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