There is no proof that breastfeeding while pregnant increases the risk
of miscarriage.  Really though, with the high rate of miscarriage
anyhow, this is a tough thing to figure out.  But what about if
miscarriage is threatening--does this increase the risk of completion of
the miscarriage.  I don't think anyone knows.  But personally I doubt
the minimal hormonal changes that occur when a 4 year old nurses
occasionally are likely to do much.

Jack Newman, MD, FRCPC