Why should we avoid chemical products during pregnancy?

Birth defects: Since we don't know all the possible effects of all the chemical products at various amounts & timing of exposure, it's best to avoid as best one can to minimize risk of birth defects and even miscarriage. If you can't, you can't. But if you can and don't, you might have regrets later on since we're talking about a lifetime.

Unknown toxicities.: Because with many chemicals the potential toxicities to the developing fetus are not completely known. To be honest everything is a chemical. And of course your body does not know a natural chemical from a not natural chemical. Only your brain "knows" the difference. Better advice: do not expose yourself to out of the ordinary products needlessly.