Menstruation is the end result of an elegant and intricate chain reaction of chemical signals via hormones that trigger the physical processes that make up the menstrual cycle.
The cycle begins with the hypothalamus region of the brain sending the gonadotropin releasing hormone (GnRH) to the pituitary gland. In turn, the pituitary gland releases follicle stimulating hormones (FSH), which stimulates the production of an egg in the ovarian follicle and also generates the production of estrogen in the ovaries. Estrogen is a hormone responsible for stimulating new cell growth along the uterine lining, or endometrium. Another chemical, leutinizing hormone (LH), triggers the release of the egg (ovulation), which should ultimately attach to the endometrium. Once the egg is released, the follicle produces estrogen and another hormone, progesterone, both of which stimulate new cell growth along the endometrium to ensure that the egg attaches to the uterine wall.
If the egg is fertilized, the follicle continues to pump out progesterone and estrogen to ready the endometrium for the egg's growth into an embryo -- and eventually, into a fetus.
It's here that we learn why breast-feeding can prevent pregnancy. As we discussed, the hypothalamus secretes the gonadotropin releasing hormone that sets off the whole sequence of menstruation. The release of that chemical appears to be repressed by the prolactin hormone, which encourages the production of breast milk. Without the GnRH release, the menstrual cycle won't begin anew in the event of a pregnancy. As long as prolactin levels are high -- during a postpartum period of exclusive breast-feeding, for example -- then GnRH generally won't be released.
So, it was reasonable for old wives to conclude that a woman can't get pregnant while breast-feeding, especially considering that this observation would've been made before the advent of any supplements or alternatives to breast-feeding. Since breastfeeding was the only option for nourishment back then, the blood of nursing mothers would've had high levels of prolactin -- and probably wouldn't have been able to become pregnant during the breast-feeding period.
What may have once been true no longer necessarily holds, however. Due to the widespread availability of baby formula, women tend to breast-feed less frequently. Instead, physicians often recommend that nursing women who want to avoid pregnancy use a barrier method of birth control, like a condom or diaphragm [source: Parenting].