The 10days rule is related to

The "10-day rule" recommended that, in women of child-bearing potential, non-urgent x-ray examinations that entailed pelvic irradiation should be restricted to the first 10 days of the menstrual cycle. Its rationale was to avoid irradiating a fetus before the mother realised that she was pregnant. In 1984, however, the International Commission on Radiological Protection repoed that there was little, if any, the risk of damaging the fetus during the first two weeks of gestation--that is before the first period was missed.