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Technically, she cannot. However, it is possible to have a heat with signs that are so mild as not to be easily seen by an owner. Heat in dogs is different from menstruation in people. Heat is a slang term for pro-estrus and estrus. Most dogs start to bleed and get a swollen vulva as pro-estrus starts. Approximately 10 - 14 days after pro-estrus begins, the female dog enters a fertile period (now called estrus), when the influence of estrogen is highest. At this time, the bleeding is usually present but more dilute-looking, or even straw-colored, and the vulva may be slightly less swollen. After ovulation, the ovaries secrete progesterone, which influences the uterus to prepare for pregnancy, whether pregnancy occurs or not. That is why some dogs go through "false pregnancy" or pseudocyesis, and may act maternal and even produce milk. The vast majority of female dogs go through this whole cycle every 6 months.
Just for curiosity's sake, what made you ask the question? Is your dog possibly in false pregnancy?