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