I found this on the web, I don't know how reliable it is.
http://www.uihealthcare.com/depts/med/pediatrics/iowaneonatologyhandbook/procedures/insertionumbilicalvessel.html
XIII. Grasping the catheter between the thumb and forefinger or with a forceps, the catheter can be inserted into the lumen of the dilated artery. Supporting the stump is usually necessary. Once the catheter has ben inserted, it may encounter resistance at the level of the anterior abdominal wall or at the bladder. This resistance can usually be overcome by application of gentle steady pressure. Repeated probing movements or excessive pressure must be avoided. If unsuccessful, wait 2-3 minutes until the vasospasm ceases, or attempt the other umbilical artery.