Although ovulation usually happens before menstruation this is not always the case with the first period after childbirth. This period is often infertile ? a sort of priming the system to get to all in working order again. The complication arises with the longer the first period is delayed the more likely it is for the mother to have ovulated. So whereas if your periods return at say, 12 weeks postpartum, the period is likely to be infertile but if the first period returns at, say 12 mouths, then the first period is likely to be fertile.
I believe that there are other things which are likely to affect an individual mother and her likelihood of getting pregnant while breastfeeding, obviously the first is how much and how often the baby sucks, but other things like family history and diet may also play a part, especially the amount of animal protein the mother eats.
My first paragraph here is supported by research the second however, are ideas I?ve pulled together from various research I?ve read but most is dated and it is more my personal conclusions.
Best wishes,
SARAH