Female fertility

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The fertility of an individual female is defined as the number of live births of babies to that female. Note that the term fertility rate is typically used when talking about aggregating fertility values across populations, but we simply talk of fertility when referring to a particular individual female. The fertility of an individual female is a nonnegative integer, i.e., it could take values 0, 1, 2, ... Due to biological considerations, it typically would not exceed 20 (there are only about 32 females in recorded history who have had more than 20 children, according to Wikipedia's list of people with the most children).

Fertility is a loose umbrella term. The more technical terms are described below.

Note that in day-to-day parlance, fertility may be used in the sense of "ability to produce children." In demographic jargon, we use the term fecundity for that sense of the word fertility, and reserve the use of the term fertility for actual children produced so far or in the future.

More precise jargon

The following three terms are closely related:

Term Meaning is it constant or does it change with time? If so, how does it change?
Parity The number of live births to a given female that have occurred so far. The parity of a female may change over time. Note that parity can either stay constant or increment by integer amounts over time, with the increments typically being by 1. Higher increments occur if the female gives birth to twins.
Completed fertility The parity of a female who has completed her childbearing years. It is constant (but can only be known once the female has completed her childbearing years).
Expected fertility The value that the female expects for her completed fertility. This equals the sum of her parity and the number of future children she expects to have. The expected fertility of a female may change with time, as her parity as well as her expectations and goals for the future change.

There are many different ways of aggregating fertility across populations.

Population-level fertility measures

Fertility measure Definition
Crude birth rate Number of live births per 1000 members of the population.
General fertility rate Number of live births per 1000 women in their childbearing years.
Age-specific fertility rate Number of live births to women at a given age, per 1000 women at that age.
Total fertility rate Sum of age-specific fertility rates for all ages in childbearing years, but scaled to a denominator of 1 rather than 1000.
Cohort completed fertility Arithmetic mean of the values of completed fertility for all women in an age cohort, typically one that has completed its childbearing years.

Biological issues, exceptions, and branch cases

Premature destruction of the fetus

The typical path to a live birth is:

Conception \to Child grows in the womb for about nine months \to Child is born

The following are some subtleties associated with measurement:

  • Many fetuses that are conceived spontaneously abort, often even before the to-be mother is aware that she is pregnant.
  • In some cases, fetuses are aborted either due to induced abortions or due to some physical accident or medical problem.
  • In some cases, fetuses that survive to the point of birth die during the process of birth.

If any of the above cases occur, the fetus is not counted towards the female's fertility.

Multiple births

In some cases, a female may give birth to twins (identical twins or fraternal twins), triplets, or higher forms of multiple births. Each live baby is counted as one, so giving birth to twins adds a total of 2 to the female's fertility.

Egg donors, surrogate pregnancy, and adoption

Three functions related to children can be conceptually separated, even though they are usually done by the same woman and/or family:

  1. Providing the egg that is used for conception, i.e., genetic parentage
  2. Bearing the fetus through the term of pregnancy and giving birth to it
  3. Taking care of the baby after it is born

The definition of fertility does not make a clear distinction between these tasks, though since births are what is recorded, (2) is the definition that is most likely to be used in practice. A few points:

  • If either surrogate pregnancy or the use of external egg donors becomes quantitatively significant, then the distinction between (1) and (2) will start mattering.
  • For the distinction with (3) to start mattering, adoption of babies and/or the use of both egg donors and surrogate pregnancies would need to be quantitatively significant.
  • If we are measuring averages (i.e., rates) across a population, then the distinction between (1), (2), and (3) is not relevant if all the people involved are members of the same population. If, however, the people involved are in different populations, it might matter. For instance, if there are adoptions in country A of babies from country B, then the distinction between (2) and (3) would matter in computing the fertility rate of country A. Similarly if people from country A use surrogate mothers from country B.