Fertility While Nursing

I found this a couple weeks ago and it explains rather reasonably how fertility returns post-partum, while breastfeeding:

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
From: http://www.kathydettwyler.org/detfert.html

The author, Kathy Dettwyler, has studied a lot of different aspect of nursing. Her site is pretty interesting to poke around:
http://www.kathydettwyler.org/

I found her mentions about the natural ages of weaning amongst other large mammals to be affirming of what I have been observing with my little one.