Hi ladies! I wanted to post this (I know it is long but super informative), because this question often gets asked. I have tried to explain to people about the fact that some women can get pregnant while they are nursing and others can't. I just thought this was great information to pass along to this community. Anyway, this was written by Katherine A. Dettwyler, Ph.D, in 1995, but is still great information. Hope this helps someone!
There is a chapter in Breastfeeding: Biocultural Perspectives on "Breastfeeding, Fertility and Maternal Conditon," by Peter Ellison. He is an anthropologist and head of the anthropology department at Harvard University. This chapter takes a historical look at the research that has been done on understanding the links between breastfeeding and fertility, from the earliest days up to 1993, when he finished his chapter (it takes forever to get a manuscript through all the stages in to print).
Here's my brief synopsis of his thorough chapter. Suckling by the baby causes the mother's pituitary to release prolactin. It used to be thought that prolactin directly affected ovulation/fertility, but new research suggests that there is another hormone intermediate between prolactin and the ovaries. So that high levels of prolactin lead to either high or low levels of this other factor, which then affects fertility. Fertility is not an "either/or" sort of phenomenon. Post-partum, a woman does not ovulation for a while, even if she isn't breastfeeding. If she is breastfeeding frequently enough to keep her prolactin levels above her individual critical threshhold for fertility (and women vary in this threshhold) then her fertility is suppressed.
The greatest level of suppression is not ovulating, but as your prolactin levels go up, your fertility will gradually return. First you will ovulate, but not have the proper hormone levels for fertilization; then you will ovulate and fertilization may occur, but you still may not have the proper hormone levels for implantation; finally, you may ovulate, be fertilized, and implant, but not have the proper hormone levels for continuing the pregnancy, so you have a very early miscarriage, probably along the lines of minutes or hours after implantation, so you wouldn't know you had been pregnant. It is also possible to ovulate without having the right hormonal levels in the right combinations for the uterus to have been preparing for implantation, so yes, it is possible to ovulate without menstruating. For all of these stages, there seems to be incredible individual variation between women. Some women get pregnant again the first time they ovulate, with no intervening menstrual periods. I knew a woman in Indiana years ago who had three children in six years with no menstrual periods! Her doctor couldn't figure out when to predict her due date :)
Also, would you believe there is no research out there yet, none at all, on whether it is possible for the trajectory of gradually returning fertility to be reversed as a result of increased nursing? I specifically asked Peter Ellison to include this research in his chapter, and he assures me there isn't any, though as he puts it "Logically, it makes sense." That is, if the baby nurses more frequently again,, after the mother's periods have returned, it would raise the mother's circulating levels of prolactin, presumably high enough to affect fertility again.
I am sure that this works, both from personal experience and from anecdotal reports from other nursing moms. To give a specific example, with my third child Alexander, I worked mainly from home the first year of his life, and spent many hours at my computer writing with me logged on and him latched on! The summer of 1992, when he was a year old, I was at home for the summer (not teaching classes) and he nursed a lot. When he was 13 months old, and fall semester was starting, I put him in day care 6 hours a day, so I could have more time at the office, and my periods promptly returned the next month. I had a period in October, and one in November, then we finished for the semester in mid-December and I was at home with him all day for 5 weeks (what a job :)). I didn't have a period in December or January, then resumed again for good in February. Sigh. I wish someone would develop a pill that mimicked the actions of lactation amenorrhea so I didn't have to have periods for the next who knows how many years. And I don't have PMS or menstrual cramps or anything, so I know I shouldn't complain. Back to the subject at hand -- my understanding is that a woman is born with thousands of eggs, and that menopause has nothing to do with using up all your eggs.
I would imagine that there could be some connection between continued breastfeeding and failure to sustain a pregnancy, especially in a woman who released large amounts of prolactin in response to nursing, or whose ovaries were especially susceptible to whatever level of prolactin (or other hormone) she did produce. You should consult a reproductive endocrinologist to see if this is the case. I don't think the oxytocin levels that cause uterine contractions could be leading to miscarriages that long after the birth of the first child, but the prolactin levels might be interfering with implantation or continuation of the pregnancy.
You can find much of Peter Ellison's information in his already published works. If you have access to a university library, look him up on the computer and see what they have by him. I hope this helps.