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ORIGINAL RESEARCH |
From the Department of Community Medicine, The University of Hong Kong Medical Center, Hong Kong, China.
Address reprint requests to: Gabriel M. Leung, MD, MPH, University of Hong Kong, Department of Community Medicine, 21 Sassoon Road, Pokfulam, Hong Kong, China; E-mail: gmleung{at}hku.hk.
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ABSTRACT |
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METHODS: We conducted a prospective, population-based birth cohort study in 1997. Data were collected on breast-feeding history, household smoking habits, method of delivery, and other demographic, obstetric, behavioral, and potential confounding variables via a standardized self-administered questionnaire. Multivariable logistic regression was used to examine the association between method of birth (cesarean versus forceps or vacuum delivery versus normal vaginal birth) and either not initiating breast-feeding or doing so for less than 1 month. Among women who breast-fed for 1 month or more, multivariable survival analysis was employed to study the relationship between method of delivery and breast-feeding duration. We repeated these analyses to examine the link between parental smoking habits and breast-feeding initiation and duration.
RESULTS: A total of 7825 mother-infant pairs were followed up for 9 months. Cesarean delivery was a risk factor for not initiating breast-feeding, for breast-feeding less than 1 month, and remained a significant hazard against breast-feeding duration. Assisted delivery with forceps or vacuum, although not associated with breast-feeding initiation, was a significant risk against breast-feeding duration. Conversely, current parental smoking habits only affected breast-feeding initiation but were unrelated to breast-feeding duration.
CONCLUSION: This study indicates a possible effect of forceps or vacuum delivery on breast-feeding and of cesarean on long-term breast-feeding duration. The findings provide additional evidence in support of the avoidance of unnecessary obstetric interventions.
There has been convincing and accumulating evidence that breast-feeding is associated with lower rates of infant illness in both developing1,2 and industrialized countries.3–6 More recently, investigators have reported longer-term protective effects of breast milk, including lower incidences of diabetes mellitus,7 recurrent wheeze at age 6,8,9 and higher intelligence quotients10,11 later in childhood. From an economic viewpoint, Ball and Wright estimated that each never-breast-fed baby cost the health care system between $331 and $475 during the first year of life.12 The American Academy of Pediatrics has recommended exclusive breast-feeding for the first 6 months of life, and that breast milk be retained as an integral component of an infant’s diet for at least 1 year.13 Unfortunately, consistently fewer than 30% of infants born in Hong Kong are breast-fed at all, according to population-based estimates since the early 1980s.14,15
The impact of cesarean delivery on breast-feeding has been studied extensively.16–31 However, the results are conflicting. Six studies suggest an inverse association between these two variables,22,25,27–30 but ten have been unable to establish such a link.16–21,23,24,26,31 Most studies have not distinguished between the initiation (ever versus never) and the duration of breast-feeding. Previous work in this area has stressed the importance of distinguishing between initiation and duration because it allows for greater conceptual clarity and better predictive possibilities.32,33 The factors associated with the decision to breast-feed may not necessarily be the same as those affecting the duration of infants being nursed. There is also some evidence to suggest that the benefits of breast-feeding accrue with the duration an infant remains on the breast.4,7,11,13 In addition, whether the potential link between cesarean and breast-feeding extends to assisted instrumental delivery has not been examined in the literature. Any empirical evidence about this newly hypothesized link would be of important public health interest. As vacuum and forceps techniques are a major avenue for reducing cesarean rates in difficult labors, this information also has strong clinical implications.
The relationship between maternal smoking and breast-feeding has also been studied previously. There appears to be solid evidence pointing to a negative relationship.31–41 Fewer smoking mothers attempt breast-feeding than their nonsmoking counterparts,31,34–36 and for those smokers who begin breast-feeding, they continue for a shorter period than women who do not smoke.33,37–41
To date, only four studies have attempted to examine the important effects of both cesarean and smoking, and control simultaneously for the known confounders of various demographic and maternal behavioral factors such as age, socioeconomic status, employment, and education level.31,33,42,43 None of these studies covered Asian, and in particular Chinese, populations. In addition, two were not population based,33,42 and among these, one suffered from a small sample,33 whereas the other was based on data collected in the 1970s.42 Of the two population-based studies, one found no association between the method of delivery and breast-feeding and reported exclusive breast-feeding as the sole outcome of interest, disregarding the potentially significant pool of women who supplement breast milk with formula feeds.31 The other showed similar rates of breast-feeding between mothers who smoked and those who did not, as well as between infants delivered by cesarean and those born per vagina.43
The objective of this paper is to examine the effects of cesarean and instrument-assisted birth and parental smoking habits on the initiation and duration of breast-feeding in a population-based birth cohort of Hong Kong Chinese infants in 1997.
