Dating of the pregnancy is one of the main aims of first trimester ultrasound in twin pregnancies. The main objective of dating in multiple pregnancies is to make ultrasound examinations later in pregnancy more effective in the screening and management of complications either specific to, or more frequent in, these pregnancies. The dating of twin pregnancies should ideally be based on that of singletons and should be performed when the crown-rump length (CRL) measurement is between 45 and 84 mm (i.e. 11+0 to 13+6 weeks of gestation) using the same reference chart, at the time of the routine 1st trimester ultrasound scan. If the woman presents after 14 weeks of GA with a CRL measurement above 84 mm, the dating of pregnancy should be based on the head circumference measurement. One of the recurrent questions is the choice of the fetus upon which to base dating, especially when there is a significant difference between the size of the two twins. The recent ISUOG guidelines on the role of ultrasound in twin pregnancies have outlined the most common practice: the larger CRL should be used for pregnancy dating as it may protect against overlooking the diagnosis of a subsequent selective IUGR of the smaller twin. As is the case for singleton pregnancies, twin pregnancies conceived via ART should be dated using the oocyte retrieval date or the embryonic age from fertilization.