Well obviously before clamping, after clamping there isn’t much flow! In this study with full term babies they were delivered and placed on the mother’s chest, and then an ultrasound probe was placed to measure umbilical cord flow. (Boere I, et al. Umbilical blood flow patterns directly after birth before delayed cord clamping. Archives of disease in childhood Fetal and neonatal edition. 2014).
This is great study, really important in helping to understand what is actually happening during delayed clamping, and what probably happened in the past as we were evolving and didn’t have cord clamps, although exactly how the cord was managed in the distant past is unknown, was it bitten by the mother? left to dry?
The babies were all vaginal deliveries at term who breathed spontaneously. The cord was clamped at a time determined by the midwife doing the delivery, the local standard was to wait at least one minute and after the cord stopped pulsating. Clamping occurred on average at 5 minutes after delivery, and the researcher checked to see if there was still pulsation palpable just before the cord was clamped.
Arterial flow often stopped before cord pulsation disappeared, and often continued longer than venous flow. Venous flow continued for an average of 4.5 minutes in babies in whom is stopped before cord clamping, and as long as over 8.5 minutes (that is the third quartile, so the maximum duration isn’t reported); some babies still had some pulsation when the cord was clamped, which was at 5.5 minutes and up to over 9.5 minutes.
The duration of flow was extremely variable, but the fact that venous flow often stopped first suggests that prolonged delays in cord clamping might sometimes decrease placental transfusion.
The infant’s breathing pattern affected flows, in particular vigorous crying reduced or even reversed venous flow, but large inspirations increased flow.
This technology doesn’t allow volume measurements, as you would need to be able to measure accurately the vessel diameters, which are probably themselves changing. A similar studies in preterms, and during resuscitation would be fascinating.