It wanes fairly quickly--and perhaps can never be fully revived.

The dynamic relation between parent and child may fairly easily fall into quiescence, a static condition.
For a full dynamic relationship it is necessary that there be actual contact.

The nerves run from the four primary dynamos, and end with live ends all over the body.

And it is necessary to bring the live ends of the nerves of the child into contact with the live ends of corresponding nerves in the mother, so that a pure circuit is established.

Wherever a pure circuit is established, there occurs a pure development in the individual creation, and this is inevitably accompanied by sensation; and sensation is the first term of mental knowledge.
So, from the field of the breast and arms, the upper circuit, and from the field of the knees and feet and belly, the lower circuit.
And then, the moment a child is born, the face is alive.