That's an interesting find, but the real story is that modern dental work is extremely toxic to pregnant women and their fetuses. And it's not just the radiation from x-rays, either: dentists use a great number of toxic chemicals and substances in modern dental work, and many such substances -- such as mercury fillings -- are left in the patients' mouths for decades, where they slowly poison them. Dentists don't call them mercury fillings, of course: they use the safer-sounding "silver fillings" description, to hide the fact from patients that they're actually putting an extremely potent nerve toxin into their mouths.
Getting back to x-rays, I always find it amusing to see the x-ray operator at the dentist office fleeing from the machine before they zap patients. If x-rays are so safe, why are these people always making sure they're as far away from the machine as possible? The truth is that x-rays are simply bad for your health. The more x-rays you get, the worse the effect. Certainly, x-rays are sometimes necessary for good dental work, but I also suspect there are a whole lot of x-rays taking place for no reason other than to create a billable activity. Studies show, for example, that people with dental insurance receive a far greater number of x-rays than those who pay for dental care out of their own pocket. Coincidence? Yeah, right...