Dave
Well-Known Member
Female dogs and cats can be spayed when in heat or pregnant. This can usually be done up until a few days before delivery. These surgeries can take longer, and can therefore cost more. Spaying before having a first litter or heat cycle is usually a simpler procedure. The heat cycle for dogs is once or twice a year starting as early as 6 months of age. Duration is 3 weeks. Heat cycles in cats start as early as 6 months and occur every 3-4 weeks during spring through early fall. The gestation period for both dogs and cats is 63 days. Female cats can become pregnant again as soon as 10 days after giving birth (while still nursing the first litter).
http://www.doghause.com/neuter01.html