The alleged rapist, whose identity has not been revealed, is now under house arrest, police in Thanh Tri District announced on Sunday. He is a neighbor of the girl, now 12.
In early January, the girl's family detected abnormal signs in her physical health, including a bigger belly, so they took her to a hospital for a checkup.
Only then did they learn she was six-months pregnant.
The girl later said the neighbor had raped her on several occasions.
On April 17, she was taken to Hanoi Obstetrics & Gynecology Hospital for a C-section.
Those found guilty of raping a minor can receive a jail sentence of 7 to 15 years. They can receive a death sentence for gang rape, if the act causes serious physical or mental damage, or if the victim is less than 10 years old.