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MATERIALS AND METHODS |
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We employed Pearson 
2 and multivariable logistic regression analyses45 to study the association between cesarean and the initiation of breast-feeding (ever versus never breast-fed) (n = 7266, missing data for 559 [7.1%] mother-infant pairs). The same statistical techniques were used to examine the hypothesis that cesarean is a risk factor for short breast-feeding duration (ie, breast-feeding for less than 1 month versus 1 month or more) among mothers who elected to breast-feed (n = 2344, missing data for 276 [10.5%]). A multivariable survival analysis using the Cox proportional hazards model46 was used to study the relationship between cesarean and breast-feeding duration (n = 2344). This model allows joint estimation of the effects of predictor variables on the "hazard," the risk of cessation of breast-feeding, rather than the duration itself. We only included mothers who breast-fed for at least 1 month because we wanted to determine whether there was any impact of cesarean beyond the period when lactation is usually established. For women who were still breast-feeding at the time of the 9-month follow-up, the duration of breast-feeding was calculated using the age of the infant at the time of the survey. In such cases (12.8%), breast-feeding duration was identified as a censored value in the Cox model. We selected 9 months as the cutoff to allow the study of long-term breast-feeding duration (ie, beyond the 3-month follow-up) but did not extend it to 18 months to minimize possible recall bias of mothers because of time lag and memory lapse. Further, there was little additional information to gain from extending the observation period to 18 months, as most (87.1%) breast-feeding mothers terminated the feeding practice by the end of 9 months. We based our analysis primarily on cumulative data from the first baseline recruitment visit up to the 9-month follow-up, but also used information from the 3-month visit to supplement missing values in cases (n = 353, 4.5%) where subjects were lost to follow-up after the 3-month appointment.
For the multivariable analyses, the main independent variable was method of delivery (normal vaginal, vaginal assisted with vacuum or forceps, cesarean). Potential predictors were included if they were associated with a P < .05 at bivariate analyses, or were selected based on known confounders documented in previous reports.16–41 The following independent variables were included in the multivariable models: gestational age (less than 37 weeks, 37–41 weeks, more than 41 weeks), birth weight (less than 2500 g, 2500–2999 g, 3000–3499 g, 3500–3999 g, 4000 g or more), mother’s current smoking habit (yes versus no), father’s current smoking habit (yes versus no), mother’s age (24 or older, 25–29, 30–34, 35 or older), mother’s and father’s education level (grade 6 or less, grades 7–9, grade 10 or more), full-time job held by mother and father (yes versus no), birth order of infant (first, second, third or more), gender of infant (male versus female), and residential region (Hong Kong Island, Kowloon, New Territories).
Similarly, we repeated these analyses to examine the link between parental smoking habits (at first Maternal and Child Health Center baseline visit) and breast-feeding initiation and duration. We constructed Kaplan-Meier survival curves to examine breast-feeding duration at 9 months between women who had normal vaginal delivery, vaginal delivery with the assistance of forceps or vacuum, and those who underwent surgical delivery, and compared them using the log rank test. All analyses were conducted using Stata 6.0 (Stata Corp., Santa Monica, CA).
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RESULTS |
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). Thirty-four percent initiated breast-feeding; however, at 9 months, only 355 (4.5% of total sample) infants were still on the breast. The median duration was 8 weeks. Vaginal delivery with assistance accounted for 16.7% of births, whereas 27.1% of infants were delivered by cesarean. Of the mothers, 2.7% were smokers, whereas 33.0% of fathers smoked at the baseline visit (ie, first Maternal and Child Health Center appointment). The majority of mothers (60.1%) and fathers (57.0%) continued formal education beyond the 9-year compulsory duration (ie, up to junior secondary level of grade 9) required and provided free of charge by the government. About half (47.0%) of the mothers and most (93.9%) fathers held a full-time job. Most (89.3%) infants were either the first or second child in the family.
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) and multivariable (Table 3) analyses (odds ratio [ORadjusted] 1.52, 95% confidence interval [CI] 1.34, 1.73). However, breast-feeding initiation for infants delivered using forceps or vacuum was not different from those born vaginally without instrument assistance (ORadjusted 1.09, 95% CI 0.95, 1.26). This observation was supported by the bivariate results in Table 2, which demonstrated no significant difference in the proportions of ever-breast-fed infants who were delivered vaginally without assistance compared with those who had forceps or vacuum assistance (ORcrude 0.90, 95% CI 0.79, 1.03). Mothers and fathers who were current smokers at the first Maternal and Child Health Center visit contributed to not initiating breast-feeding (ORcurrent smoker—mother 3.10, 95% CI 1.98, 4.85; ORcurrent smoker—father 1.27, 95% CI 1.13, 1.42). Of the three factors linked to the noninitiation of breast-feeding, maternal smoking was the strongest risk factor for failing to breast-feed, followed by paternal smoking and cesarean delivery, respectively. Other predictors for not starting breast-feeding included maternal and paternal full-time employment, increasing birth order, and living in Kowloon and the New Territories. On the other hand, increasing maternal age (P for trend < .001) and higher levels of education for both mothers (P < .001) and fathers (P < .001) were all positively associated with the initiation of breast-feeding.
