Cohort study confirms link between adolescent obesity and later fertility problems

Published 23 August 2021

Girls affected by obesity during their mid-childhood and adolescence are found at risk of reduced fertility and greater likelihood of remaining childless. Prevention of obesity in school-age girls recommended as a measure to reduce the risk of infertility.

A remarkable study which followed up almost 6000 births for 50 years has confirmed what has long been suspected, that adolescent girls with obesity have an increased risk of infertility and childlessness in their later adult years.(1) The study examined the effect of obesity in three childhood phases – early childhood (3-6 years), prepubertal (7-10 years) and pubertal (11-15 years) – and found significant associations with decreased fecundability in the latter two age groups.

This prospective study was based on the 1966 birth cohort of the Northern Finland registry, whose female subjects were followed until the age of 50 with postal questionnaires on reproductive capacity as evaluated by decreased fecundability, a need for infertility investigation and fertility treatment by the age of 46. With 1507 women of the cohort excluded for never having attempted pregnancy, the final study population totalled 4382 women who before the age of 46 had attempted to achieve pregnancy. Childlessness and the number of children by age 50 were cross-checked with national registers, while BMI data (height and weight) were collected by trained staff at all stages as part of Finland’s national child health programme. There were on average seven measurements during infancy and 16 during childhood for each child. The authors explain that their three time-frames of assessment correspond with a model of childhood obesity, moving from an adiposity peak at nine months which decreases until age 4-6 years, then an adiposity rebound usually beginning at 5-7 years.

More than 1104 women in the cohort reported childlessness or some infertility problems, although results here showed no association with age at adiposity peak nor following adjustments with BMI during this period. However, at ages 7–10 and 11–15 years obesity was significantly associated with decreased fecundability (adjusted OR 2.05 and 2.04 respectively) and a lower number of children. Indeed, at age 11–15 years both overweight and obesity were significantly associated with a higher risk of childlessness (adjusted OR 1.56 and 1.77 respectively) when compared with normal weight and even after excluding women with PCOS.

In explaining this exclusion, the authors note that there were around 500 women with assumed (oligo/amenorrhea and hirsutism) or diagnosed PCOS and its exclusion allowed calculation without an effect of this strong confounder. Indeed, the authors acknowledged that ‘PCOS is strongly associated with overweight and obesity and is also the most frequent cause of anovulatory infertility in women’. The results of this study would thus suggest that the adult infertility associated with childhood obesity is not simply an effect of PCOS – and that ‘the findings corroborate the body of evidence for a causal relation between early adiposity and the reproductive functions in women’.

The study thus appears to offer good evidence that being obese in mid-childhood and during adolescence is associated with an increased risk of fertility problems in later life. Indeed, even overweight girls in the 7–10 years age group had fewer children than their normal weight peers. Similarly, women who were obese at age 11–15 years were more likely to remain childless, to have fewer children and to suffer more often from decreased fecundability than their normal weight counterparts.

Of course, the link between childhood obesity and later health risks has been substantially studied, particularly in cardiovascular disease. Earlier this year, the CDC listed five ‘immediate health risks’ from obesity in childhood (hypertension and other metabolic disorders, type 2 diabetes, asthma, musculoskeletal problems, liver diseases) and, not least of them, obesity in later life. It is the latter association which has raised controversies over some childhood obesity studies – that in a non-prospective design the effect of adult obesity cannot always be untangled from the childhood association.

On the matter of infertility, the authors of this new study describe adult obesity as ‘a well-known risk factor for infertility and childlessness’. But the few studies of a similar effect from childhood obesity have produced conflicting results. For example, an Australian cohort study of 1544 female births in 1985 found that childhood obesity before 12 years of age (but not after) ‘appears to increase’ the risk of female infertility in later life.(2)

As a biological explanation for the association the authors propose that adolescent girls with obesity have a ‘more pronounced and longer state of physiological insulin resistance’ than normal weight girls, and that insulin resistance in adulthood is strongly associated
with PCOS and anovulatory infertility. However, as the authors here insist, the association of obesity and overweight with childlessness remained significant even after excluding women with PCOS, suggesting that the adverse effect of a higher BMI at adolescence on fertility cannot solely be caused by PCOS.

What’s to be done? Despite the growing catalogue of adverse health effects resulting from childhood obesity, public health campaigns have proved of limited value, and childhood obesity continues as a ‘worldwide epidemic’. While some progress has been made in food labelling, regulatory measures to limit the promotion of junk food and sweetened drinks to children have proved a step too far in most jurisdictions. And here too in the conclusion to this study the authors can say little more than repeat the mantra of active prevention and treatment of obesity and maintaining a normal weight, especially during adolescence, to sustain fertility.

1. Laru J, Nedelec R, Koivuaho E, et al. BMI in childhood and adolescence is associated with impaired reproductive function—a population-based cohort study from birth to age 50 years. Hum Reprod 2021; doi:10.1093/humrep/deab164
2. He Y, Tian J, Oddy WH, et al. Association of childhood obesity with female infertility in adulthood: a 25-year follow-up study. Fertil Steril 2018; 110: 596-604. doi:10.1016/j.fertnstert.2018.05.011.

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