Answer:
during the first trimester
Step-by-step explanation:
Because The development of the umbilical cord begins in the third week of embryologic formation. The developing embryo consists of a trilaminar disc attached to the decidua basalis by the connecting stalk, the primitive umbilical cord.[1][2] The connecting stalk is a thick stalk of the extraembryonic membrane extending from the caudal end of the embryo to the center of the developing placenta on the decidua basalis.[3] The process of body folding occurs during week four with rapid growth amnion and embryonic disc compared to the yolk sac. Cranial caudal folding causes approximation of the connecting stalk and yolk sac on the ventral surface of the embryo.[1][2] The amnion expands to cover the entire embryo except for the rudimentary umbilical ring, where the connecting stalk and yolk sac emerge.[1][2] During this time, the allantois, an outpouching of the endodermal hindgut, forms and extends into the connecting stalk.[1][2][4] Between the fourth and eighth weeks, there is an increase in amniotic fluid production, which causes the amniotic cavity to swell and fill the chorionic space. This increase in the amniotic fluid also causes elongation of the connecting stalk, and the yolk sac is compressed down within the connecting stalk to form the omphalomesenteric or vitelline duct.[1][2][5] The expansion of the amniotic cavity causes the amnion and the chorion to come into contact, and the extraembryonic mesoderm covering these two layers fuses. As such, the chorionic cavity disappears, leaving the umbilical cord, the composite of the connecting stalk and vitelline duct surrounded by the amnion, floating in the amniotic fluid.[1][2][4]
Starting in week three, endothelial precursor cells in the mesoderm surrounding the allantois coalesce to form small capillaries. Vasculogenesis continues, and by the end of the third week, the capillaries have grown to establish a functional vascular network within the connecting stalk. During the same period, the arterial and venous systems within the embryo are developing. The arterial system is initially established as the paired dorsal aortae from which the aortic arches originate. The primitive venous system is initially made up of the umbilical, vitelline, and cardinal systems. Early in the fourth week, two umbilical arteries branch from the paired dorsal aortae to become connected to the vascular network of the umbilical cord.[1] During the fifth week, this connection is obliterated as the umbilical arteries develop their connection to a branch of the fifth pair of lumbar intersegmental arteries that will later become the internal iliac arteries.[1][2][4] The umbilical veins are originally bilateral and drain into the right and left sinus horns of the sinus venosus. The connections of the umbilical veins to the sinus horns regress in the second month with complete regression of the right umbilical vein as the left umbilical vein persists and forms its connection to the ductus venosus within the developing liver.[2][4] With the initiation of fetal heart pumping around week four, the umbilical arteries carry deoxygenated blood to the placenta, and the umbilical vein carries oxygenated blood back to the fetus from the placenta.[4]
By week seven, the intestines begin to herniate out of the embryo through the umbilical ring and into the umbilical cord.[4][6][2] This physiologic herniation is necessary for proper rotation of the intestines and adequate growth of the fetus to house the expanding intestines.[6] The rapid development of the intestines causes elongation of the umbilical cord.[6] Between weeks ten and twelve, the intestines leave the umbilical cord and return to the abdominal cavity.[4][2][6] During this time, the extraembryonic mesoderm develops a rich extracellular matrix to protect the cord called Wharton’s jelly.[2][7] The umbilical cord continues to elongate during the second trimester with a length comparable to the crown-rump length of the fetus.[1][8] By term, the vitelline duct and allantois have typically completely involuted.[1][5] However, in some cases, remnants of the allantois and vitelline duct can be found in the umbilical cord proximal to the neonate.[2][5] At birth, the cord typically measures an average of 50 to 60 cm in length and 2 cm in diameter with up to 40 helical turns.[4][8] After the birth of the neonate, the umbilical cord is clamped and then cut as the neonate now breathes on its own, and the remainder of the umbilical cord is delivered along with the placenta .
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