Final answer:
Umbilical vessels can be accessed immediately after birth until natural occlusion, usually occurring within 20 minutes post-delivery, after which they atrophy and become fibrotic remnants.
Step-by-step explanation:
The umbilical vessels may be accessed immediately after birth before natural occlusion occurs. The umbilical cord normally is clamped and cut within seconds of the baby's delivery. It contains two arteries and one vein that are responsible for circulating blood between the fetus and the placenta. Following the cutting of the cord, the umbilical stump left on the newborn dries and falls off, usually within three weeks, to become the navel. Without medical intervention, the vessels would naturally occlude within 20 minutes after birth due to the Wharton's jelly in the cord swelling and the blood vessels constricting in response to the cooler external temperature. After occlusion, the vessels will collapse and eventually atrophy, becoming fibrotic remnants within the circulatory system.