Fig. 1

a Measurement of mean sac diameter at 8 weeks’ gestational age using transvaginal ultrasound scan. Gestational sac diameter was obtained by placing the calipers inner-to-inner on the sac wall, excluding the surrounding echogenic rim of tissue. MSD was calculated by first adding the longitudinal, anteroposterior and transverse dimensions of the chorionic cavity. Thereafter, the sum of the three measurements was divided by three. b Measurement of crown–rump length with transvaginal ultrasound at 8 weeks’ gestational age. CRL was measured as the maximal straight-line length of the embryo, obtained along its longitudinal axis, with the embryo neither too flexed nor too extended