Too Old to Have Children?
Too Old to Have Children?
STUDY QUESTION. Is it possible to construct an age curve denoting the ages above which women are biologically too old to reproduce?
SUMMARY ANSWER. We constructed a curve based on the distribution of female age at last birth in natural fertility populations reflecting the ages above which women have become biologically too old to have children.
WHAT IS KNOWN ALREADY. The median age at last birth (ALB) for females is ~40–41 years of age across a range of natural fertility populations. This suggests that there is a fairly universal pattern of age-related fertility decline. However, little is known about the distribution of female ALB and in the present era of modern birth control, it is impossible to assess the age-specific distribution of ALB. Reliable information is lacking that could benefit couples who envisage delaying childbearing.
STUDY DESIGN, SIZE, DURATION. This study is a review of high-quality historical data sets of natural fertility populations in which the distributions of female age at last birth were analysed. The studies selected used a retrospective cohort design where women were followed as they age through their reproductive years.
PARTICIPANTS/MATERIALS, SETTING, METHODS. Using a common set of eligibility criteria, large data files of natural fertility populations were prepared such that the analysis could be performed in parallel across all populations. Data on the ALB and confounding variables are presented as box and whisker plots denoting the 5th, 25th, 50th, 75th and 95th percentile distribution of the age at last birth for each population. The analysis includes the estimation of Kaplan–Meier curves for age at last birth of each population. The hazard curve for ALB was obtained by plotting the smoothed hazard curve of each population and taking the lowest hazard within a time period of at least 5 years. This lowest hazard curve was then transformed into a cumulative distribution function representing the composite curve of the end of biological fertility. This curve was based on the data from three of the six populations, having the lowest hazards of end of fertility.
MAIN RESULTS AND THE ROLE OF CHANCE. We selected six natural fertility populations comprising 58,051 eligible women. While these populations represent different historical time periods, the distribution of the ages at last birth is remarkably similar. The curve denoting the end of fertility indicates that <3% of women had their last birth at age 20 years meaning that almost 98% were able to have at least one child thereafter. The cumulative curve for the end of fertility slowly increases from 4.5% at age 25 years, 7% at age 30 years, 12% at age 35 years and 20% at age 38 years. Thereafter, it rises rapidly to about 50% at age 41, almost 90% at age 45 years and approaching 100% at age 50 years.
LIMITATIONS, REASONS FOR CAUTION. It may be argued that these historical fertility data do not apply to the present time; however, the age-dependent decline in fertility is similar to current populations and is consistent with the pattern seen in women treated by donor insemination. Furthermore, for reproductive ageing, we note that it is unlikely that such a conserved biological process with a high degree of heritability would have changed significantly within a century or two.
WIDER IMPLICATIONS OF THE FINDINGS. We argue that the age-specific ALB curve can be used to counsel couples who envisage having children in the future. Our findings challenge the unsubstantiated pessimism regarding the possibility of natural conception after age 35 years.
STUDY FUNDING/COMPETING INTEREST(S). No external funding was either sought or obtained for this study. There are no conflicts of interest to be declared.
Since the general availability of effective contraception in the late 1960s, motherhood has become an issue of personal preferences instead of biology (van de Kaa, 1987). Women could decide to continue schooling and acquire a profession before thinking of having children. One of the consequences of this choice was the significant postponement of childbearing in Western societies (Lutz et al., 2003). The problem with this delay is that fertility—the biological capacity to reproduce—starts to decline around age 25–30 years (Wood, 1989; Van Noord-Zaadstra et al., 1992). Most women still decide to delay having children and the question is: how long can they wait?
The age at which fertility declines to a level where it becomes impossible to conceive a pregnancy leading to live birth is associated with the biological age at last birth (ALB): the age at last birth when no contraceptive measures have been used to limit family size. In the present era of modern birth control, it is impossible to assess the age-specific distribution of ALB. In developed countries many fertile women and men are childless by choice and if not, most of them want a family of one or two children only and prevent conception thereafter. Accordingly, we do not observe in modern populations the ages when the biological capacity to have children ends.
