There’s a wonderful web page by Nobuo Yamamoto, who creates successively better and better equations, and then matches them to a photo of an actual egg. After lots of work, he finally came up with the following equation:

     (x2+y2)2=x3+310xy2$(x^2+y^2{)}^2=x^3+\frac{3}{10}x{y}^2$

You can see a plot of it on Wolfram Alpha . Better yet, you can see this comparison of the graph of the equation and an actual egg on Nobuo’s website. Here it is, used with permission: