First, your son-in-law should refuse to submit to a polygraph "test." The polygraph cannot detect lies. It is little more than a prop for an interrogation. Moreover, the "test" is fundamentally dependent on trickery, and truthful persons run a considerable risk of being falsely accused of deception.

By all means, have your son-in-law read Chapter 3 of The Lie Behind the Lie Detector. to find out about the trickery that is going to be practiced against him if he agrees to be polygraphed.

With regard to pregnant women not being polygraphed, I suspect that this is a liability issue. I know that in polygraph studies conducted with volunteers at the Department of Defense Polygraph Institute, pregnant women are not allowed to participate.

In any event, you should not be eager for your son-in-law's accuser to be polygraphed, too, because anyone -- truthful or not -- can pass a polygraph "test" by augmenting their physiological responses to the "control" questions. See Chapter 4 of The Lie Behind the Lie Detector for details.