Fig. 1

Sonographic differential diagnosis between intrauterine pregnancy and caesarean section scar pregnancy in the early first trimester. Interrupted yellow line on a sagittal section of the uterus – imaginary line dividing uterus in two portions described by Timor-Tritsch et al. [9]. Continuous blue line on a sagittal section of the uterus – imaginary endometrial line connecting internal cervical os with the uterine fundus described by Cali et al. [14]. The gestational sac is situated below the yellow line, implanted at the level of CS scar. Less than two thirds of the SID line are above the endometrial line, towards the anterior uterine wall. The uterine cavity is empty. The myometrial layer thickness is 2 mm. CS, caesarean section; GS, gestational sac; SID, superior-inferior diameter of gestational sac