Are you charting your cycles at all? That will give you a better idea of whether nursing could be a "problem." The primary cause of breastfeeding infertility is the lowered estrogen levels, which supresses ovulation altogether. However, once you start ovulating again, breastfeeding often causes lowered progesterone levels. If your progesterone levels are too low to maintain the uterine lining long enough, you may start menstruating before a fertilized egg has a chance to implant. The best way to see if this is an issue is to chart your basal body temperature to see how long your luteal phase (the time from ovulation to menstruation) is. (I apologize if you know all this already.) The rise in progesterone after ovulation causes an observable increase in your temperature, and you can count the number of days of elevated temperature you have to check the length of your luteal phase -- your temperature will drop again right before your period starts. Ten days is usually the shortest luteal phase to allow a good chance at implantation -- an average length luteal phase is around 14 days. So if you are having luteal phases of less than 10 days, you may not be able to maintain a pregnancy.
I did not start cycling again until my dd was 19 months old, and my first 2-3 cycles were very long, but with extremely short luteal phases (my first cycle was 52 days long, in which I did not ovulate until about day 46!) By my 5th and 6th cycles I was up to a 10 day luteal phase (I had a 14 day luteal phase pre-pregnancy) and we successfully conceived again on our first try during my 7th cycle post-partum, while my dd was still nursing several times day and night -- dd just weaned within the past few weeks and I am uneventfully in my second trimester.
It seems from all that I have read (and I read a lot
) that nursing during pregnancy is only a concern in the very early stages, for the reasons mentioned above, and in women with a history of pre-term labor. As another poster mentioned, the oxytocin receptors are not formed until later in pregnancy, so the contraction-causing aspects of nursing should not cause an increased risk of first-trimester miscarriage. Low progesterone levels might not even be recognized as causing miscarriages, but rather in difficulty conceiving while nursing.
Progesterone is also important in maintaining pregnancy even after implantation, as the corpus luteum has to maintain progesterone production until the placenta can take over around the beginning of the second trimester. However, an important thing to note is that you can have your OB or midwife check your progesterone levels, and progesterone supplements can be given if they are low.
I think the average woman should not worry about nursing during a pregnancy, as it is not usually a problem. I think it is good to know about the progesterone issue however, so that if a woman is having trouble conceiving or suffers early miscarriages while nursing, she can look into her progesterone levels.