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), even after adjusting for potential confounders (ORadjusted 1.25, 95% CI 1.00, 1.56) (Table 3). Similarly, an instrument-assisted birth was associated with breast-feeding for less than 1 month (ORadjusted 1.32, 95% CI 1.04, 1.68) despite its lack of association with breast-feeding initiation previously. In fact, bivariate analyses suggest there was no difference in the proportions of infants who were breast-fed for less than 1 month between those born with instrumental assistance and those delivered by cesarean (48.0% versus 44.7%, P = .28). However, smoking status of both mothers and fathers at the first Maternal and Child Health Center visit did not show a link with short-term (less than 1 month) breast-feeding duration. Women who attended school beyond grade 10 were more likely to breast-feed their children for less than 1 month (ORadjusted 1.67, 95% CI 1.12, 2.48), and so were mothers who had a full-time job (ORadjusted 2.98, 95% CI 2.43, 3.67). The trend regarding maternal education level was significant (P = .009). Older women also tended to breast-feed their infants for more than 1 month (P = .044).
In the Cox proportional hazards model, cesarean birth and, to a lesser degree, assisted delivery maintained their association with breast-feeding as risk factors against the duration of infants remaining on the breast (hazards ratiocesarean 1.16, 95% CI 1.04, 1.30, hazards ratioassisted 1.12, 95% CI 0.99, 1.26) (Table 3). Maternal characteristics such as better education (P for trend = .012) and full-time employment (P for trend < .001) also remained important hazards against breast-feeding for a long period. The hazards ratios related to parental smoking habits were close to unity in the Cox model.
Findings about the method of delivery being a significant risk for breast-feeding duration (P < .001 for both normal vaginal versus vaginal with assistance, and normal vaginal versus cesarean) are confirmed by survival analysis using the Kaplan-Meier method (Figure 1). We did not find any significant difference between the vaginal with assistance and cesarean delivery survival curves (P = .90).
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DISCUSSION |
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In two similar analyses of data from nationally representative samples in the United States and Mexico, investigators also reported cesarean as a risk factor for not initiating breast-feeding. However, neither demonstrated an association with long-term breast-feeding duration, although the Mexican study had found that surgical delivery remained a significant risk for short-term breast-feeding duration (ie, less than 1 month versus at least 1 month).28,30 Prolonged mother-infant separation, an increased length of stay in hospital, as well as maternal endocrinologic changes induced by surgery have all been postulated as reasons for failure to initiate breast-feeding in mothers who undergo a cesarean.30 Our study confirmed these findings and extended the period of potential harm from surgical delivery to 9 months postpartum.
Interestingly, we found that assisted delivery did not affect the initiation of breast-feeding but posed a significant risk against the duration mothers nursed. This suggests that in addition to the usual factors (eg, rooming-in, early mother-infant contact) in the immediate postpartum period influencing the decision of whether to start breast-feeding, a separate mechanism is at work to determine the short- and long-term duration of breast-feeding once lactation has been initiated. This novel pathway also appears to be proportionately related to the degree of obstetric intervention, as shown by our trend analyses.
According to a search of MEDLINE (1966–2001) using key words/MeSH terms such as "cesarean" and "breast-feeding," the enduring effect of cesarean on long-term (more than 1 month) breast-feeding duration demonstrated in our study has not yet been described in the literature. However, the dose-response relationship observed across the three methods of delivery, the consistency with a previous study revealing an association between short-term breast-feeding duration and cesarean birth,30 and the tight CIs around the point estimates in both multivariate models examining breast-feeding duration are all highly suggestive of a real link between cesarean, or more broadly the degree of obstetric intervention in delivery, and the length of time infants remain breast-fed once the practice has been initiated. Again, this probably operates through the same pathway as noted for the effects of assisted delivery on breast-feeding duration.
Taking these two pieces of evidence together, we propose the mechanism whereby breast-feeding duration is curtailed may work through a delayed psychologic reaction of the mother to an eventful delivery. Although she may have started breast-feeding in the postpartum period, negative feelings about and, more generally, the adverse impact of an instrumental or surgical birth preclude long-term breast-feeding duration. The precise elucidation of this mechanism is beyond the scope of this report, and our data are insufficient to answer this question. Information on the indications for assisted or surgical deliveries as well as on maternal and neonatal peripartum complications would also be helpful in the delineation of the causal pathway. For example, one could postulate that mothers with significant complications from a difficult assisted vaginal delivery would be less likely to initiate or maintain breast-feeding compared with women with an elective cesarean. In addition, infants with significant intrapartum complications leading to an operative delivery might be less likely to be breast-fed because of the need for neonatal care. Our findings should be confirmed by additional studies in other populations. Future studies should distinguish between elective and emergency cesarean deliveries as well as prospectively collect more comprehensive information about a full range of potential risk factors associated with breast-feeding duration, with particular attention paid to the psychologic reactions of mothers to an eventful delivery.
