During their periods, women jettison one (usually) egg, but also, for the most part, the lining of their uterus (the endometer) . The egg itself is lost amidst the deluge of blood and meat. Also the egg can’t be taken out and just exposed to moderate heat to produce a child. You need the fertilised egg to stay inside for nine months.

On the other hand, a fertilised chicken egg just needs heat. It’s a self contained unit that can develop without the mother.

So I reckon a chicken egg is closer to a human baby really. Also it’s easier for the joke, visually (an ovum is tiny)