Sex is safe during pg, unless like Tamara said, there is a medical reason not to. When the book spoke of inducing labour, it would have meant when you are at or close to term with your pg. The semen can act as a natural prostoglandin and can help get things going in there.

I am naturally not one of these extra amorous women during pg, but the extra raging hormones can certainly make it fantastic sometimes. So I say if you feel up to doing it, then by all means go for it, but just don't go swinging from chandeliers LOL.