Breastfeeding has considerable benefits for both mothers and infants. Current research evidence is demonstrating that events during labour and birth may have an effect on establishing exclusive breastfeeding (Forster & McLachlan, 2007). This study used a retrospective cohort methodology to assess for an association between prophylactic ecbolic medication used for the third stage of labour and exclusive breastfeeding at two weeks postpartum. Method: The study took advantage of data that had been collected and stored as part of routine maternity care. Two data sets were merged: one from a New Zealand District Health Board maternity database (Ecbolic data); and one from the New Zealand Ministry of Health maternity database (Breastfeeding data). After exclusions the final cohort consisted of 5988 well women giving birth to a normal healthy infant with no medical interventions. Binary regression analysis was used to evaluate an association between the confounding variables and exclusive breastfeeding at two weeks, and then multivariate analysis was performed with prophylactic ecbolics and the significant confounding variables to examine the relationship between exclusive breastfeeding and prophylactic ecbolics. Results: Univariate analysis revealed that ethnicity was significantly associated with exclusive breastfeeding at two weeks, with Chinese women OR 0.49 95% CI (0.37 – 0.64) and Samoan women OR 0.81 95% CI (0.66 – 0.99) being less likely than New Zealand/European women to be exclusively breastfeeding at two weeks. Multivariate analysis established that use of prophylactic ecbolics was not an independent factor predicting exclusive breastfeeding at two weeks in this cohort. Conclusions: Further research is needed to understand why women elect to stop exclusive breastfeeding prior to the recommended 6 months and what factors may be successful in influencing changes in behaviour in this regard. Given that Chinese and Samoan women are most likely to stop exclusive breastfeeding this would be a priority area for both health intervention and research.