Should you fix your female dog while she is in heat or starting to come in heat?
During the estrus or heat cycle there is increased blood flow to the uterus and it’s size is temporarily enlarged, this may require some extra surgical time to ensure that all blood vessels are properly ligated to prevent bleeding, but as long as done properly should not pose an increased risk to the patient. I would advise a pre-anesthetic blood panel to ensure the red & white blood cell counts were at normal levels and ensure that organ function is normal. In our animal hospitals all patients undergoing anesthesia have an intravenous catheter in place to recieve fluids to maintain their blood pressure during surgery and the heart rate, respiratory rate, blood pressure, EKG and oxygen saturation levels are closely monitored during surgery which minimizes the anesthetic risk. In many cases there is an additional fee to the surgery to compensate for the extra anesthesia time.
As long as there is not exposure to an intact male dog and risk of accidental pregnancy there is no harm in waiting 4-6 weeks for the uterus to return to a more normal size. Spaying female dogs prior to their 3rd heat cycle has shown to decrease the chance of mammary cancer later in life and so it is often recommended to have the ovariohysterectomy (spay) performed at 6-7 months of age before the first estrus.