There is some reference to this on the LLLI site.
A quote from LLLI:

"Many mothers wonder whether uterine contractions stimulated by
breastfeeding pose a danger to the unborn baby or increase the risk
of a premature birth. There is no documented danger to mother or
fetus when mothers breastfeed through a healthy pregnancy.
Unfortunately, miscarriage and premature birth do happen in a
percentage of all pregnancies, whether a mother is nursing another
child or not. When there are problems, it is only human to look for
somewhere to place blame. All too often, breastfeeding is a handy
scapegoat when things go wrong."

the URL for the whole article is:
http://www.lalecheleague.org/llleaderweb/LV/LVMarApr95p35.html

Also interesting is the web page for the new edition of Mothering
Your Nursing Toddler.  One of the topics discussed is nursing during

pregnancy.  There are many anecdotal stories about toddler
nursing at this site.  The URL for nursing during pregnancy is:

http://www.telepath.com/wordwrks/mynt/Motherly_advice.html#Preg

Janet Vandenberg
RN, BScN
Ontario, Canada