pregnancy is so costly, and human offspring so dependent on their mothers, early spontaneous abortion is high to ensure that energy of a pregnancy is spent on developing a fetus with a high chance of survival. Fecundity and fertility Human reproductive ecology considers fecundity and fertility from a demographic perspective. In this view, fecundity is the reproductive potential of an individual and fertility is the actual reproductive output of an individual. Fecundity Fecundity is determined by the biological limitations of the individual and can be reduced when biological and ecological factors impact an individual's reproductive capabilities. The key components of fecundity are a person's reproductive maturation and the maintenance of their reproductive system. In humans, the timing of female reproductive maturation is particularly variable and is heavily influenced by ecological considerations. In addition, the age at menarche has decreased over time in many global populations. This phenomenon is referred to as the secular trend. Age at menarche is one measure of the fecundity of an individual female. Male reproductive maturity is less subject to environmental and ecological factors, and does not follow the secular trend that female puberty does. In adults, fecundity is determined by the biological processes of reproduction. Female fecundity is heavily influenced by reproduction and energetics. The ovarian cycle limits the potential of conception to a brief period of fertility roughly once a month. Successful egg maturation, fertilization, and implantation must be able to occur for a reproductively mature female to be fecund. Changes in energy levels, diet, and hormones can all interfere in this process. During breastfeeding, a period of lactational infertility also reduces female fecundity. The metabolic load hypothesis in human reproductive ecology describes how the energetic expenditure of lactation acts to inhibit ovarian cycling. With the majority of available energy going towards milk production, energy is not expended on reproductive effort. Male fecundity is primarily determined by the quality of sperm and the availability of fertile female mates. Individual variation in sperm load, pH, lifespan, and morphology creates varying fecundity in males. As males do not gestate, their contribution to fecundity is less well established post-reproduction. A lack of fecundity in adults can be described as infertility. Infertility occurs in about 10-15% of couples, with the causes of infertility shared equally between males and females. Fertility Fertility is the measure of an individual's actual reproductive output, rather than just their potential for reproductive success.