Strengths and Limitations
This study used a large population-based sample from a validated
provincial registry, comprising all hospital births, within a publicly
funded health care system. The ability to categorize women by type of
conception, including subfertility and type of infertility treatment is
another strength. One limitation could be nondifferential
misclassification of the exposure: Subfertility was based on a single
OHIP billing code of ICD-9-628 in the 2 years prior to the index
conception date, in the absence of receipt of infertility treatment
during that time. In those who conceived by infertility treatment, this
OHIP billing code appeared among 90% of women with non-invasive
infertility treatment, and 96% of those with invasive infertility
treatment, indirectly suggesting that clinically subfertile women were
being captured herein. On the other hand, exposure to infertility
treatment was captured by chart review by trained abstractors at the
time of birth, and is likely to be accurate as are other data elements
in the Ontario birth registry.22 However, it is
possible that some pregnant individuals who conceived by non-invasive or
invasive infertility treatment were not captured by this approach
resulting in nondifferential misclassification and a possible
underestimate of the effect on PTB. In fact, capturing infertility and
fertility treatment status from birth certificate records has been found
to be highly specific yet poorly sensitive, suggesting that while some
females with infertility are missed with this approach, false positives
are unlikely.23