<P> The umbilical cord develops from and contains remnants of the yolk sac and allantois . It forms by the fifth week of development , replacing the yolk sac as the source of nutrients for the embryo . The cord is not directly connected to the mother 's circulatory system , but instead joins the placenta , which transfers materials to and from the maternal blood without allowing direct mixing . The length of the umbilical cord is approximately equal to the crown - rump length of the fetus throughout pregnancy . The umbilical cord in a full term neonate is usually about 50 centimeters ( 20 in ) long and about 2 centimeters ( 0.75 in ) in diameter . This diameter decreases rapidly within the placenta . The fully patent umbilical artery has two main layers : an outer layer consisting of circularly arranged smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundant ground substance staining metachromatic . The smooth muscle cells of the layer are rather poorly differentiated , contain only a few tiny myofilaments and are thereby unlikely to contribute actively to the process of postnatal closure . </P>
<P> The umbilical cord develops from and contains remnants of the yolk sac and allantois . It forms by the fifth week of development , replacing the yolk sac as the source of nutrients for the embryo . The cord is not directly connected to the mother 's circulatory system , but instead joins the placenta , which transfers materials to and from the maternal blood without allowing direct mixing . The length of the umbilical cord is approximately equal to the crown - rump length of the fetus throughout pregnancy . The umbilical cord in a full term neonate is usually about 50 centimeters ( 20 in ) long and about 2 centimeters ( 0.75 in ) in diameter . This diameter decreases rapidly within the placenta . The fully patent umbilical artery has two main layers : an outer layer consisting of circularly arranged smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundant ground substance staining metachromatic . The smooth muscle cells of the layer are rather poorly differentiated , contain only a few tiny myofilaments and are thereby unlikely to contribute actively to the process of postnatal closure . </P>
where is the umbilical cord attached to the mother
where is the umbilical cord attached to the mother
the placenta
the placenta