P1130031

j 1.3.2.4 Day 41 to 60 of pregnancy

Around Day 40 the embiyo reachcs its highesl posi-uan within the cmbryonic \eside {Fig. 1.66 and 1.67). Froto here it gradualh sinks, suspended by its iimbiScai cord, fcack down to the fioor of the vesicłe (KAHN and Lehm. 1984). After this time movements of the fetuscan be obserwed frwjuentły (Allen and Goddard 1984).

The umbiłical cord normally attaches to the dorsai pole of the aflantochoaion. Very rarely it attaches lat-etafly or ventrafly. Cases have been observed where the yoik sac was positioned ventraily and the ailantois dor-safiy to the embryo leadtng to an embryonic migration front top to bottom within the veside (Fig. 1.68). Itisas-sumed that this kmd of disorientation of the embryo is a setjuel to an eariier twin conception (Ginther 1984c). In singieton pregnandes there seetrts to be a mechanism

which cnsures that the conceptm is onented witb the embiyonic disc at its ventral aspcct at the time of frea-don between Days 16 and 20. This aiso leads to the embryo rising from ventral to dorsai during its ascent between Days 20 and 40 and to the attachment of the umbilicus to the dorsai aspect of the allantochorion. This process sometimes appears to be disturbed in twin pregnandes when the two vesicles inteifere with one anothefs orientation within the uterus.

The fetus has a crown ramp iength of 25 mm on Day 45 of gestation (Fig. 1.69) and grows to a Iength of 40 mm by Day 60. The placentai vesicle reaches a diameter of 6 cm between Days 45 and 50. Snbsequently h ex-ceeds the scanning width of the 5 MHz scanner and can onły be depicted in sections (Fig, 1.70). From Day 50 to 60 a penetradon depth of 10 cm is also inadequate to show the yesicle in its entirety.

Fig. 1.70: Pregnancy on Day 54. The diameter of the concep-tus ejtcecds the scanning width of the ultmsound probe. The head (H) of the fetus lies to the left. The umbilical cord rans towąrds 3 o'clock out of the picture.

Fig. 1.69: Pregnancy on Day 46. The fetus lies on its back on the fioor of the vestclc. Its crown ramp Iength mcasures 33 mm. The head with the eyes (B) lies to the left.