Doctors who have puzzled over the reason why mammogram screening can often be inaccurate may finally have the answer.

It seems that the density of the breast fluctuates during the menstrual cycle, particularly among women aged between 40 and 49, the very time when screening is recommended to begin.

The time to screen is during the first two weeks of the cycle when the breast is not so dense, compared with the latter two weeks, researchers have discovered.

A team from the Fred Hutchinson Cancer Research Center in Seattle tested 2,591 premenopausal women aged between 40 and 49, and discovered a 4 per cent difference in breast density between the two phases of the menstrual cycle.

Team leader Emily White believes the difference is significant, and could lead to more accurate testing (J Natl Cancer Inst 1998; 90: 906-10).

Radiation levels from mammogram screening may cause one death from breast cancer for every eight it detects, a computer model has worked out. It was estimating the risks and benefits of widening the screening programme to include women aged between 40 and 49 (Journal of Medical Screening, 1998; 5: 81-7).