There are recommended ranges for weight gain during pregnancy and these depend on whether you are under weight, within a "normal' weight range, or overweight to begin with (and to know this your height has to be factored in). Typically, recommended weight gain ranges from about 15-35 Ibs with overweight people being at the lower end of the spectrum and underweight people being near the upper end of that range.
When weighing yourself, be sure to weigh at the same time of day without any clothes on and using the same scale- a good time do to this is in the morning before you have anything to eat. Weight can fluccuate throughout the day by several pounds (easily) and clothes can effect the number too so its important to be consistent.
All that said, if you really did gain 5 Ibs in a week (is it possible that the number might be skewed from some of the things I just mentioned?) that is something you need to watch closely. If you continue to gain at that rate you need to call your doctor because certain conditions during pregnancy can cause rapid weight gain (water weight) and may require treatment.
Between the 6-7 months, I gained 8-10 pounds each month. It seems to be the time in the pregnancy for the most weight gain (at least for me).