Below are Dr Cheung's comments on the original study by David Dunson et al. (2004) quoted by this article and the basis of the authors arguments.
Study population –
- Most participants were trying to avoid pregnancy initially (so a very specific sample)
- While it is appropriate to separate age groups by 5-year increments in demography, i.e., 30-34, 35-40 etc., it is a wide range for the 35-39 age group.
- The proportions of subjects age 35, 36, 37, 38…were not given and results could be skewed by a disproportionate larger representation of women of say, age 36 and 37 rather than 37 and up. There is a big difference in fecundity between a woman of age 35 or 36 and one of 39!
- The methodology of the study was heavily based on statistical modeling of fecundity using field data collected on menstrual cycles from participants so the actually rates of conceiving need to be put in perspective. As mentioned, most participants were trying to avoid pregnancy initially and would very likely have different characteristics.
The authors stipulated that the focus of the study was “to address the important question of whether declines with age in male and female fertility throughout the 20s and 30s are primarily attributable to an increasing proportion of sterile couples in older age groups or to most couples becoming gradually less fertile” where sterility is defined as the inability to conceive a pregnancy naturally in the absence of clinical interventions. I think the answer is so intuitive that older age groups are just becoming gradually less fertile that talking sterility (as in menopause) seems to be irrelevant!
These interesting population-based epidemiological / modeling studies are generally rather “dirty” in their data, requiring a lot of post-hoc statistical adjustments. They did show a lower chance of achieving a pregnancy with age but for the reasons discussed above, I would not take the actual rates seriously. Otherwise, those women in their late 30s (closer to 40 rather than, say, 36) are given the wrong message!
Nobody is saying that women age 35, 36, or 37 are not able to conceive. However, the “margin of safety” is much reduced, particularly if the perspectives of the original studies are not interpreted appropriately or taken out of context by the referred layperson article.
—Dr Anthony Cheung