My understanding is that breakthrough bleeding occurs when the pill is not
taken at the SAME time every day.
As for progesterone only BC methods. We've seen dramatic drops from the
mini-pill, depo-provera and norplant in our clinic. So much so, that if a
BF'ing mom wants hormonal birth control we tell her to first use the
mini-pill for a month before the depro-provera. Once depo-provera is given it
is in the body for months.
Marie