OK, I tried looking stuff up. I had a really hard time finding anyhting. I usually search Medline, but my dh must have changed his password. From what I have found, they don't really know, BUT they do know the placenta stops supplying oxygen shortly after birth b/c if babies don't breath right away they have problems. If the placenta kept supplying O2, there wouldn't be any problems, right?
Here's what I did find:
http://www.springerlink.com/content/w254270608n13qv7/
In this article, it mentions that low O2 sats at birth indicate thta it is normal for the placenta to start detaching even before birth. It's also normal for it not to, indicated by the higher O2 sats at birth. So maybe it is possible for some women to continue passing oxygen through the placenta to the baby after birth, but not for long.
This is the 2nd thing I found.
http://www.madsci.org/posts/archives...9642.An.r.html The 2nd part is more helpful, imo. It mentions how if the cord is not cut right away, it constricts on its own, effectively cutting off all circulation. If you talk to a midwife about cord clamping and cutting, they'll likely tell you that it's not really necessary to clamp the cord before cutting in most cases, b/c it is already constricted if you wait long enough (if you wait until after the placenta is delivered is probably long enough.) But there are rare cases where it will continue to seep or bleed, so they clamp all babies just in case.
I also found some articles talking about how it's important for breathing fo rthe baby to receive all of the blood from the cord/placenta, so don't cut it right away. But this is because the baby needs the "extra" blood in there b/c now with lung function, it's no longer "extra" blood, but necessary blood for the lungs to function properly. They don't use their lungs in the womb, so the blood in the cord isn't in their body and that's fine but after birth, they need that blood in their bodies.
Btw, my 2 younger boys have von Willebrand Disease, a very common mild bleeding disorder. They both had problems with the cord site bleeding after the cord had fallen off of their belly button. So when the cord is seeping or whatever if it is cut but not clamped, it's probably due to a case like my boys' where the baby's blood is not clotting correctly, not because there is still blood flowing between the placenta and baby.