The book Adventures in Tandem Nursing is a really good resource. I will consult the Breastfeeding Answer Book for you too:
"In its 2002 positions statement on breastfeeding, The American Academy of Pediatrics asserted: Breastfeeding during a pregnancy is not unusual. If the pregnancy is normal and the mother is healthy, breastfeeding during pregnancy is the mother's decision."
The book also states that a difficult pregnancy may be a reason to consider weaning.
In my personal experience, I only know 2 people (LLL leaders) who nursed through their pregnancies, though both did night wean at that time just so as to get better sleep. One experienced pain and a decrease in milk supply and had a difficult pregnancy with bedrest recommended at 2 or 3 different stages due to threat of preterm labor or spotting, but continued to breastfeed her toddler. The other experienced no pain or reduction in supply and nursed without issue through her pregnancy. Another friend and mother of 6 only breastfeeds until she conceives because she had 2 miscarriages, both of which were while nursing and feels that there is a correlation in her. I had a miscarriage while nursing and while I don't feel there was necessarily a correlation, I still hop to wean before we TTC. Again, sure to be an unpopular sentiment on this board and of course not saying that they WERE related, just my own feelings.
Book lovin librarian nerd mama to Caleb 6/06 and Aiden 4/09: and 1 angel 11/07. "No one cries alone in my presence."