Fecundity Decline and Pregnancy Postponement Impact on Number of Children
Fecundity Decline and Pregnancy Postponement Impact on Number of Children
Background: Over the past decades, the proportion of couples who resort to infertility treatment has tremendously increased, and fertility (the final number of children) sharply declined. We explored the roles of two potential causes of these trends: a temporal decline in the couples' fecundability and a postponement of age at initiation of pregnancy attempts.
Methods: We conducted a Monte–Carlo simulation for the reproductive history of 100000 women based on the fertility and socio-demographic characteristics of the 1968 birth cohort in France. Declines in fecundability of various amplitudes have been implemented, as well as increases in the distribution of age at initiation of pregnancy attempts.
Results: A decline in fecundability by 15% implied a decrease in fertility by 4%, and an increase in the proportion of couples eligible for infertility treatments by 73%. An increase in the mean age at initiation of first pregnancy attempt by 2.5 years from 25 years entailed a decrease by 5% in fertility and an increase by 32% in the proportion of couples eligible for infertility treatments.
Conclusion: A relatively important decrease in fecundability and an increase by 2.5 years in age at first pregnancy attempt are likely to have only a limited impact on fertility. However, they may have a large impact on the proportion of involuntarily infertile couples, likely to resort to assisted reproduction techniques.
The average number of children born per woman has been declining sharply in most developed countries during the last decades. This was mainly due to the changes in behavior of couples: having children is now seen as competing with many other aspirations of the woman, the man and the couple (Van De Kaa, 1987; Lesthaeghe and Surkyn, 1988). Another major behavioral change in many countries is the postponement of childbearing. Contemporary couples are not only wishing fewer children than their forerunners, they also want to start having them later in their life. The mean age at first birth rose by 2–4 years in many countries over the last 20–30 years (Sobotka, 2004) and the mean age at childbearing (all parities) is now exceeding 30 years in Denmark, Finland, Italy, Netherlands, Sweden or Switzerland (Council of Europe, 2005). Such averages are still in the best ages of the reproductive period, but their increases mean that some couples are trying to have children at older and older ages. Some of them could fail to procreate. Berrington (2004) suggested to call such couples 'perpetual postponers': they still want one (more) child but can never decide when it is appropriate to start trying to have it. They can thus end their reproductive life with fewer children than wished.
In this paper, we will use the word 'fertility' as a synonym for the actual number of children for a woman or a group of women. This is in agreement with the demographic usage, but more restrictive than the medical usage (see Habbema et al., 2004). The word 'fecundity' will refer to the ability of men and women to bear children. This ability depends on fecundability (the monthly probability of pregnancy among sexually active non-contraceptive couples), on the rate of fetal mortality (the probability that a pregnancy does not result in a live birth) and on the probability of being permanently sterile (i.e. unable to conceive).
In addition to the above-mentioned trends of fertility, a decline in fecundity can be suspected. Several reports indicated a possible decline in semen parameters over the last decades in developed countries (Carlsen et al., 1992; Auger et al., 1995; Swan et al., 2000). This possible decline has been accompanied by clear increases in the incidence of testis cancer, a pathology associated to decreased fertility before and after the onset of the disease (Moller and Skakkebaek, 1999; Jacobsen et al., 2000), and led to the hypothesis of an increased incidence of testicular dysgenesis syndrome (Skakkebaek et al., 2001). How such adverse temporal trends in male reproductive health may translate at the level of the couple is unclear. Sperm concentration and morphology are clearly associated with fecundability (Bonde et al., 1998a,b; Slama et al., 2002), however, their ability to predict 1-year involuntary infertility is limited (Guzick et al., 2001). Moreover, since only the changes in sperm concentration values in the range from 0 to 50 million sperm/ml are associated with the probability of pregnancy at the population level (Bonde et al., 1998a,b; Slama et al., 2002), important variations in sperm concentrations may have a limited effect on the probability of pregnancy. Indeed, a decline by 21% in median sperm concentration, which corresponds to the decline observed among French semen donors extrapolated over a 15-year period, would only translate into a relative decline by 7% in fecundability (Slama et al., 2004). Similarly, a decline by 47% in sperm concentration (as extrapolated for French semen donors over a 45 years period) would reduce fecundability by 15% (Slama et al., 2004). From the point of view of public health, however, the relevant endpoint is the occurrence of a live birth (fertility) or the occurrence of involuntary infertility for 12 months or more, a duration above which expensive and painful fertility treatments may be initiated.
Our aim is to assess the potential impact on fertility (the final number of children) and on the demand for assisted reproduction technologies (ART) of the following biological and demographic trends: a decline in fecundability with an amplitude coherent with what is expected because of a temporal deterioration of male reproductive health, and a further postponement of births in line with current demographic trends. We will also estimate the possible impact of the recourse to ART on fertility.
