Gaya APA

Callen, P, W. (1994). Ultrasonography in Obstetrics and Gynecology . United State of America: W.B. Sounder Company.

Gaya MLA

Callen, Petter, W. "Ultrasonography in Obstetrics and Gynecology". United State of America: W.B. Sounder Company, 1994. Text.