Guinness also has a second category for world's smallest dog when measured by length. That title is held by Heaven Sent Brandy, a Chihuahua in Largo, Florida, that measures 6 inches (15.24 centimeters) long.
When she was born, Miracle Milly weighed less than an ounce and fit in a teaspoon, Semler said. Her mouth was too tiny to nurse from her mother, so Semler gave her milk every two hours through an eyedropper.
She slept in a doll's crib next to Semler's bed, growing stronger as the months passed.
The Chihuahua now sleeps in a baby's crib and will eat nothing but food cooked by humans.
"She really likes salmon and chicken," Semler said, noting that she eats four times a day.
Miracle Milly is close to her two sisters, both which are of normal size, but she prefers the company of people.
"She does not understand that she is a dog," Semler said. "She thinks she's a kid."
She doesn't bark and likes playing with the plants in Semler's backyard. If there are birds to chase, even better.
If she's playing inside, she gravitates toward Paco, a yellow Chihuahua plush toy twice her size. The stuffed animal is among dozens lining her crib.
"We give her a new toy almost every week," Semler said. "She likes to cuddle with them."
Miracle Milly is one of 10 Chihuahuas that Semler owns, and is easily the most popular.
"People are amazed when they see her because she is so small," Semler said. "And she has a big personality. People love her."
