Sonography during the first trimester of pregnancy is essential in twin pregnancies. This is due to the fact that a short two minute scan can reliably determine chorionicity and amnionicity. However, if the scan is not performed until the second or third trimester, the determination of chorionicity and amnionicity may be tedious and time consuming at the end of which many competent sonographers will frequently mis-assign chorionicity and amnionicity. A first trimester scan can therefore determine the appropriate management of the patient for the entire duration of the pregnancy.