Tongsong, Theera, and Supatra Sirichotiyakul. 1993. “Transvaginal Sonographic Measurements of Crown Rump Length as a Predictor of Gestational Age”. Thai Journal of Obstetrics and Gynaecology 5 (1). Bangkok, Thailand:1-5. https://he02.tci-thaijo.org/index.php/tjog/article/view/257817.