Armstrong, who won the Tour de France seven straight years after overcoming testicular cancer, announced the birth of his son on the social-networking site Twitter late Thursday night. The proud papa even posted a picture of the newborn, playfully sticking his tongue out at the camera.
"Wassup, world? My name is Max Armstrong and I just arrived. My Mommy is healthy and so am I!" Armstrong posted to his account at about 11:15 p.m. EDT.
Armstrong and his girlfriend, Anna Hansen, announced in December they were expecting a baby in June. Armstrong also has three children with his ex-wife, Kristin.
About 15 minutes after the initial post, Armstrong sent another message praising the doctor and nurse who helped deliver the newborn, saying they were amazing. He then posted that Max weighed 7 pounds, 5 ounces and was 20 inches long.
When the pregnancy was announced in December, a spokeswoman for the Lance Armstrong Foundation said that the baby was conceived naturally. Armstrong had three children with his ex-wife, Kristin, using in vitro fertilization with sperm he had banked before undergoing chemotherapy.
Three years after retiring following his last Tour de France win, Armstrong announced in September his return to competition and cycling's marquee race in 2009. He called his comeback an attempt to raise global awareness in his fight against cancer.
Armstrong competed in the Giro d'Italia in May, finishing 12th overall. He did not speak to the media for the final two weeks, but announced on his Twitter page after the race that he was headed home to await the baby's arrival.
He is expected to be riding in the Tour de France when it begins July 4.
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Copyright 2009 The Associated Press.