The pattern of fertility decline can best be studied in so-called natural fertility populations where no deliberate attempt is made to limit family size (Henry, 1961, 1965). These populations lived before the so-called demographic fertility transition (Chesnais, 1992) when couples intentionally restricted the number of children, first by measures such as withdrawal, abstinence, the use of condoms, prolonged breastfeeding or induced abortion (McLaren, 1990), and since the 1960s by modern contraceptive methods including the 'pill', intrauterine device and sterilization. As we demonstrated previously (Te Velde and Pearson, 2002), the evolving practice of birth control enabling parents to prevent the birth of additional children after they have achieved their preferred family size first resulted in earlier ALB. Demographic fertility transition started at the end of the 18th century in France and spread throughout Europe and the rest of the Western world during the 19th and beginning of the 20th century. It was related to increasing life expectancy and a greater valuation of children. This pattern started gradually and was adopted first by the educated residents in cities and then later among the working class and in rural areas. Given that this transformation occurred slowly, it is impossible to identify a specific year or period when it started or was completed. Natural fertility populations existed all over the world until the 18th–19th centuries. Also in the recent past as well as at present, there have been populations where contraception is not practised for religious and cultural reasons (Tietze, 1957; Rahman and Menken, 1993; O'Connor et al., 1998).
The similarity of the median female ALB at ages 40–41 years in most natural fertility populations (Bongaarts, 1982) suggests that there is a universal pattern of age-related fertility decline. However, this has only been assessed in small data sets (Tietze, 1957; Trussell and Wilson, 1985; Menken et al., 1986). Recently, electronic databases describing the reproductive events of large natural fertility populations have become available and offer the opportunity to study the distribution of ALB more reliably. These data sets are not only larger but also they allow analysts to apply identical sample inclusion criteria and identical methods of analysis.
The aim of this study is 2-fold. The first is to review and analyse data files from natural fertility populations with respect to the distribution of ALB. Apart from shared biological determinants, the ALB distribution in natural fertility populations may be confounded by behavioural, socio-medical and historical factors such as ('period') effects, age at marriage, birth spacing, the level of secondary infertility and parity. These factors have to be taken into account as they may change the distribution of ALB (Leridon, 1977). The second aim is to construct an age curve that describes ages above which women are not biologically able to have children. To achieve this aim, we combine several data sets matching with the lowest chance of reaching ALB. The resulting curve is likely to be least influenced by confounding factors and therefore best reflects the biological process of fertility decline. Hereafter, we call this the biological ALB curve.
Abstract and Introduction
Abstract
STUDY QUESTION. Is it possible to construct an age curve denoting the ages above which women are biologically too old to reproduce?
SUMMARY ANSWER. We constructed a curve based on the distribution of female age at last birth in natural fertility populations reflecting the ages above which women have become biologically too old to have children.
WHAT IS KNOWN ALREADY. The median age at last birth (ALB) for females is ~40–41 years of age across a range of natural fertility populations. This suggests that there is a fairly universal pattern of age-related fertility decline. However, little is known about the distribution of female ALB and in the present era of modern birth control, it is impossible to assess the age-specific distribution of ALB. Reliable information is lacking that could benefit couples who envisage delaying childbearing.
STUDY DESIGN, SIZE, DURATION. This study is a review of high-quality historical data sets of natural fertility populations in which the distributions of female age at last birth were analysed. The studies selected used a retrospective cohort design where women were followed as they age through their reproductive years.
PARTICIPANTS/MATERIALS, SETTING, METHODS. Using a common set of eligibility criteria, large data files of natural fertility populations were prepared such that the analysis could be performed in parallel across all populations. Data on the ALB and confounding variables are presented as box and whisker plots denoting the 5th, 25th, 50th, 75th and 95th percentile distribution of the age at last birth for each population. The analysis includes the estimation of Kaplan–Meier curves for age at last birth of each population. The hazard curve for ALB was obtained by plotting the smoothed hazard curve of each population and taking the lowest hazard within a time period of at least 5 years. This lowest hazard curve was then transformed into a cumulative distribution function representing the composite curve of the end of biological fertility. This curve was based on the data from three of the six populations, having the lowest hazards of end of fertility.
MAIN RESULTS AND THE ROLE OF CHANCE. We selected six natural fertility populations comprising 58,051 eligible women. While these populations represent different historical time periods, the distribution of the ages at last birth is remarkably similar. The curve denoting the end of fertility indicates that <3% of women had their last birth at age 20 years meaning that almost 98% were able to have at least one child thereafter. The cumulative curve for the end of fertility slowly increases from 4.5% at age 25 years, 7% at age 30 years, 12% at age 35 years and 20% at age 38 years. Thereafter, it rises rapidly to about 50% at age 41, almost 90% at age 45 years and approaching 100% at age 50 years.
