Hi,
Well, I wish I could give references, but all I can do is anecdotal info, and
personal at that.  I have had 2 miscarriages, one while NOT BF, and one while
BF.  Both were by the 10th week of pregnancy.  I am currently BF a 27 mo old
and am also 8 mo pregnant.  I also was warned by several people to wean
because of fear of miscarriage.  However, that would have been traumatic to
my son and to me, emotionally, and I have some really great midwives who said
that to wean would be silly.  The only time they reccommend weaning to
prevent miscarriage is when the mother has a history of several miscarriages,
especially later than the 1st trimester and when the miscarriage is due to
contractions(early labor) brought on by stimulation of some sort (nipple or
sex).
SO after my second miscarriage (while BF) they told me that if I experienced
a lot of contractions while nursing, then I may want to consider either
weaning completely or partially for the sake of the baby.  With a healthy
pregnancy, and me being healthy, there has been no need for any weaning on my
part.  Now my toddler cut back quite abit when my supply crashed, but he
still nurses at least 2X per day, and I very rarely ever have contractions
while BF him.
If I can find any references, I will post them!
Good luck to this mom!

Jay