I'm still breastfeeding my 20 month old son so this is a question that's been on my mind for a while. It's a common question and there's lot's of conflicting info about it. I've been doing some light research (you know, in between all the mom things) and I've concluded that the simple answer is YES, it is safe as long as you choose the right polish.
And how do you know what is the right polish? For starters, pick polishes that are Big 3 Free. That means that they don't contain dibutyl phthalate, formaldehyde and toluene. From the Campaign for Safe Cosmetics website:
Dibutyl phthalate (DBP) adds flexibility and a moisturizing sheen, and helps dissolve other cosmetic ingredients. DBP is a reproductive and developmental toxin that has been linked to feminizing effects in baby boys.
Toluene helps suspend the color and form a smooth finish across the nail. It also affects the central nervous system and can cause headaches, dizziness and fatigue. Toluene is a possible reproductive and developmental toxin.
Formaldehyde is found in some nail products such as nail hardener. Formaldehyde is a known human carcinogen. It is also an irritant to the eyes, nose and throat, and can lead to skin irritation and an allergic rash called dermatitis.
Some of the most popular brands are Big 3 free, like Essie, OPI, and Sally Hansen.
But you can go even further and get "5 free", which means that in addition to the toxic trio they also don't contain formaldehyde resin and camphor. Formaldehyde resin is similar to regular formaldehyde and is a nail strengthener. Unfortunately, it is also a skin allergen and can remain active for up to three days after your polish has dried. While no link to cancer has been established for formaldehyde resin (in contrast to formaldehyde), it can cause dermatitis. Camphor exposure in large doses, especially through inhalation, can be toxic and cause irritation, nausea, dizziness, and headaches.
Some 5 Free polish brands are high end like Chanel and Dior but you might be surprised to learn that some great drugstore brands are too like NYC, Wet n Wild, Sally Hansen Insta-Dri and Sinful Colors. For a more complete list check out All Lacquered Up.
Now you know some brands that are non toxic but did you know that in addition you can also buy vegan and animal cruelty free polishes? Zoya is the most popular,100% vegan and 5 Free (alas it isn't available to me locally, only online through second hand retailers).
This site has great info on non toxic and vegan polishes. http://www.choosingraw.com/you-ask-i-answer-all-about-vegan-nail-polish/
The Vegan Polish Resource is another detailed list including emails from the companies confirming if their polishes are non toxic and/or vegan. Sally Beauty Supply store polishes Sally Girl and FingerPaints are on this list. Nail polish fan favorites China Glaze (black label) and Seche Top Coats plus OPI and Orly are here as well. A note about OPI as there's some controversy. They maintain an animal cruelty free policy but their parent company does test on animals. The polishes themselves are non toxic (3 Free), it's up to you to decide.
So there you have it. Yes, you can paint your nails during pregnancy and breastfeeding as long as you choose a non toxic polish, and if it's vegan, even better. Always remember to paint in a room with good air circulation, most of the issues from nail polish toxins are caused by inhaling them.
(Disclaimer: I'm just a mom with an art degree, not a scientist or a doctor and this is my opinion based on my research. I hope it's useful but please do your own research as well.)