The medical examiner's office said Veronica Sotelo, 16, died of hypertrophic cardiomyopathy.
Sotelo collapsed Oct. 26 at Polytechnic High School in Fort Worth. She later died at a hospital.
According to the Mayo Clinic, hypertrophic cardiomyopathy is a disease that makes the hard muscle become abnormally thick. This thickened heart muscle can make it harder for the heart to pump blood and may also affect the heart's electrical system, according to the clinic.
The condition often goes undiagnosed because there are few, if any, symptoms, according to medical experts. Symptoms can include shortness of breath and abnormal heart rhythms.
"Most people with hypertrophic cardiomyopathy lead normal lives with no significant problems," MayoClinic.com posted.
Classmates said Sotelo was acting normally and showed no signs of health problems.
Doctors at Cook Children's Hospital delivered Sotelo's unborn child, who died four days later while in intensive care.
As of Thursday, the medical examiner's office had not determined a cause of death for the infant, who was named Ginger.