The ovarian cycle and the beginning of pregnancy

The normal cattle estrous cycle averages about 21 days, with a usual range of 18–24 days. Estrus, or heat, is the shorter period of sexual receptivity within that cycle. Ovarian follicles contain eggs and cells that produce estrogen. Several waves of follicles can develop within a cycle; only the appropriate dominant follicle proceeds to ovulation.

After ovulation, the remaining follicular tissue forms the corpus luteum, which produces progesterone. If pregnancy does not develop, signals from the uterus cause this structure to regress, progesterone falls and another cycle can begin. An early embryo alters that signaling so progesterone production continues. Pregnancy and cycling are therefore different physiological states, not simply visible behavior. Puberty and the return to cycling after calving also depend on the animal’s condition and circumstances. A cycle average is useful teaching information, but it cannot confirm pregnancy or predict a particular cow’s reproductive health.