A friend of a friend recently asked me a question that I couldn’t answer. She’s in her late 30s, has a two year old daughter, and has been breastfeeding on demand for the past two years. She and her husband have been trying to have another child, but she just recently learned that she miscarried after their first attempt. She has only recently started getting her period again, and was wondering if the breastfeeding could negatively impact her body’s ability to get pregnant again. I speculated that the high levels of prolactin which occur during breastfeeding might inhibit ovulation, just as high levels of oestrogen inhibit breastmilk supply by competing with prolactin for binding sites in breast tissue, but I told her I wasn’t really sure and that I would investigate. I thought that somehow oestrogen and prolactin were counter opposites: one could not exist in high levels while the other was around. Turns out I was waaaaay off base. Here’s what I found:
During pregnancy, the corpus luteum, acting on instructions from the placenta, secretes the oestrogen and progesterone necessary to maintain the pregnancy. These high levels of steroid hormones simultaneously suppress Follicle Stimulating Hormone (FSH) and Leutenizing Hormone (LH), the two hormones most responsible for ripening an egg and then triggering ovulation—after all, if you’re already pregnant, there’s no need to ovulate. After delivery, once the placenta is removed, the high levels of oestrogen and progesterone no longer exist, and the levels of FSH and LH gradually begin to rise again, preparing the body for ovulation. Eventually, as the levels creep up, the pituitary takes notice again, and begins to release more FSH and LH through a negative feedback loop, which eventually will trigger ovulation.
“Most nonlactating women resume menses within 4 to 6 weeks of delivery, but about one-third of the first cycles are anovulatory, and a high proportion of first ovulatory cycles have a deficient corpus luteum that secretes sub-normal amounts of steroids. In the second and third menstural cycles, 15% are anovulatory and 25% of ovulatory cycles have luteal-phase defects…Lactation, or breastfeeding, further extends the period of infertility and depresses ovarian function. Plasma levels of FSH return to normal follicular phase values by 4 to 8 weeks postpartum in breastfeeding women. In contrast, pulsatile LH stimulation is depressed…in the majority of lactating women throughout most of the period of lactational amenorrhea.” 
In other words, after not menstruating for so many months, it takes the body a few tries to get the delicate hormone balance back up to speed again. The first few cycles either don’t release an egg, or if an egg is released, the corpus luteum, which is responsible for secreting enough progesterone to maintain the pregnancy until the placenta can take over, isn’t quite up to the task. This is called a luteal phase defect, and it’s a very common cause of early miscarriages. In women who are breastfeeding, the process of returning to normal ovarian cycles takes even longer.
In breastfeeding women, FSH, the hormone responsible for ripening an egg, returns to normal pre-pregnancy values fairly early, but LH, the hormone responsible for triggering egg release, continues to be suppressed due to the breastfeeding. (However, contrary to popular belief, prolactin is not at all responsible for this suppression. It’s the constant suckling and stimulation of the nipple itself which actually suppresses ovarian function, which is why on demand breastfeeding is so essential to maintaining lactational amenorrhea.)
So, there you have it. To answer the question: it will probably just take a few more cycles for your body to get back into full swing in terms of ovulating, but continued breastfeeding did not contribute or cause the miscarriage in any way, and will not prevent conception. Most likely, the miscarriage was caused by a short luteal phase or corpus luteum that just wasn’t quite ready to maintain a pregnancy, and this will no longer be a problem once your body goes through a few more cycles and gets used to ovulating again.
 Hatcher, R.A. et. al. (2011) Contraceptive Technology, 20th Revised Edition. Ardent Media, Inc.: New York.