There are two figures to be aware of when talking about birth rates. The first is the fertility rate. This rate measures the number of children per woman. Obviously in nature you cannot have one and a half, or two and a quarter children. But in statistics, just as obviously, you can. The fertility rate is calculated based on official data on the number of children born each year in a given country, proportional to the number of women of childbearing age, that is, between 15 and 49, resident in said country.
Since women make up 50 per cent of the population of each country, and men cannot procreate (in the sense of giving birth, at least not if they are assigned male at birth), the 'perfect' fertility rate if the population is to remain numerically unchanged is 2 (technically 2.1). This means that for every two parents required biologically to 'create' a child, when they die there will be two children to replace them.
Of course, 2 is not a magic number, nor is it valid in every situation. There may be situations where it is preferable to reduce the population, others where it is better to increase it.
Curiously, however, 2 seems to be the magic number for people.
The second figure that we need to be aware of when talking about the birth rate is the average age when having the first child. This figure expresses the average age of all women who give birth for the first time in a given country in a given year. So if in a country in a given year three children are born to women who have no previous children, and these women are aged 21, 30 and 42, the average age at first child in that country for that year will be 31 (21+30+42 divided by 3).
The importance of the average age when having the first child is closely linked to that of the length of childbearing age and carries a range of health implications. Too early an age when having the first child implies a higher probability of death in childbirth or other critical issues related to pregnancy, childbirth and the puerperium for girls who are too young. Too old an age when having the first child implies a greater likelihood of using artificial insemination/fertilisation procedures when attempting to become pregnant, as well as a greater likelihood of health problems for the child and complications in pregnancy and childbirth.
On a purely mathematical level, furthermore, the higher the average age when the first child is born, the narrower the window for bringing more children into the world. The fertile years at potential mothers’ disposal are reduced. If, biologically speaking, women remain fully fertile more or less until the age of 35 (and then, to a lesser extent, between 36 and 40, and even more so after 40), this means that an average age for having a first child of 27 implies a window of another eight + five years of fertile age to have another child (or others), plus another three to five additional years. An average age at first child of 33, on the other hand, reduces this prospect of time for more children by six years .