Hiya! It is one of the possible explinations for being late. I would say go and see your GP as it may be that you have low levels of the pregnancy hormone, or there may be another reason why you are late. Either way, they should be able to help. My sister had 11 negative tests and the first one didn't turn positive til she was nearly 3 months gone! she had her pregnancy confirmed by her GP before this but carried on testing out of curiosity! Hope all is well. daffi x