The fall of fecundity with age

Women are having children at increasingly late ages...

In the last 30 years, the average age at which women have their first child has been rising regularly in France, as in most European countries. Around 1970, French women were having their first child at the average age of 24; today that age is 28.5 years. Though the 1970 figure was reached after a period of decreasing average age (during the baby boom, women had more children at increasingly young ages), the rise over the last three decades has been spectacular. Fertility (fécondité) at 35-39 years rose steeply over the same period, while fertility at 40-44 years has also risen, though less sharply. The phenomenon appears to represent a major behavioural change. For an entire set of reasons, women today want to postpone having a family. As they see it, a set of conditions has to be met before trying to conceive: finding a stable partner who will be an “acceptable” father, reaching agreement with him on having a child, finishing one’s studies, ensuring a minimum of occupational stability for oneself, having sufficient resources—all legitimate conditions that can only work to postpone the arrival of the first child.

...and perhaps too late?

But if women wait too long, is there a risk that they won’t be able to have that child for physiological reasons? As we have just seen, female fertility (that is, the biological ability to conceive) is certain to fall with age—male fertility also falls, but later in life and to less effect. Here we concentrate on female fertility.

Three factors play a role. First, as we all know, menopause is an absolute limit beyond which a woman cannot get pregnant (unless she turns to highly artificial methods, as we shall see). Menopause occurs around 50 years, and there are no recent signs that this age is changing. In reality, menopause is the “outermost” limit: a woman’s child-conceiving ability begins to diminish several years before menopause definitively sets in. In societies where people did not practice birth control, women who were still married at age 50 had had their last child well before that age—at around 40 on average. We can conclude that most women become infertile well before 50 years of age.

The age at which women become infertile varies of course by individual woman, or rather by couple. Furthermore, total infertility is preceded by a period in which “childbearing ability” is simply declining. The likelihood of conceiving falls while the risk of miscarriage rises. The likelihood of conceiving can be measured by the time required to conceive. Conception is not always achieved during the first month of sexual relations without contraception. The average time lapse may be from 3 to 5 months for a young couple and it increases with age—which is particularly difficult for older couples, who are usually the most impatient to “get pregnant”! And whereas the risk of miscarriage at 25 years of age is estimated at 12%, the figure rises to 20% at 37 years, 30% at 43, and sharply thereafter. This risk is therefore a major obstacle to having children at a late age.

Quantifying the fall in fertility with age

By combining all these data in a simulation model that also takes into account the age at which a couple tries to have its first, second, third, etc. child and the number of children they wish to have, we can reconstruct the history of a generation living in contemporary France (Leridon 2004). We can estimate, for example, that a women trying to have a child at around age 30 has a 75% chance of succeeding in twelve months’ time, a 66% chance if she starts trying at 35 and a 44% chance if she begins at 40. The risks of total failure are, respectively, 8%, 15% and 36% (see Figure 1, upper curves).

ART (Assisted Reproductive Technology): an uncertain remedy after 35 years

These results do not take into account possible recourse to ART. Many couples turn to this technology (several tens of thousands in France every year) but success is hardly guaranteed. Worse yet, ageing complicates the problem, and IVF success, for example, falls sharply after age 35 and is nearly nil after age 45 (Figure 1, lower curve). Why? Because medical methods work above all to improve ovulation and fertilization but they cannot counteract the underlying physiological factors. We know, for example, that a woman’s oocyte stock is already completely determined at birth; as a woman ages, her oocytes also age and the stock diminishes. This is probably why fertilisation becomes more difficult and why the risk of chromosome anomalies (which may induce spontaneous abortion) rises with age. There are only two techniques that can overcome this difficulty: performing IVF with oocytes from a young woman (but oocyte donation is rare in France) or engaging a younger woman as “surrogate mother,” thereby enabling a woman over 50 to “have” a child (but this practice is illegal in France). In most cases, then, ART is not a reliable remedy for the difficulties that may prevent a woman from having a child after age 35 or 40.

Contact: Henri Leridon

Second unions and the desire to have children

To the above-cited reasons women may have for postponing the arrival of their first child should be added the current number of divorces and separations, followed by an increase in the number of persons who “repartner,” becoming part of a new couple. The development of second unions is definitely another reason for wanting to have children at a relatively late age.

In France, for example, in the generations born from 1935 to 1954, 34.3 % of repartnered women and 39.4% of repartnered men had children—in a union that was therefore not their first.

The probability is even higher after childless first unions (48.2% for women and 55.2% for men). People usually form new couples and have children in them at relatively late ages, and children born to persons in their second union represent a considerable proportion of later-age fertility.

As shown in Figure 2, the proportion of children born in second unions to women over 35 has greatly increased. In 20 years it rose from 6% to 18% for women aged 35-39 years, and from 10% to over 25% for women aged 40-44. For fathers in their forties the figure more than doubled in 20 years, from 14% to 32%.

 

 

Contacts: Eva Beaujouan,  Anne Solaz

Online: October 2008

In French demographic terminology, fécondité is distinguished from fertilité.

Fertilité is the overall biological ability to conceive while fécondité is a demographic notion pertaining to the number of children a women actually bears.