The so-called 10-day rule.
The 10-day option aimed at restricting all radiography of potentially fertile women to the first third of the menstrual cycle was introduced in 1959 without any valid supporting evidence. No earlier or later experimental evidence has indicated that diagnostic radiography involving the early embryo might carry a measurable risk to the individual developing in utero.
D005260:Female; D005333:Fetus / Q000528:radiation effects; D006801:Humans; D008597:Menstrual Cycle; D011247:Pregnancy / Q000528:radiation effects*; D011859:Radiography / Q000379:methods*; D013997:Time Factors