I'm pregnant and nursing for the fifth time...and am tandem nursing through pregnancy for the fourth time. OBs have told me I'd experience everything from preterm labor (nursing causes the uterus to contract ... see below) to low birthweight baby. I've also hear moms say they were told their breastmilk would go bad (it doesn't go bad, sour, or have an expiration date!) or that they would be robbing their newborn of colostrum (colostrum comes in toward the end of pregnancy and not only is great for the newborn, but gives the older nursling(s) an extra "dose" of all those immune factors and such).
This is something I recently posted in another forum:
Breastfeeding causes uterine contractions, but the uterine contractions it causes are much weaker than those caused by an orgasm, so if it's ok for you to have sex while pregnant, it should be ok for you to nurse through a healthy pregnancy.
As for preterm labor - I was told that as well. I have nursed through four pregnancies (this is the fifth pregnancy I've nursed through) and of those four pregnancies, I tandem nursed through three. My second child was 8 days "late", my third was 3-4 days "late", my fourth was one day "late" and my fifth was 9 days "late." I nursed while in labor and it only caused contractions to pick up while the nursling was actively nursing and then the contractions calmed down to where they were before the nursling latched on.
Some doctors will also tell you that pregnant nursing will cause low birthweight babies. The first pregnancy I nursed through produced a 9.5 lb baby (my firstborn was 7lbs 2oz), and the pregnancies I tandem nursed through produce 8lbs 4 oz, 10lbs 14oz, an 9 lbs 14oz babies!
Resources you might be interested in:
"Adventures in Tandem Nursing" by Hilary Flower
"Mothering Your Nursing Toddler" by Norma Jane Bumgarnerhttp://www.kellymom.com/nursingtwo/faq/index.htmlhttp://www.llli.org/FAQ/bfpregnant.htmlhttp://www.unmc.edu/obgyn/education/preg_bfng.htmhttp://www.llli.org/FAQ/tandem.htmlhttp://www.associatedcontent.com/art...fe.html?cat=52
ETA: Whenever a doctor tells me that there is some sort of danger in pregnant nursing, or something that goes against my research (pregnancy, birth, breastfeeding, whatever), I say something like, "That's interesting to hear, since my research doesn't show that at all. I would love to see your resources on the subject - can you email them to me or can you give me copies at my next appointment?"
Most people don't realize that doctors spend 0-2 hours learning about breasts in medical school, and even then, it's about diseases of the breast, not human lactation. A lot of what they "learn" about breastfeeding their either learn from their predecessors or they make assumptions. I've spoken with MANY OBs, and most admit that they have no idea where they "heard" some of the things they thought were true about breastfeeding and don't have research to back up what they're telling patients. Don't get me wrong, there are some awesome and knowledgeable doctors out there, but more often than not, they don't know much more about breastfeeding than the average person.