von Willebrand factor (vWF) multimers were examined in fetal, umbilical cord, and neonatal platelet-poor plasma (PPP) specimens. Sixty-five of 65 (100%) fetal PPP samples aged less than 35 weeks and seven of ten (70%) fetal samples aged greater than 35 weeks had unusually large vWF (ULvWF) multimers. Thirty of 46 (65%) cord PPP samples from neonates ranging in gestational age from 34 to 41 weeks had ULvWF. There was no significant relationship between either gestational age at time of delivery or birth weight and likelihood of finding ULvWF multimers in cord PPP samples. No maternal PPP sample contained ULvWF multimers. Serial heelstick samples from 16 preterm and term neonates were analyzed for 8 weeks. ULvWF multimers disappeared from the PPP of ten of the neonates during this time. The PPP of four neonates had vWF patterns similar to those in normal adult PPP throughout the sampling period. The ULvWF multimeric forms of fetal and neonatal PPP samples were similar to those constitutively released from endothelial cells. They were not as slowly migrating in a very porous 0.5% agarose gel system as the ULvWF multimers released from Weibel-Palade bodies in response to the calcium ionophore A23187. A vWF protomer was present in 97% of fetal samples, 83% of cord blood specimens, and 11% of neonatal heelstick samples, but was not found in any maternal sample. These results indicate that control mechanisms operative in older children and adults to prevent circulation of ULvWF multimers and vWF protomeric forms are normally acquired late in uterine life or during the neonatal period. ULvWF multimers, which are normal components of fetal, most cord, and some neonatal plasma samples, may contribute to in utero and postnatal hemostasis.

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