Mammomgram screenings during periods

Are the results of a mammogram taken during one's period reliable? The medical services dept of my employer (who gives free annual screenings) told me no. Then, the technicians at a local diagnostic center told me it was no problem. This screening is routine. Can anyone give me a somewhat clear answer?
Thanks.

[CCF-RN,MSN-JS]

Dear Marie-Teresa, In April of this year there was a report of a study looking at comparing mammograms of women taken during their period and early in the menstrual cycle (first week after).  Breast tissue is often less dense early in the menstrual cycle, and less density could make it easier to detect breast cancer on mammogram films. The study that was reported was small, comparing mammograms of only 11 women none with breast cancer history, they did note the decrease in density early in the menstrual cycle.  The American Cancer Society position is that having a mammogram is more important than scheduling it to coincide with the early phase of the menstrual cycle.