Fecundity is the physiological capability of a woman, man, or couple to reproduce, that is, to produce a live birth. Unless both partners are fecund, no birth can occur. In contrast, fertility is the actual reproductive output of an individual, couple, or group. Considerable confusion results from the fact that in French and other Romance languages, the meanings of fecundity and fertility are reversed; for example, the French fécondité is equivalent to the English fertility and the French fertilité is equivalent to the English fecundity. English-speaking physicians also use fertility to mean fecundity. Confusion also exists because demographers have defined fecundity as the capacity to reproduce but have defined fecundable as the capacity to conceive, and fecundability as the percycle probability of conception, regardless of whether that pregnancy results in a live birth.
Whereas fertility can be directly observed and measured, fecundity cannot. Demographers, statisticians, and epidemiologists have developed techniques for indirectly estimating the incidence of sterility (the inability to produce a live birth), for directly estimating the incidence of fetal loss, and for estimating conception probabilities by cycle day of intercourse. The measures of sterility, fecundability, and conception probabilities necessarily pertain to a couple and not to an individual.
Logically, fecundity depends on a sequence of events. The female must produce an egg capable of being fertilized, the male must produce sperm that can fertilize the egg, fertilization must occur, the fertilized egg must survive to implant in the uterus, and–once implantation has occurred–the pregnancy must result in a live birth. Successful progression along this sequence can be influenced by many factors.
Fecundity varies among individuals and couples of a given age. The fecundity of groups declines with
the aging of women as increasing percentages of women become sterile because they are unable to become pregnant and due to fetal loss. This increase is modest through the 30s and rises sharply thereafter until virtually all women are sterile at about age 50. It is plausible that the fecundity of individual women also declines with age, although that decline is likely to be less pronounced before a rather rapid loss in reproductive capacity is experienced. In contrast, fecundity of males does not appear to decline until well after age 50. The risk of fetal loss rises with age only from the mid-30s or early 40s. There is considerable heterogeneity in the risk of fetal loss–some women are highly prone whereas others are not–thereby creating heterogeneity in fecundity.
Intercourse and Pregnancy-Related Factors
Among ovulating women having intercourse, whether a particular cycle will result in pregnancy depends on the frequency and timing of intercourse. If intercourse does not occur in a fairly narrow time segment extending from five days before ovulation to the day of ovulation, then the risk of pregnancy is exceedingly low (Figure 1).
The more often intercourse occurs within this time segment, the more likely it is that pregnancy will occur; however, the maximum probability is surprisingly small, only about 40 percent. (The maximum probability of conception from a single intercourse optimally timed within a cycle is about 30 percent.) This per-cycle probability of conception, technically known as fecundability (introduced by the Italian demographer and statistician Corrado Gini in 1924), can be directly estimated without reference to conception probabilities by cycle day among regularly menstruating women not using contraception; a typical value would be about 20 percent among young women. Even when the timing and frequency of intercourse are held constant, fecundability can be reduced by both involuntary and voluntary factors. It is reduced by irregular ovulation around menarche and menopause, lactation (both because ovulation is suppressed and–when ovulation is resumed–because of a decreased likelihood of successful implantation), by smoking, by the sexually transmitted infections chlamydia and gonorrhea (due to tubal scarring), by strenuous physical activity among women, by extreme malnutrition, probably as women get older (at least above age 40), and by use of contraception. Worldwide, the most important of these would be contraception in countries in which deliberate birth control is widespread. When this is not the case, lactation can be an important factor reducing fecundability. Elective abortion reduces fecundity worldwide to a far greater extent than fetal loss.
Sexually Transmitted Diseases
Sexually transmitted infections have major effects on fecundity (and fertility) in certain populations. Syphilis is an important cause of fetal loss among women with primary or secondary infections and may be an important factor contributing to low fertility among certain tribal groups in Burkina Faso and the Central African Republic. Untreated pelvic inflammatory disease caused by chlamydia and gonorrhea is a major cause of tubal scarring and sterility. The low fertility characteristic of Central Africa (a belt extending from the west coast of Cameroon and Gabon through northern Congo into southwest Sudan) in the 1950s and 1960s was attributed to a high prevalence of gonorrhea, long before the additional role of chlamydia was recognized. In sub-Saharan Africa, gonorrhea and chlamydia are still common infections. Widespread in equatorial regions, yaws and pinta, while not sexually transmitted, are closely related to syphilis and are also treatable with penicillin. Mass penicillin campaigns against gonorrhea (New Guinea), yaws (Martinique), and yaws and pinta (Cameroon, Burkina Faso, Congo, and Zambia) were followed by substantial increases in fertility. It is possible that improved diagnosis and treatment of sexually transmitted infections in sub-Saharan Africa as a component of AIDS prevention programs will also result in increased fecundity.
A link between nutrition and fertility has been postulated as a relatively simple explanation for variations in marital fertility in populations that do not use contraception. It is suggested that the lower the nutritional status of a population, the lower the fecundity and hence fertility. Chronic malnutrition probably does result in a delay in menarche, but the reduction in fecundity among adolescents resulting from that delay is unlikely to have an important effect on fertility. When food supplies are so short that there is outright famine and starvation, fecundity and hence fertility are sharply reduced. But when malnourishment is chronic and food intake is above starvation levels, there does not appear to be an important nutrition–fertility link.
In the not so distant future, the use of current and new technologies in reproductive biology and genetics could greatly modify the situations described above, rendering the infecund fecund.
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Fecundity comes from the Latin word fecundus, meaning fruitful, rich, or abundant. It is the rate at which individual organisms in the population produce offspring. Although the term can apply to plants, it is typically restricted to animals.
There are two aspects of reproduction: 1) fertility, referring to the physiological ability to breed, and 2) fecundity, referring to the ecological ability to produce offspring. Thus, higher fecundity is dependent on advantageous conditions in the environment that favor reproduction (e.g., abundant food, space, water and mates; limited predation, parasitism, and competition ). The intrinsic rate of increase (denoted as "r") equals the birth rate minus the death rate. It is a population characteristic that takes into account that not all individuals have equal birth rates and death rates. It therefore refers to the reproductive capacity in the population made up of individual organisms. Fecundity, on the other hand, is an individual characteristic. It can be further subdivided into potential and realized fecundity. For example, deer can potentially produce four or more fawns per year, but they typically give birth to only one or two per year. In good years with ample food, they often have only two fawns.
Animals in nature are limited by environmental conditions that control their life history characteristics such as birth, survivorship , and death. A graph of the number of offspring per female per age class (e.g., year) is a fecundity curve. This can then be used to interpret the individuals of a certain age class who contribute more to the population growth than others. In other words, certain age classes have a greater reproductive output than others. Wildlife managers often use this type of information in deciding which individuals in a population can be hunted verses those that should be protected so they can reproduce.
As the number of animals increase, competition for food may become more intense and, therefore, growth and reproduction may decrease. The result is an example of density-dependent fecundity. Fecundity in predators typically increases with an increase in the prey population. Conversely, fecundity in prey species typically increases when predation pressure is low.
Some scientists have found that fecundity is inversely related to the amount of parental care given to the young. In other words, small organisms such as insects and fish which typically invest less time and energy into caring for the young usually have higher fecundity. Larger organisms such as birds and mammals which expend a lot of energy on caring for the young through building of nests, feeding, protecting, and caring have lower fecundity rates.
[John Korstad ]
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