Baby Centre. https://www.babycentre.co.uk/a1014526/doppler-scans#section4Diakses pada 22 November 2019Medline Plus. https://medlineplus.gov/lab-tests/doppler-ultrasound/Diakses pada 22 November 2019Healthline. https://www.healthline.com/health/pregnancy/ultrasoundDiakses pada 22 November 2019