Background: Over the past decades, the proportion of couples who resort to infertility treatment has tremendously increased, and fertility (the final number of children) sharply declined. We explored the roles of two potential causes of these trends: a temporal decline in the couples' fecundability and a postponement of age at initiation of pregnancy attempts.
Methods: We conducted a Monte–Carlo simulation for the reproductive history of 100000 women based on the fertility and socio-demographic characteristics of the 1968 birth cohort in France. Declines in fecundability of various amplitudes have been implemented, as well as increases in the distribution of age at initiation of pregnancy attempts.
Results: A decline in fecundability by 15% implied a decrease in fertility by 4%, and an increase in the proportion of couples eligible for infertility treatments by 73%. An increase in the mean age at initiation of first pregnancy attempt by 2.5 years from 25 years entailed a decrease by 5% in fertility and an increase by 32% in the proportion of couples eligible for infertility treatments.
Conclusion: A relatively important decrease in fecundability and an increase by 2.5 years in age at first pregnancy attempt are likely to have only a limited impact on fertility. However, they may have a large impact on the proportion of involuntarily infertile couples, likely to resort to assisted reproduction techniques.
The average number of children born per woman has been declining sharply in most developed countries during the last decades. This was mainly due to the changes in behavior of couples: having children is now seen as competing with many other aspirations of the woman, the man and the couple (Van De Kaa, 1987; Lesthaeghe and Surkyn, 1988). Another major behavioral change in many countries is the postponement of childbearing. Contemporary couples are not only wishing fewer children than their forerunners, they also want to start having them later in their life. The mean age at first birth rose by 2–4 years in many countries over the last 20–30 years (Sobotka, 2004) and the mean age at childbearing (all parities) is now exceeding 30 years in Denmark, Finland, Italy, Netherlands, Sweden or Switzerland (Council of Europe, 2005). Such averages are still in the best ages of the reproductive period, but their increases mean that some couples are trying to have children at older and older ages. Some of them could fail to procreate. Berrington (2004) suggested to call such couples 'perpetual postponers': they still want one (more) child but can never decide when it is appropriate to start trying to have it. They can thus end their reproductive life with fewer children than wished.
In this paper, we will use the word 'fertility' as a synonym for the actual number of children for a woman or a group of women. This is in agreement with the demographic usage, but more restrictive than the medical usage (see Habbema et al., 2004). The word 'fecundity' will refer to the ability of men and women to bear children. This ability depends on fecundability (the monthly probability of pregnancy among sexually active non-contraceptive couples), on the rate of fetal mortality (the probability that a pregnancy does not result in a live birth) and on the probability of being permanently sterile (i.e. unable to conceive).
In addition to the above-mentioned trends of fertility, a decline in fecundity can be suspected. Several reports indicated a possible decline in semen parameters over the last decades in developed countries (Carlsen et al., 1992; Auger et al., 1995; Swan et al., 2000). This possible decline has been accompanied by clear increases in the incidence of testis cancer, a pathology associated to decreased fertility before and after the onset of the disease (Moller and Skakkebaek, 1999; Jacobsen et al., 2000), and led to the hypothesis of an increased incidence of testicular dysgenesis syndrome (Skakkebaek et al., 2001). How such adverse temporal trends in male reproductive health may translate at the level of the couple is unclear. Sperm concentration and morphology are clearly associated with fecundability (Bonde et al., 1998a,b; Slama et al., 2002), however, their ability to predict 1-year involuntary infertility is limited (Guzick et al., 2001). Moreover, since only the changes in sperm concentration values in the range from 0 to 50 million sperm/ml are associated with the probability of pregnancy at the population level (Bonde et al., 1998a,b; Slama et al., 2002), important variations in sperm concentrations may have a limited effect on the probability of pregnancy. Indeed, a decline by 21% in median sperm concentration, which corresponds to the decline observed among French semen donors extrapolated over a 15-year period, would only translate into a relative decline by 7% in fecundability (Slama et al., 2004). Similarly, a decline by 47% in sperm concentration (as extrapolated for French semen donors over a 45 years period) would reduce fecundability by 15% (Slama et al., 2004). From the point of view of public health, however, the relevant endpoint is the occurrence of a live birth (fertility) or the occurrence of involuntary infertility for 12 months or more, a duration above which expensive and painful fertility treatments may be initiated.
Our aim is to assess the potential impact on fertility (the final number of children) and on the demand for assisted reproduction technologies (ART) of the following biological and demographic trends: a decline in fecundability with an amplitude coherent with what is expected because of a temporal deterioration of male reproductive health, and a further postponement of births in line with current demographic trends. We will also estimate the possible impact of the recourse to ART on fertility.