The timing of a woman's fertile days within the menstrual cycle is highly unpredictable, public health researchers say.
Dr Allen Wilcox and colleagues (National Institute of Environmental Health Sciences, North Carolina, United States) found that more than 70 per cent of women were in their fertile window before day 10 or after day 17 of their menstrual cycle. In addition, between 1 and 6 per cent were fertile on the day that their next period was due to start.
"Women should be aware that no calendar method is completely effective. Our data suggest there are few days in the menstrual cycle during which some women are not potentially capable of becoming pregnant," they say (
British Medical Journal 2000; 321:1259).