There have been few long-term studies on the risks of using nail polish during pregnancy. A single small study by an environmental group has suggested that phthalate, a chemical used in much nail polish, has caused birth defects in laboratory animals. No human studies have yet been done studying this link.
The FDA, EPA, CDC, Health Canada and several European scientific bodies have examined phthalates and have not come to the same conclusions. These groups have all indicated that phthalates pose no danger.
As always, if you have concerns about using a product speak to your physician. If you are afraid of using phthalates, check your product labels for dibutyl phthalate, also referred to as DBP. There are products available that are free of DBP as well as other potentially harmful ingredients such as toluene or formaldehyde.