The BAB lists six references for this information on page 352 (including
Lawrence, Riordan & Auerbach, and an article in JHL).  I think what Dr.
Newman said recently about this was most interesting (and I am not quoting,
just recalling) that if all it took was the little bit of oxytocin released
during breastfeeding to cause a miscarriage, then doctors would use that
little bit of oxytocin to do abortions - and they do *not* do this!  Just
attach a breastpump for a few minutes to a mother who wants to end a
pregnancy, and there you have it...

Lisa Jones, LLLL in Wellington Florida