Why are nail paints removed when women are in labour?

It's usually for hygiene and medical monitoring reasons. If you opt for birthing pool then you may be advised to wear no makeup or nail polish/varnish for hygiene reasons - to prevent contamination. Some hospitals' policy might also require you to remove nail polish as it is believed to interfere with the blood pressure and pulse machines. (http://www.sciencedirect.com/science/article/pii/S0964339706001121) That being said, a friend of mine gave birth and had a french manicure but none of her doctors/midwife/nurses said anything. Best to check with the hospital beforehand. If you are undergoing a C-section you may be requested to remove all signs of nail polish and make-up prior to the procedure so they can monitor your skin tone when surgery is in progress.