I see what they're saying (this was my second choice) but at the same time I feel like a backup of blood would activate the baroreceptors and cause decreased sympathetic activity to the SA & AV node.
Blood flow in series increases the resistance; blood flow in parallel decreases the resistance (TPR).
By blocking the umbilical veins you have in respect limited the excess flood flow to the placenta.
This reduces the flow in parallel circulation; thus increases the TPR. (MAP = cardiac out put times TPR)
This is because the total cross sectional area is reduced.
From this, there is an increased pressure in the fetal circulation.
The baroreceptors located immediately distal to the bifurcation of the common carotid artery would sense a high pressure and increase their afferent signal via CN9.
This Reduces the sympathetics and increases the parasympathetics via CN10 (vagus).
Thus, reducing the heart rate!
From lnsetick on reddit: "This was my reasoning: you're compressing either the blood supply to the baby, or the blood flow away. If you compress the supply to the baby, pressure would tank and regulatory systems would get its heart to work harder. If you compress the blood flow away, then SVR would rise and regulatory systems would reflexively slow the heart down."