Repeated measurements over several months may be required to figure out whether a woman is having high cholesterol level, given that the lipid profile fluctuates during menstrual cycle, says a new study.

During the menstrual cycle women's cholesterol levels vary because depending on the rise and fall in levels of the female sex hormone estrogen.

"Doctors who are looking at women (for) high cholesterol have to take into account the phase of the menstrual cycle they are at when they take the measurement," says study co-author Enrique F. Schisterman, chief of the Epidemiology Branch at the Eunice Kennedy Shriver National Institute of Child Health and Human Development.

To make cholesterol readings more consistent and reliable, measurements should be taken at the same time each month for a couple of cycles.

"Practically, it's easier to recognize the beginning of a cycle. So if you do it consistently at the beginning of the cycle then you will get consistent measures over time," Schisterman notes in the report published online in the Journal of Clinical Endocrinology and Metabolism.

To reach this conclusion, the researchers compared levels of estrogen with cholesterol and triglyceride levels in 259 healthy women, aged between 18 and 44.

While 94 percent of the women had 14 or more tests taken over two menstrual cycles, the women also measured the phases of their cycles using fertility monitors that detect hormone levels indicating ovulation.

Only five percent had cholesterol levels higher than 200 mg/dL, which is borderline high-risk for heart disease. But, cholesterol levels among 19.7 percent of the women reached 200 mg/dL at least once. A few of the obese women over 40 had greater fluctuation in cholesterol levels compared to the rest of the group.

The researchers noted that as estrogen levels rise, HDL, or "good" cholesterol also rises. The estrogen levels reach the maximum at the time of ovulation. However, they found that as estrogen levels increased, total and LDL, or "bad" cholesterol, as well as levels of triglycerides, fell. The reduction of bad cholesterol began in the days after ovulation.

Levels of total cholesterol, LDL cholesterol and triglycerides were lowest just before the start of menstruation, the researchers found. "This is more recognition that hormones play a very important role in women's lives on all levels, including basic tests, like the test for cholesterol," Schisterman says in his report.