Toluene, dibutyl phthalate (DBP) and formaldehyde are the three chemicals that a. Are known to cause birth defects and b. Are used in nail salons. Or stuff like methyl methacrylate and acetone is also 'on the radar'
There hasn't been much research on these compounds 'in the wild' as it were. Yes they are capable of causing various defects (heart defects and endocrine disruption seem to be the main ones) but what we don't know is if this translates to a real, quantifiable risk to workers or users of nail salons at the doses they are exposed to.
The only papers I've been able to find on this suggest a small increase in heart defects and things like hypospadias in workers exposed to organic solvents in the first three months of pregnancy.
It's very difficult to do studies on exposures and prenancy - you can't just divide a thousand pregnant women up and give half of them a beaker of toluene, for example :) so the advice is necessarily based on the precautionary principle.
The trend to treat women as dumb wombs on legs is absolutely concerning, what is needed is solid advice based on larger studies. Unfortunately we don't have this, so I can see where the avoidance comes from. I'd also say that in my old lab, a pregnant worker would be absolutely banned from going near any of the chemicals on our risk list ( which meant data crunching for nine months, as so much of our stuff was seriously nasty.) However, that doesn't neccessarily translate to avoiding getting your nails done, due to exposure level and frequency.
So no, you can't fix that bit of intestine in a methacarn bath, no way, and no, you can't go near the hot lab, but you probably can get your nails done ;)