a. The umbilical vein is obliterated and becomes the round ligament of the liver.
b. The umbilical arteries are obliterated and ultimately become ligaments.
c. The ductus venosus is obliterated and becomes a ligament. Anatomic
closure is completed at the end of 2 months. The ductus venosus is superficially
embedded in the wall of the liver.
d. The ductus arteriosus is obliterated and becomes a ligament. Functional
closure takes 3-4 days; anatomic closure is completed by 3 weeks. The constriction
seems to be stimulated by a substance called Bradykinin, which is released from the
lungs during their initial expansions.
e. The foramen ovale closes after the umbilical cord is tied and cut. A large
amount of blood is returned to the heart and the lungs. With the lungs now functioning,
there is equal pressure on both atria as the vessels constrict. Failure of the foramen
ovale to close spontaneously results in an atrial septal defect, which may or may not
require surgery later.
2-13. PRINCIPLES OF FETAL IMMUNOLOGY
a. During the third trimester, passive immunity to some diseases is provided by
b. Diseases that the fetus receives temporary protection from include:
c. Passive immunity is short term and infants must begin immunization against
the above diseases by the age of 2 months.
2-14. MULTI-FETAL PREGNANCIES
a. Multi-fetal pregnancy is a pregnancy involving two or more fetuses.
b. Twin fetuses may originate several ways (see figure 2-11).