Women who undergo regular mammograms may want to consider scheduling their screening for the first week of their menstrual cycle, according to a new study.

The breast tissue may be less dense during this week, so mammograms conducted at this time may be more accurate for some women, the researchers said.

Mammograms are known to be less accurate in younger women, in general, than in older women, and one reason for this might be that younger women have denser breast tissue, which makes tumors harder to spot. (A separate line of research has linked denser breast tissue with a higher risk of breast cancer in women of any given age.)

However, before the new research, very few studies had looked at the impact of breast density on the sensitivity of mammogram results, and some had suggested the density varies too little over the course of a woman's menstrual cycle to have any impact, said study researcher Dr. Diana Miglioretti, a senior investigator at Group Health Research Institute in Seattle.

Miglioretti and her colleagues examined mammogram results from 387,218 pre-menopausal women, most of whom were in their 40s. Of those, 1,283 mammograms revealed breast cancer. [Related: Q & A with Dr. Miglioreti: Can Mammograms Cause Cancer?]

They found that, among women who'd had a mammogram within the last two years, mammograms were more sensitive at detecting breast cancer if a women was in the first week of her menstrual cycle.

For this group, mammograms were nearly 80 percent sensitive during the first week of the woman's menstrual cycle, meaning a physician could identify breast cancer from the mammogram 80 percent of the time when cancer was present. For other weeks of the cycle, the sensitivity of the screening ranged from 67 percent to 73 percent, leaving more cancers unseen.

The researchers aren't sure why menstrual cycle phase did not seem to affect mammograms for women who had not had a mammogram in the previous two years. But Miglioretti noted that breast cancers found in women at their first mammogram tend to be larger, while the cancers found in women who've been screened before tend to be smaller. It may be that the lower breast density experienced during the initial part of the menstrual cycle is only helpful in increasing the detection rate of these smaller tumors, she said.

The rate of mammograms turning up false positives — meaning a mammogram seems to reveal cancer that is actually not present — was not affected by the phase of a woman's menstrual cycle.

False-positive results may lead to unnecessary subsequent testing, and mammograms that miss true cancers —called false-negative results — are more common among women in their 40s than women who are older.

The American Cancer Society recommends women with an average risk of breast cancer get mammograms starting at age 40, and the National Cancer Institute recommends that women age 40 and older get a mammogram every year or two. The U.S. Preventive Services Task Force recommends women between 50 and 74 years old receive mammograms every two years, and women younger than 50 should speak with their doctor about when to begin breast cancer screening.

The new findings do not apply to women who have received a mammogram for diagnostic reasons — that is, to follow up on symptoms such as a lump in the breast.

"If a woman has a breast lump or some other concern, she should contact her physician immediately and not wait to schedule that appointment," Miglioretti said.

Another benefit to receiving mammograms in the first week of the menstrual cycle may be reduced discomfort — many women report experiencing tenderness of the breasts in the second half of their cycle, Miglioretti said.

The study was published Dec. 3 in the journal Radiology.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @Rachael_MHND.

This article was provided by MyHealthNewsDaily, a sister site to LiveScience.