"Trimethylaminuria is a genetic mutation that causes the body to produce too much trimethylamine, a compound found in fish. Particular foods, medication and hormones can exacerbate the condition."
This unusual condition is caused by a mutation in the FMO3 gene, and leads to an accumulation of trimethylamine because it cannot be converted to trimethylamine N-oxide. trimethylamine is a pungent compound found abundantly in fish.
However there exist milder variant polymorphisms, and even "normal" woman can be subject to the fish smell during menstruation and have transient trimethylaminuria because of a decrease in FMO3 activity (perhaps an evolutionary deterrent for mating?):
"In comparison, three healthy control subjects that harbored heterozygous polymorphisms for [Glu158Lys; Glu308Gly] FMO3 or homozygous for wild FMO3 showed normal (> 90%) metabolic capacity, however, on days around menstruation the FMO3 metabolic capacity was decreased to ~60-70%. CONCLUSION: Together, these results indicate that abnormal FMO3 capacity is caused by menstruation particularly in the presence, in homozygous form, of mild genetic variants such as [Glu158Lys; Glu308Gly] that cause a reduced FMO3 function"
Like a few other syndromes (such as maple syrup urine), trimethylamineuria can be diagnosed a a simple whiff of body odours, but it may be confused with halitosis. Thankfully this group from Philadelphia produced a better diagnostic protocol to distinguish the two:
"Because of our basic research into the nature of human body odors, our lab has received referrals of people with idiopathic malodor production, from either the oral cavity or body. We developed a protocol to help differentiate individuals with chronic halitosis from those with the genetic, odor-producing metabolic disorder trimethylaminuria (TMAU)."