It's most likely related to rising progesterone levels. I used to think it was estrogen, but studies have shown that progesterone and testosterone have a very strong effect on the function of the chloride ion channel (but not estrogen).
Non-genomic effects of sex hormones on CLC-1 may contribute to gender differences in myotonia congenita.
Author information: Fialho D1, Kullmann DM, Hanna MG, Schorge S.
Abstract
Myotonia congenita is caused by mutations in the voltage-gated chloride channel ClC-1. It is more severe in men than women and often worsens during pregnancy, but the basis for these gender differences is not known. We show here that both testosterone and progesterone rapidly and reversibly inhibit wild-type ClC-1 channels expressed in Xenopus oocytes by causing a prominent rightward shift in the voltage dependence of their open probability. In contrast, 17beta-estradiol at similar concentrations causes only a small shift. Progesterone and testosterone also profoundly inhibit ClC-1 channels containing the mutation F297S associated with dominantly inherited myotonia congenita. The effects of sex hormones are likely to be non-genomic because of their speed of onset and reversibility. These results suggest a possible mechanism to explain how the severity of myotonia congenita can be modulated by sex hormones.
I use a small amount of the herb Black Cohosh which seems to help (you should not take >40 mg per day). I also use small amounts of licorice root extract (250 mg per day).
But then you can get into the problem of estrogen dominance which can stimulate the growth of breast cancer. As far as I know, only the herbs bloodroot and mandrake have been shown to both block progesterone and suppress estrogen receptor positive breast cancer.
Since progesterone is produced in large amounts by the ovaries, sometimes a complete hysterectomy or ovarianectomy can give some relief, but the adrenals can still produce progesterone. And we definitely need it, but not in large enough amounts to inhibit the chloride channel function. During pregnancy progesterone is produced by the placenta, which is why myotonia often improves for women who were miserable during their pregnancy shortly after they give birth.
I forgot to mention that fat cells also produce progesterone, so the less body fat you have, the lower your progesterone should be. A good reason for those of us with MC to keep our weight down :-)