[JnA (OP)]

My daughter (7) is reading a book on human reproduction. Most answers are easy but sometimes she throws me a curly one..

Today we were discussing how a baby gets 'fed' inside the womb. After a short explanation about the umbilical cord and that we cut it she asked, "why cut it? why not let the baby keep growing like that?"

...

I guess there is a time period where the umbilical cord still continues to  function.. what is that?  minutes? hours?

I suspect the expulsion of the placenta ceases the umbilical function, but can that be stopped, and, if so, how long could you keep a newborn healthy in this way?

[CliffordK]

The umbilical cord isn't designed to continue functioning outside of the womb.  I would imagine it would be a potential source of infection for both the child and the mother.

The womb would be generally sterile until the water breaks.  After the water breaks, then the child is born, the umbilical cord, and residual placenta would provide a perfect path for microbes to enter the mother's body, and could make her very sick.

Animals, of course, can be born without tying and cutting the umbilical cord. 

With the pigs, we would often leave the umbilical cords long.  Within an hour or so they would naturally dry up and fall off. 

In the case of Rh incompatibility, doctors will sometimes use the veins in the umbilical cord for a blood transfusion.

Anyway, once the placenta (afterbirth) is expelled, the umbilical cord would dry up.  This would also indicate that it isn't getting arterial blood through the cord, so somehow both the infant and mother are turning off the cord circulation.

[evan_au]

The umbilical cord functions in a sterile environment, and it is fairly short. The baby places a fairly heavy load on the mother's metabolism, and the carrying capacity of the placenta.

As CliffordK notes, once the baby is born, the placenta is no longer in a sterile environment so it will be exposed to all sorts of pathogens, as well as providing a path for pathogens to attack the mother.

Due to its length, the umbilical cord would not allow the baby to be easily carried, once the baby is born.

Transitioning from a semi-shared circulation to breathing and breast feeding is a first step towards independence of the infant.

There are some doctors advocating storing the placenta in liquid nitrogen, as future technologies may allow extraction of compatible stem cells which could be used for "spare parts", later in life.

[JnA (OP)]

Anyway, once the placenta (afterbirth) is expelled, the umbilical cord would dry up.  This would also indicate that it isn't getting arterial blood through the cord, so somehow both the infant and mother are turning off the cord circulation.

maybe the same process as deciduous trees use... abscission cells?

[RD]

... storing the placenta in liquid nitrogen, as future technologies may allow extraction of compatible stem cells which could be used for "spare parts", later in life.

or alternatively used for a "placenta burger"  [xx(] ...
http://www.sciencebasedmedicine.org/index.php/eating-placentas-cannibalism-recycling-or-health-food/

[JnA (OP)]

... storing the placenta in liquid nitrogen, as future technologies may allow extraction of compatible stem cells which could be used for "spare parts", later in life.

or alternatively used for a "placenta burger"  [xx(] ...
http://www.sciencebasedmedicine.org/index.php/eating-placentas-cannibalism-recycling-or-health-food/

yeah, I think I prefer the old wives tale version - a pint of Guinness 

[cheryl j]

The blood vessels in the umbilical cord clamp shut at the same time  the baby's new circulatory system route kicks in, and it happens amazingly fast. When the baby is inside the mom, blood takes a different path through the baby's body. Because the deflated fetal lungs are not used to breath, no blood is pumped through the veins and arteries that go from the heart to lungs and back, and most of it is rerouted by a hole in the heart called the foramen ovale and a short vessel called the ductus arteriosis. The umbilical vessels constrict, the ductus arteriosis constricts, and the hole in the heart closes as the baby starts to breath on its own.

[chris]

The placenta and umbilical cord contains foetal blood; the two umbilical arteries in the cord are branches of the iliac arteries in the abdomen, and the umbilical vein, which returns blood from the placenta, flows to the liver where it joins the portal vein (that supplies the liver) and also creates a structure called the ductus venosus, which flows into the inferior vena cava and thence back to the heart.

After a baby is born, blood flow automatically ceases within the placenta within about 3-5 minutes without any external intervention. Customarily, however, a clamp is usually placed across the cord close to the umbilicus (belly button) to reduce the risk of foetal haemorrhage.

The physiological (automatic) cessation of blood flow in the cord is believed to be a response to temperature change. The birth of the baby exposes the cord to "room-temperature" air, triggering vasoconstriction (closure) of the two umbilical arteries. This is thought partly to be a function of the "Wharton's jelly", a gelatinous carbohydrate rich material similar to the vitreous humour found inside the eyeball.

The result of the vasoconstriction is occlusion of the vessels and blood flow stops.

However, before this closure occurs, it is possible to tap off blood from the cord and placenta, which is how neonatal umbilical cord blood stem cell harvests are performed for stem cell storage. The cord is first clamped and then the vessels cannulated and the blood drawn off by syringe. The stem cells are then isolated and stored on liquid nitrogen.

Chris