Objectives: This study sought to explore hormonal and postnatal factors, which are associated with the return of menstruation in breastfeeding mothers.
Design: A prospective cohort study in Bristol, UK. Ninety-one breastfeeding mothers of mixed parity were recruited at community-based antenatal clinics. Eighty-five mothers completed all stages and informed us about the return of menstruation. Data were collected from interviews and questionnaires antenatally, immediately postnatally, and 1 week, 4 weeks and 6 months later. Blood samples were taken at all time points except at 6 months. Associations between hormone levels, postnatal factors and the return of menstruation were explored using univariate and multivariate statistical survival analysis.
Results: Sixty-five (76%) mothers were still breastfeeding at 6 months and 46 (54%) of these were fully breastfeeding (not giving formula milk); 21 (32%) mothers started menstruating in the first 6 months whilst they were still breastfeeding and 14 were fully breastfeeding at 6 months. Survival analysis modelling showed that formula use (RR=4.27; 95% CI=1.89, 9.67), pacifier (dummy) use (RR=3.26; 95% CI=1.35, 7.84) and lower postnatal progesterone levels (P=0.006) each made a significant contribution to the chance of earlier resumption of menstruation. Prolactin and oestradiol levels showed no associations with the return of menstruation.
Conclusions: Higher postnatal progesterone levels are associated with delayed menstruation; while the use of pacifiers and infant formula milk are associated with an earlier return to menstruation.