The Menstrual Cycle:
Most women can relate to at least one if not all of the symptoms of PMS:

Craving chocolate during the menstrual cycle is more natural than one might imagine.  Estrogen and progesterone rise and fall during the menstrual cycle.  The peaks of these hormones influence a drop in magnesium levels.  During ovulation, magnesium drops dramatically as that is when these hormones are at their highest.  Chocolate is a source of energy, delivering glucose AND, in the case of dark chocolate, magnesium.  (a 1-oz. piece of Dark Chocolate 80% cacao contains more magnesium than most other food sources)

PMS is a group of symptoms that mirror low energy and low magnesium levels.

Infertility:
There is also information that suggests that spasms in the fallopian tubes could be one of the causes of infertility preventing the fertilization of the egg and transport to the uterus for implantation.  Magnesium helps with the reduction of spasms in smooth muscle tissue.

Pregnancy:
Pregnancy is energy consuming, and a time when your body needs more magnesium, along with other important nutrients.  It may lessen the likelihood of pre-eclampsia, which is important in fetal development.  Magnesium allows the fetus to absorb more nutrition through the placenta.

Weight:
Magnesium helps with weight control in several ways.  It helps in the process of digestion and using the proteins, carbohydrates and fats that we eat.  It improves insulin function, otherwise the food gets stored as fat, especially around the belly.

It is difficult to obtain your full daily magnesium requirement solely through the food sources, so supplementation (either by transdermal or oral means) helps to restore the balance your body needs.