As expected,31–41 there was a strong association between maternal and paternal smoking and the failure to start breast-feeding, independent of other potential confounders. Smoking habits reflect a person’s attitude towards health. Mothers who smoke are, therefore, potentially less knowledgeable and less enthusiastic about breast-feeding than their nonsmoking counterparts.38 Similarly, fathers who smoke may be a proxy for a lower level of health consciousness in the household and, therefore, adversely affect their wives’ decision about breast-feeding.47 Our results support this hypothesis and suggest that maternal smoking is the strongest risk factor for the noninitiation of breast-feeding, even though our prevalence of maternal smoking is much lower than that reported in the West.31,38,41
However, we failed to identify a relationship between parental smoking habits and the duration of breast-feeding. The absence of such a relationship in our study is not incompatible with previous documentation of a noncausal association. Eriksen suggested the observed link was probably a result of uncontrolled confounding by social and psychologic factors.38 Given that only 0.9% of mothers who initiated breast-feeding smoked and the very widely associated CIs from the two breast-feeding duration multivariate analyses, we may not have had adequate power to detect a subtle difference. Further, it is also possible that the manner in which we recorded parental smoking habits (ie, through a single measurement at the first Maternal and Child Health Center visit) may have precluded us from detecting a true association between smoking behavior and the length of time infants were breast-fed. Recent studies from the United Kingdom and New Zealand have reported maternal smoking as a significant risk factor for a shorter duration of breast-feeding, and were also able to demonstrate a clear dose-response gradient.31,41
As shown previously,31,38,41 maternal age and education attainment were positively associated with the initiation of breast-feeding in a dose-response manner. However, our results revealed that better education adversely affected breast-feeding duration, also demonstrating a clear inverse gradient. We postulate that more educated women are more aware of and receptive to the benefits of breast-feeding, and hence the decision to breast-feed. On the other hand, better educated women are also more likely to be in full-time jobs, which in turn is a major barrier for breast-feeding, especially in a culture like Hong Kong’s where there is little sympathy and provision for nursing mothers. Therefore, breast-feeding duration was shortened for working mothers and those with more years of schooling in our sample. We searched for interaction between these two variables in the multivariable models, and significance testing yielded borderline but with insignificant P values, suggesting that, though statistically not significant at the conventional .05 level, there might have been a moderate amount of interaction between maternal education and full-time employment.
To put our results into a broader population perspective, 12.4% of all infants who are never breast-fed in Hong Kong can be attributed to cesarean, irrespective of the indication or need for such a procedure. Similarly, the population-attributable risk for breast-feeding for less than 1 month was 6.3% (PAR = Pe[OR - 1]/{1 + Pe[OR - 1]}), where Pe is the prevalence of cesarean in the overall sample. Assuming a reduction of cesarean rates to 15%, the upper limit recommended by the World Health Organization,48 the two population-attributable risks would become 7.2% and 3.6%, respectively. If confirmed by other studies, results of the present report will support the avoidance of cesarean as a useful adjunct for breast-feeding promotion. This is particularly relevant for Hong Kong, where cesarean delivery rates have climbed steadily from 17% to 28% between 1987 and 1999.49 As vacuum extraction and the use of forceps are major substitutes for cesarean deliveries in difficult births, but both are also implicated as being independent risk factors against breast-feeding from our results, this poses a challenge to the obstetric community to come up with ways to reduce such instrument-assisted deliveries, and also drive down cesarean delivery rates and maintain good mother-infant outcomes. The answer may lie in strategies to mitigate their impact on breast-feeding initiation and duration, rather than in the reduction of absolute intervention rates. Although yet to be confirmed by evidence in the literature, women who deliver by cesarean or with instrument assistance should perhaps be specifically targeted for special breast-feeding counseling and support postpartum and throughout the first months after delivery. The government should also institute macroeconomic and policy strategies to encourage breast-feeding, paying particular attention to lowering cesarean delivery rates. Obstetric facilities in Hong Kong can follow successful examples of intervention programs such as the Baby Friendly Hospital Initiative,50 promoting breast-feeding soon after birth, providing rooming-in facilities with infants for mothers, as well as giving psychosocial support from a doula.18,30
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Footnotes |
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We wish to thank Jacqueline Wong and Simmy Wan for providing assistance in literature searches and data presentation.
Received August 7, 2001. Received in revised form November 27, 2001. Accepted December 18, 2001.
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REFERENCES |
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