Here's a quote from the CFA:
The average length of pregnancy in the cat is 65 days, but it can vary from 61 to as long as 71 or 72 days. Over their breeding life, most queens will establish a fairly predictable pattern for length of gestation. A litter delivered before 61 days will have a high mortality rate since the kittens may not have lungs mature enough to support life outside the uterus. In dogs, the fetal heart rate shows a significant decrease in the five days before whelping, and this can be used to predict delivery. This cannot be used to predict delivery for queens, however, since the fetal heart rate of kittens stays steady throughout pregnancy. For some queens the rectal temperature may be used to monitor for impending delivery as it is in dogs, although it can be unreliable. Start taking the temperature at about day 61. When the temperature has dropped one full degree (usually to about 99°F or less), labor has begun and obvious signs should appear in 12 to 24 hours. This decline in temperature is triggered by falling progesterone levels. Another sign that active labor will begin within about 24-48 hours is the presence of milk in the mammary glands, although in some queens milk comes in many days before delivery of the litter.
Of course if you are concerned, always call the veterinarian. Please let us know when the kittens are born. I hope all turns out well, and you soon have healthy kittens. I know the last few days are nerve wracking, but all of a sudden labor starts, and you're on your way! I would not wait 71 days. That would be a rare case. 65-67 is average.
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A dog, I have always said, is prose; a cat is a poem. ~Jean Burden