In humans, an adult woman's fertility peaks for a few days during each roughly monthly cycle. The frequency and length of fertility (the time when a woman can become pregnant) is highly variable between women, and can slightly change for each woman over the course of her lifespan. Humans are considered to have concealed ovulation because there is no outward physiological sign, either to a woman herself or to others, that ovulation, or biological fertility, is occurring. Knowledge of the fertility cycle, learned through experience or from educational sources, can allow a woman to estimate her own level of fertility at a given time (fertility awareness). Whether other humans, potential reproductive partners in particular, can detect fertility in women through behavioral or invisible biological cues is highly debated. Scientists and laypersons are interested in this question because it has implications for human social behavior, and could theoretically offer biological explanations for some human sexual behavior. However, the science here is weak, due to a relatively small number of studies.