An excerpt from Marilyn M. Shannon’s book, Fertility, Cycles & Nutrition.
The experience of a nursing mother — as she follows a continuum from complete infertility to normal fertility — is instructive for non-nursing women with irregular cycles or unexplained infertility.
At the beginning of breastfeeding
When the hormones of ovulation — FSH and LH — are consistently low, as in a nursing mother with extended amenorrhea (the absence of periods), follicle development does not occur and the endometrium [uterus lining] does not develop. The result is that neither ovulation nor menstruation occurs.
As mucus returns
As she begins to produce more of the reproductive hormones, cervical mucus appears and the endometrium develops. If the nursing mother does not ovulate, eventually the endometrium will be shed away, causing spotting or a full, sometimes prolonged episode of bleeding. Such a “period” without previous ovulation indicates that an infertile, “anovulatory cycle” has occurred. It is common for a breastfeeding mother to have one or several such anovulatory cycles and periods.
As the inhibition of fertility (caused by the hormones of breastfeeding) continues to subside, increased FSH and LH may stimulate an ovarian follicle to develop to some extent, and an immature egg, probably incapable of fertilization, may be ovulated. If so, a basal temperature shift occurs, although it may be less than the usual shift of 0.4° F. Since the levels of FSH and LH in the pre-ovulatory phase [of each cycle] set the stage for the function of the corpus luteum after ovulation, luteal function [Phase II] may be reduced.
A sympto-thermal chart, on which the signs of fertility are recorded, may show this with a luteal (post-ovulatory) phase which is quite a bit shorter than the normal twelve to sixteen days of elevated basal temperatures. Pregnancy is less likely to occur if the duration of elevated temperatures in [Phase II] is less than nine to twelve days.
Spotting may also occur before the actual period begins, while the temperatures are still elevated. Such “irregular shedding” may be yet another manifestation of luteal insufficiency. The entire pattern at this stage of the nursing mother’s return to fertility commonly includes an extended patch of “more fertile” mucus prior to ovulation, as well as a delay in ovulation that may cause a long cycle.
Back to normal
Finally, the fertility pattern of the nursing mother returns to its familiar norms, and she may continue to nurse for many months while having fertile ovulatory cycles, and she may even conceive again while doing so.
— Marilyn M. Shannon is a tenured, part-time instructor of biology at Indiana University Purdue University at Fort Wayne, Indiana, where she has taught human anatomy and physiology for more than 30 years. Her book, Fertility, Cycles & Nutrition is a staple for many NFP families and is available here.