A 22-year-old man pleaded guilty Tuesday to a sexual assault charge filed after he impregnated and then married a 14-year-old girl, and he could face up to 50 years in prison.

Matthew Koso legally married the girl in Kansas, which has no minimum age for marriage, after she became pregnant. The girl, Crystal Koso, now 15, gave birth Aug. 24 to a 7-pound, 1-ounce girl named Samara.

Nebraska, however, requires people to be at least 17 before they can marry and prohibits people 19 or older from having sex with anyone under age 16.

Koso had pleaded not guilty in August. As he left the courtroom with his wife, he would not say why he changed his plea.

The Associated Press generally avoids naming victims of sexual assault, but the couple has spoken freely about the charges, and the girl's name has been widely reported.

His attorney, Willis Yoesil, said he believed possible Koso could be sentenced to probation. He would not say why he advised Koso to change his plea.

Nebraska Attorney General Jon Bruning said his office would not make a recommendation to the judge about Koso's sentence.

Sentencing was set for Feb. 7. The judge ordered a pre-sentence investigation, which would include an evaluation of Koso's mental capacity.

Bruning filed first-degree sexual assault charges against Koso in July and a trial was scheduled to begin Thursday.

Bruning, who was criticized by some for pursuing the case given that the sex was consensual and the couple eventually married, defended his decision.

"The law clearly states that it is illegal for a grown man to have sex with a child," he said.

Assistant Attorney General Matt Enenbach said Tuesday that evidence would show that Koso had sex with Crystal when she was 13.

After the girl became pregnant, her mother gave permission for Koso to take her daughter across the state line to Hiawatha, Kan. There is no minimum age for marriage in Kansas, but state law requires anyone under 18 to have permission to marry from both parents, a guardian or a judge.

Koso's lawyer, Yoesil, has said the girl's mother and Koso's parents approved of the marriage. The girl's father has not lived with the family for some time.

Kansas Gov. Kathleen Sebelius recently said she was asking legislators to establish a strict ban on marriage involving anyone younger than 16. The governor said the legislation also should retain the requirement for parental or judicial consent for anyone age 16 or 17.