Smoking during Pregnancy Is Associated with a Decreased Incidence of Obstetric Anal Sphincter Injuries in Nulliparous Women

Abstract

BackgroundSmoking is a modifiable lifestyle factor that has been shown to be associated with adverse perinatal outcomes and to have adverse health and dose-dependent connective tissue effects. The objective of this study was to examine whether smoking during pregnancy was associated with the incidence of obstetric anal sphincter injuries (OASIS) among six birthweight groups in singleton vaginal deliveries, considering nulliparous and multiparous women separately between 1997 and 2007 in Finland.MethodologyA retrospective population-based register study. Populations included women with spontaneous singleton vaginal deliveries, consisting of all 213,059 nulliparous and all 288,391 multiparous women. Incidence of OASIS (n = 2,787) between smoking status groups was adjusted using logistic regression analyses.Principal FindingsOf the nulliparous women, 13.1% were smokers, 3.6% had given up smoking during the first trimester of their pregnancy and 81.1% were non-smokers. Among these groups 0.7%, 0.9% and 1.1%, respectively suffered OASIS (p≤0.001). Nulliparous women who smoked had a 28% (95% CI 16–38%, p≤0.001) lower risk of OASIS compared to non-smokers, when adjusting for background variables. In multiparous women, the overall frequencies of OASIS were much lower (0.0–0.2%). A similar inverse relationship between OASIS rates and smoking was significant in pooled univariate analysis of multiparous women, but multivariate analysis revealed statistically insignificant results between non-smokers and smokers.ConclusionsNulliparous women who were smokers had a 28% lower incidence of OASIS. However, smoking during pregnancy cannot be recommended since it has shown to be associated with other adverse pregnancy outcomes and adverse health effects. The observed association warrants clinical repetition studies and, if confirmed, also in vitro studies focusing on connective tissue properties at a molecular and cellular level.

Familial risk of obstetric anal sphincter injuries: registry‐based cohort study

“…Additionally, we reported a 40% reduction in the occurrence of OASIS among women who smoked until the end of their pregnancy; however, the results were not significant when we adjusted for a number of variables, including birthweight. Our results were in line with a recent study from Finland that showed nulliparous women who smoked during pregnancy had a 28% lower incidence of OASIS than non‐smokers, but this association was observed regardless of birthweight . It is well established that women who smoke deliver smaller infants, and this may explain why smoking protects against OASIS .…”

“…A weakness was that the lack of data precluded adjusting for factors like maternal smoking, delivery position, body mass index (BMI) and duration of second stage, which have been associated with OASIS and may have confounding effects on the occurrence of OASIS. 6,10,[15][16][17] We have previously reported a time trend in OASIS, 6 resulting in increasing occurrence from 1967 to 2004. Possible explanations for the increase of OASIS in this period include: (1) improved routines for the registration of OASIS in the medical birth registry; (2) improved diagnostic attention and routines; (3) a change of classification during this time period; and (4) changes in obstetric and demographic risk factors.…”

“…Our results were in line with a recent study from Finland that showed nulliparous women who smoked during pregnancy had a 28% lower incidence of OASIS than non-smokers, but this association was observed regardless of birthweight. 17 It is well established that women who smoke deliver smaller infants, and this may explain why smoking protects against OASIS. 17,23 Another explanation could be the influence of smoking on the connective and collagen tissue.…”

“…17 It is well established that women who smoke deliver smaller infants, and this may explain why smoking protects against OASIS. 17,23 Another explanation could be the influence of smoking on the connective and collagen tissue. Smoking appears to be highly hereditable, and could present a confounding factor in our study.…”

The Burden of OASIS Increases along with Socioeconomic Position – Register-Based Analysis of 980,733 Births in Finland

“…The demographic and obstetric risk factors for OASIS are well-established, including first vaginal birth [2], [3], high birthweight [4], [5], prolonged active second stage of birth [3], vacuum and forceps deliveries [2], [5], and midline episiotomy [6]–[8]. We previously reported that smoking during pregnancy was associated with a decreased risk of OASIS, even after adjustment for birthweight [9]. In contrast to this negative association with OASIS, smoking has typically been identified as a risk factor for pelvic floor disorders in later life, including moderate associations with a variety of bladder storage symptoms [10], urinary incontinence (UI) [11], and faecal incontinence [12].…”