LIMITATIONS, REASONS FOR CAUTION. It may be argued that these historical fertility data do not apply to the present time; however, the age-dependent decline in fertility is similar to current populations and is consistent with the pattern seen in women treated by donor insemination. Furthermore, for reproductive ageing, we note that it is unlikely that such a conserved biological process with a high degree of heritability would have changed significantly within a century or two.
WIDER IMPLICATIONS OF THE FINDINGS. We argue that the age-specific ALB curve can be used to counsel couples who envisage having children in the future. Our findings challenge the unsubstantiated pessimism regarding the possibility of natural conception after age 35 years.
STUDY FUNDING/COMPETING INTEREST(S). No external funding was either sought or obtained for this study. There are no conflicts of interest to be declared.
Introduction
Since the general availability of effective contraception in the late 1960s, motherhood has become an issue of personal preferences instead of biology (van de Kaa, 1987). Women could decide to continue schooling and acquire a profession before thinking of having children. One of the consequences of this choice was the significant postponement of childbearing in Western societies (Lutz et al., 2003). The problem with this delay is that fertility—the biological capacity to reproduce—starts to decline around age 25–30 years (Wood, 1989; Van Noord-Zaadstra et al., 1992). Most women still decide to delay having children and the question is: how long can they wait?
The age at which fertility declines to a level where it becomes impossible to conceive a pregnancy leading to live birth is associated with the biological age at last birth (ALB): the age at last birth when no contraceptive measures have been used to limit family size. In the present era of modern birth control, it is impossible to assess the age-specific distribution of ALB. In developed countries many fertile women and men are childless by choice and if not, most of them want a family of one or two children only and prevent conception thereafter. Accordingly, we do not observe in modern populations the ages when the biological capacity to have children ends.
The pattern of fertility decline can best be studied in so-called natural fertility populations where no deliberate attempt is made to limit family size (Henry, 1961, 1965). These populations lived before the so-called demographic fertility transition (Chesnais, 1992) when couples intentionally restricted the number of children, first by measures such as withdrawal, abstinence, the use of condoms, prolonged breastfeeding or induced abortion (McLaren, 1990), and since the 1960s by modern contraceptive methods including the 'pill', intrauterine device and sterilization. As we demonstrated previously (Te Velde and Pearson, 2002), the evolving practice of birth control enabling parents to prevent the birth of additional children after they have achieved their preferred family size first resulted in earlier ALB. Demographic fertility transition started at the end of the 18th century in France and spread throughout Europe and the rest of the Western world during the 19th and beginning of the 20th century. It was related to increasing life expectancy and a greater valuation of children. This pattern started gradually and was adopted first by the educated residents in cities and then later among the working class and in rural areas. Given that this transformation occurred slowly, it is impossible to identify a specific year or period when it started or was completed. Natural fertility populations existed all over the world until the 18th–19th centuries. Also in the recent past as well as at present, there have been populations where contraception is not practised for religious and cultural reasons (Tietze, 1957; Rahman and Menken, 1993; O'Connor et al., 1998).
The similarity of the median female ALB at ages 40–41 years in most natural fertility populations (Bongaarts, 1982) suggests that there is a universal pattern of age-related fertility decline. However, this has only been assessed in small data sets (Tietze, 1957; Trussell and Wilson, 1985; Menken et al., 1986). Recently, electronic databases describing the reproductive events of large natural fertility populations have become available and offer the opportunity to study the distribution of ALB more reliably. These data sets are not only larger but also they allow analysts to apply identical sample inclusion criteria and identical methods of analysis.
The aim of this study is 2-fold. The first is to review and analyse data files from natural fertility populations with respect to the distribution of ALB. Apart from shared biological determinants, the ALB distribution in natural fertility populations may be confounded by behavioural, socio-medical and historical factors such as ('period') effects, age at marriage, birth spacing, the level of secondary infertility and parity. These factors have to be taken into account as they may change the distribution of ALB (Leridon, 1977). The second aim is to construct an age curve that describes ages above which women are not biologically able to have children. To achieve this aim, we combine several data sets matching with the lowest chance of reaching ALB. The resulting curve is likely to be least influenced by confounding factors and therefore best reflects the biological process of fertility decline. Hereafter, we call this the biological ALB curve.
