Domestic cat gestation, can vary between different breeds between 58 and 70 days but the normal range is 63-65 days. You can recognise a pregnant puss the second week by the nipples which become enlarged and pink. Later mum's tum will leave little room for doubt.
Take your moggy mum to the vet for a check-up. They will not only be able to confirm (or otherwise) pussy's pregnancy by ultrasound but also will also give you a care plan with further check-ups as necessary.
Occasionally, a cat may have a false pregnancy. This is when her hormones go wonky causing symptoms that look like pregnancy. This happens for several weeks but will gradually fade away.