An OB friend shared the following wth me.  He said uterine oxytocin
receptor sites are seneitive to oxytocin only during labor and just after
delivery...for which they are specific reason. !. expel the baby, 2.
contract uterus to prevent hemmorage. He did not think nursing while pg.
could be a problem because the receptor sites wern't sensitive.  (I'll
ask him about receptor sites during orgasim...I think they work! but
maybe there is a different mechanism).  I nursed through 3 pregnancies,
nursing augmented contractions during labor, but did not induce any early
labor...all deliveries were late! Certainly some women may be more
sensitive or pregnancies more tenuous. And, of course, if the mother can
continue with sex during pregnancy, there is little evidence that nursing
would be harmful.
Reference: Moscone, Breastfeeding while Pregnant JHL, 1993 or 1994,
indicates no greater incidence of miscarriage while BFing. Anne
Andrianos, MSN, IBCLC, LLL