Police believe the girl was 13 at the time of the original incident in Trang’s Kan Tang district.
The girl’s grandmother is alleged to have continued the arrangement until the girl became pregnant.
The girl, now aged 15, gave birth to a child in March. Both she and the baby were now under the care of Songkhla Home for Children.
The girl’s mother now faces charges including human trafficking and procuring a girl under 15 for prostitution, Trang police deputy chief Colonel Triwit Sriprapa said Friday.