A baby who was born 21 weeks after conception and is believed to be the most premature infant ever born, measuring just a little longer than a ballpoint pen, is thriving and ready to leave the hospital four months later. A premature infant delivered only 21 weeks after conception will be released from a Florida hospital Tuesday (February 20) after spending four months in a neonatal intensive care unit. Doctors say the baby girl is the most premature ever to have been born, and her birth sets a new world record for a baby born at her gestational age. Amillia Sonja Taylor was only 9.5 inches (24.13 centimetres) long, just a little longer than a ballpoint pen, and weighed less than 10 ounces (284 grammes) when she was born last October. Since then, the baby girl has lived in an incubator and has received oxygen at the Baptist Children's Hospital in Kendall, Florida. Dr. William Smalling, neonatologist at Baptist Children's Hospital, says the events surrounding Amillia's birth are the first of its kind. "There has been no known survivor born this early to ever go home. And not only is the baby going home, the baby is thriving and doing well, so this is a special day for us all here," Smalling told reporters at the hospital. Despite respiratory problems and a mild brain haemorrhage, Amillia has grown to 26 inches long (66 centimetres), with a weight of 4 1/2 pounds (two kilograms). Amillia's mother Sonja Taylor said her baby was developing well despite the difficult circumstances surrounding her birth. "She smiles, so everything a baby is supposed to do for her age she is doing, which is amazing because they told me that she would be developmentally late and you would have to wait a while for these things. But she surprised us all," Sonja Taylor said. Doctors said that the baby was conceived by in vitro fertilization, making it possible to pinpoint her exact time in the womb.