Hi Tillycat, I'm not sure if I (or many of the other posters) can be of help with your query since this is a predominantly populated with PhD students from the UK. Based on my very limited knowledge, it will definitely be worth discussing with your advisor reasons why you perceive that you performance has been impaired. If you have had significant family troubles, you may get some form of dispensation. However, it would not be unreasonable of you advisor to fail to accept this unless the reasons for you case are exceptional.
I know that at my university, they often fail to accept many variants of extenuating circumstances, like a mate who broke his leg and could not attend an exam (I know, ridiculous!) at undergrad level, so I suppose it depends on how well your uni deals with such grievances.