcomes.
Other deficiencies in the area of preterm birth research are lack of basic knowledge regarding the biologic mechanisms resulting in birth before term and the incidence and prevalence of preterm symptoms and modifiable risk factors for preterm birth. The biological mechanism behind preterm birth is poorly understood and there is a dearth of reliable epidemiologic information in this area. Better research studies have to be designed to study the risk factors that contribute to preterm birth. Identifying high-risk populations would help target studies that could provide a better insight into the association between smoking and preterm birth. Clinicians and researchers need to be able to assess accurately the risk of preterm birth in women with symptoms of preterm labor using biologic markers because they are objective and reproducible and are superior to purely clinical assessment or prognostication of preterm birth. An optimal solution would be to use larger and well designed observational studies that need to confirm the association.The areas that require special attention include biologic mechanisms that result in birth before term, incidence and prevalence of preterm labor and the proportion of pregnancies that result in preterm birth, modifiable risk factors for preterm birth, and a better understanding of these basic facts among racial/ethnic minorities.
Future of Preterm research A comprehensive prevention agenda is needed to better understand the multiple and complex causes and risk factors associated with preterm birth and implement effective strategies. Future trials need to include biochemical validation of non-smoking status, collection of outcome data on birth-weight, preterm birth and perinatal deaths and a relapse prevention component for those who have stopped smoking before the first antenatal visit. Some steps that need to be taken in the future in the area of preterm birth research are –
Researching the causes and risk factors for preterm delivery – Some of the important risk factors associated with preterm birth are multi-fetal pregnancies, cervical abnormalities, medical history, smoking, alcohol use, low socio-economic status and race / ethnicity. Research studies that help identification of risk factors help in early intervention and treatment.
As smoking cessation programs have been shown to increase smoking cessation, reduce preterm birth and low birth-weight, and increase mean birth-weight, smoking cessation programs need to be implemented in all maternity care settings. Attention to smoking behavior together with support for smoking cessation and relapse prevention needs to be as routine a part of antenatal care.
Identifying women at risk early in their pregnancy : The research agenda should determine if there are ways to identify women at risk early in their pregnancy, so that they may be referred to tertiary care medical systems or provided with interventions to reduce their risk
Moving new research discoveries to public health prevention : A comprehensive research agenda is needed to evaluate drug availability, patient acceptability and adherence, and evaluate alternative routes of delivery
Expanding community-based programs – Community-based programs serve to increase awareness about preterm birth, promote early initiation and continuity of prenatal care, propagate importance of smoking cessation and promote pregnancy health at the community level. The duration, severity and consequences of rising rates of preterm birth warrant the need for an immediate and comprehensive public health response.
Prevention of preterm birth is an important public health priority. Preterm delivery is the leading cause of hospitalization among pregnant women and the second leading cause of infant death (US Department of Health and Human Services, 2003). Babies that are born prematurely face a number of problems, including low birth weight, respiratory and breathing difficulties, and underdeveloped organs and organ systems. The review of pertinent literature shows a strong association between maternal smoking and the increased risk for preterm birth. However, larger and well designed research studies need to be designed in the future to fully understand and comprehend if smoking cessation before or during the course of pregnancy leads to a decreased risk of preterm birth.
More Birth Articles