Pregnanes are a class of steroid hormones and steroids that contain a pregnane nucleus, which is a steroid core with a carbon skeleton consisting of 21 carbons. This structure includes four fused rings, labeled A through D, and is derived from cholesterol.

Pregnanes are important precursors for the synthesis of various steroid hormones in the body, including progesterone, which plays a crucial role in maintaining pregnancy and regulating the menstrual cycle. Other examples of pregnanes include cortisol, a stress hormone produced by the adrenal gland, and aldosterone, a hormone that helps regulate electrolyte balance and blood pressure.

It's worth noting that pregnanes can also refer to synthetic compounds that contain this steroid nucleus and are used in various medical and research contexts.