Discussion

This large prospective study identified factors associated with breast-feeding initiation and continuation in Canadian-born and non-Canadian-born women. Approximately one-quarter of the sample were non-Canadian-born participants, but contrary to other studies,^{(Reference Dennis, Shiri and Brown17,Reference Lok, Bai and Chan18)} we found only minimal differences in the breast-feeding duration of Canadian-born and non-Canadian-born women. We also did not find that migration characteristics affected either breast-feeding initiation or breast-feeding continuation in non-Canadian-born participants, although study power may have been limited for the current analysis. Multivariable analyses indicate that different factors affect breast-feeding initiation and continuation in the two groups of participants. In Canadian-born participants, socio-demographic characteristics, such as older maternal age, higher education, and living with a partner, were positively associated with breast-feeding initiation and continuation, while living with a smoker was negatively associated with both breast-feeding outcomes. These associations were observed less consistently in non-Canadian-born participants. Maternal smoking decreased the odds of breast-feeding initiation in Canadian-born participants and decreased the odds of breast-feeding continuation in all participants. In Canadian-born participants, those recruited in Vancouver had breast-feeding initiation rates that were higher than participants recruited at the other study sites. Among all participants, those recruited in Vancouver had approximately two to three times higher odds of continuing breast-feeding at 6-month postpartum. Early-term and caesarean birth were associated with lower rates of breast-feeding initiation and continuation in Canadian-born participants only.

The association between socio-demographic characteristics (e.g. maternal age, education and marital status) and breast-feeding initiation and duration has been a common finding in other studies^{(Reference Thulier and Mercer33,Reference Cohen, Alexander and Krebs34)} . However, in our study, the associations between breast-feeding and maternal age and education more consistent among Canadian-born women, which is similar to previous Canadian data showing that women who do not breastfeed are more likely to be younger and have less education than those who do breastfeed^{(Reference Gionet6)}. Non-Canadian-born women in our sample were more likely than Canadian-born women to have a university degree or above (69·2 % v. 60·9 %) and to initiate breast-feeding (95·5 % v. 92·7 %). Therefore, it is possible that among non-Canadian-born participants, cultural norms are more influential than socio-demographic characteristics. Studies in other countries have shown that breast-feeding practices are less associated with socio-demographic characteristics than in Western countries, and breast-feeding is sometimes more prevalent in women with lower education levels^{(Reference Tavoulari, Benetou and Vlastarakos32,Reference Tarrant, Fong and Wu35–Reference Bulk-Bunschoten, Pasker-de Jong and van Wouwe37)} .

Our findings on the differences in breast-feeding rates between study sites are consistent with national data reporting that British Columbia has the highest breast-feeding rates across Canada^{(Reference Gionet6)}. Breast-feeding rates in Canada vary substantially across the country, with an east-to-west gradient of lower to higher initiation rates ranging from 57 % in Newfoundland and Labrador to 96 % in British Columbia^{(Reference Green, Chalmers and Hanvey38)}. Furthermore, participants recruited at the Vancouver study-site were all recruited from a large tertiary hospital that was a designated Baby-Friendly Hospital Initiative (BFHI) facility, the only study site to have received that designation at the time of recruitment. Studies show that exposure to BFHI practices improves breast-feeding initiation and duration^{(Reference Tarrant, Lok and Fong39,Reference Brodribb, Kruske and Miller40)} , but further research is needed to explain why these disparities in breast-feeding practices continue within Canada.

Negative associations between maternal and household smoke exposure and breast-feeding duration have been consistently reported in previous studies^{(Reference Lok, Wang and Chan7,Reference Horta, Kramer and Platt41)} . Smoking during breast-feeding lowers prolactin levels, reduces the volume of breast milk and alters the composition and protective properties of breast milk^{(Reference Napierala, Mazela and Merritt42,Reference Baron, Bulbrook and Wang43)} . Women who smoke or who live in a smoking household are also less likely to intend to and initiate breast-feeding^{(Reference Lok, Bai and Tarrant44)}, indicating that psychosocial factors are also associated with the lower rates of breast-feeding among smoking mothers^{(Reference Donath, Amir and Team45)}. Also, research has found that when compared with non-smoking partners, fathers who smoke are more likely to prefer infant formula feeding over breast-feeding^{(Reference Lok, Wang and Chan7)}, and a father’s infant feeding preferences are strongly associated with breast-feeding initiation and duration^{(Reference Bai, Fong and Lok46,Reference Lok, Bai and Tarrant47)} .

