Only a handful of studies have examined the relationship of a woman's menstrual cycle to her blood glucose control, but they have one finding in common: menstruation's effect on blood glucose is as varied as each individual's disease. As a result, blood glucose testing remains the only way to know how a woman's monthly cycle affects her diabetes control.

Kimberly Trout, an assistant professor at Villanova University's College of Nursing who led one of the more recent studies, said that her study and others reveal decreased insulin sensitivity during menstruation as the most common issue. According to various studies and published reports, including Trout's, this decreased insulin sensitivity could be related to fluctuations in hormone levels during the different phases of the menstrual cycle, as well as inflammation and premenstrual symptoms such as mood swings.

But studies and anecdotal evidence also show opposite effects for some women, and no effect for others. In Trout's study, published in 2007, three out of five women experienced this phenomenon, but the research didn't pinpoint its etiology. And for one person in the study, insulin sensitivity improved.

"Progesterone is thought to be the main culprit in decreased insulin sensitivity during the luteal, or post-ovulation, phase," Trout said in an interview. "It's not completely understood. I also know so many women whose blood sugar levels drop. Because there are such individual and varied results in studies, everyone needs to really monitor themselves."

In some cases, fluctuations can cause DKA, according to a Review of Endocrinology article published last September. But the same article notes that extreme cases are not common and that even mild fluctuations are not always experienced by menstruating women. Trout suggested that an analogy can be made with the dawn phenomenon, an event clearly identifiable in many individuals with diabetes, but not necessarily experienced by all.

H. I. Cramer was apparently the first researcher to note fluctuations in blood glucose resulting from menstruation. His findings were published in the Canadian Medical Association Journal in 1942. In another early study on the subject, published in the British Medical Journal in 1977, 76 out of 200 women reported that menstruation changed their diabetic control. Specifically, 53 reported that their control deteriorated and their blood glucose increased. Twenty-three, however,  experienced improved control and problems with hypoglycemia.

According to the BMJ article, "Menstruation appears to be an important factor in influencing control of diabetes. The mechanism of the changes observed has not yet been determined, but it seems to be a subject worthy of further investigation." Not much has changed since then. Over the years, changes in insulin sensitivity during menstruation have been noted in anecdotal reports from patients, but these changes have been difficult to document due to the many factors that impact insulin sensitivity. Trout said it's also difficult to recruit women for studies. "Women between 18 and 45 are busy women," she said. "We screened over 100 women, and only six completed our study."

Trout is hopeful that new technologies, in the form of more precise continuous glucose monitoring, will make the phenomenon easier to track in individual women. When that occurs, monthly blood glucose patterns will be easier to identify and remedy. In terms of future study, Trout said, "The effect of premenstrual symptoms and PMS is something to take a look at."