I would query whether she is stupid... or whether, having two children already, a third could not be afforded, and about... say... six or seven months ago, she thought she might be pregnant but didn't want to face a decision, so dealt with it by not dealing with it and convincing herself she couldn't possibly be pregnant? People do stranger things. For example, they convince themselves that despite the company failing, their jobs are secure and get surprised when they are made redundant. Why shouldn't she be convinced she couldn't possibly be pregnant? Even vasectomies aren't 100% - from FHI: "A group of 573 women whose husbands had vasectomies served as the CREST controls. They were followed-up for five years by telephone interviews. Ten pregnancies were identified..." Unusual? Yes. But there's enough room for doubt that I would hesitate before jumping up and down jeering at this woman and calling her "stupid".