In the literature, the associations between breast-feeding and maternal and birth factors, such as early-term birth, caesarean section and rooming-in, are inconsistent^{(Reference Tarrant, Lok and Fong39,Reference Prior, Santhakumaran and Gale48,Reference Fan, Wong and Fong49)} . Participants in our study with early-term birth were 30–50 % less likely to initiate breast-feeding, and Canadian-born participants with early-term birth were 35 % less likely to continue breast-feeding at 6 months. Women giving birth early-term are more likely to have caesarean section^{(Reference Fan, Wong and Fong50)}, and their infants are more likely to be admitted to the neonatal intensive care unit^{(Reference Sengupta, Carrion and Shelton51)} and to have a prolonged hospitalisation^{(Reference Tita, Landon and Spong52)}, all of which can interfere with breast-feeding initiation and duration. A review of 53 studies concluded that caesarean section reduced the odds of breast-feeding initiation by over 40 % but had no effect on breast-feeding duration in women who initiated breast-feeding^{(Reference Prior, Santhakumaran and Gale48)}. However, our analysis showed that caesarean birth remained significantly associated with lower odds of breast-feeding continuation at 6 months in Canadian-born participants, even among the subset of participants who initiated breast-feeding (see online supplementary material, Supplemental Table S3).

Studies in other countries have shown that immigrant women tend to breastfeed longer than native-born women^{(Reference Lok, Bai and Chan18,Reference Rio, Castello-Pastor and Del Val Sandin-Vazquez53)} and that breast-feeding duration decreases with longer time since migration^{(Reference Lok, Bai and Chan18,Reference Barcelona de Mendoza, Harville and Theall54,Reference Harley, Stamm and Eskenazi55)} . A recent meta-analysis of twenty-nine studies found that while immigrant women were marginally more likely to initiate breast-feeding than non-immigrant women, they were twice as likely to continue breast-feeding for 12 to 24 weeks^{(Reference Dennis, Shiri and Brown17)}. However, more than one-half of the included studies (16/29) did not adjust for key socio-demographic variables, such as maternal age, education and family income. In our study, we found no significant association between migration status and migration characteristics and breast-feeding initiation or continuation and other studies in Canada have also shown conflicting findings. In one study of Canadian-born and immigrant women, Dennis et al. ^{(Reference Dennis, Gagnon and Van Hulst56)} found that migrant women were less likely to be breast-feeding exclusively at 16 weeks compared with Canadian-born women. As previously discussed, Canadian-born and non-Canadian-born participants in our study had high maternal education levels, possibly contributing to the similarity of breast-feeding rates. Furthermore, one-third of our non-Canadian-born participants immigrated to Canada before 18 years of age, possibly making them more acculturated to the Canadian lifestyle.

The current study used a large prospective national cohort with detailed data collection at numerous time points throughout the first several years of life. The dropout rate was low, and breast-feeding follow-up data were collected on 88·2 % of the sample. However, study participants had higher maternal education levels and socio-economic status when compared with the general Canadian population⁽⁵⁷⁾, which may account for the high breast-feeding initiation rates and fewer differences between Canadian-born and non-Canadian-born participants than shown in other studies. Therefore, these findings may not be generalisable to the larger population of breast-feeding women. High breast-feeding initiation rates may have affected our ability to determine factors affecting initiation. Another limitation was the exclusion of participants who did not speak and read English as this could have excluded newly arrived immigrants who may be less acculturated to Canadian society and have higher breast-feeding rates. In addition, participants without follow-up breast-feeding data were more likely to be younger with lower socio-economic status, factors which have repeatedly been shown to be associated with lower rates of breast-feeding initiation and continuation^{(Reference Cohen, Alexander and Krebs34)}. Because global breast-feeding rates vary substantially⁽⁵⁸⁾, the analysis of all non-Canadian-born participants as a single group may have affected the findings. Furthermore, variations in participant recruitment practices at different sites many limit the conclusions drawn from the regional differences in breast-feeding practices. In Vancouver, participants were recruited from a single site and all resided in the Lower Mainland of British Columbia, while in Winnipeg, participants were recruited either in Winnipeg or in a rural location in the province of Manitoba. Lastly, in an observational study, we may not be able to fully account for unmeasured confounders.