Background Long-term safety of assisted reproductive techniques (ART) is of interest as use is increasing. Cancer risk is known to be affected by parity. This study examined risk of cancer after fertility treatment, stratified by women's parity. Methods Data was obtained on all women (n=1 353 724) born in Norway between 1960-1996. Drug exposure data (2004-2014) was obtained from the Norwegian Prescription Database [drugs used in ART and clomiphene citrate (CC)]. The Medical Birth Registry of Norway provided parity status. Hazard ratios were calculated for all site cancer, breast, cervical, endometrial, ovarian, colorectal, central nervous system, thyroid cancer and malignant melanoma. Results In 12 354 392 person-years of follow-up, 20 128 women were diagnosed with cancer. All-site cancer risk was (1.14, 1.03-1.26) and (1.10, 0.98-1.23) following CC and ART exposure respectively. For ovarian cancer, a stronger association was observed for both exposures in nulliparous (HR 2.49, 1.30-4.78, and HR 1.62, 0.78-3.35) versus parous women (HR 1.37, 0.64-2.96, and HR 0.87, 0.33-2.27). Elevated risk of endometrial cancers was observed for CC exposure in nulliparous women (4.59, 2.68-7.84 vs. 1.44, 0.63-3.31). Risk was elevated for breast cancer in parous women exposed to CC (1.26, 1.03-1.54) and among nulliparous women after ART treatment (2.19, 1.08-4.44). Conclusion CC appears associated with increased risk of ovarian and endometrial cancer. Elevations in risks of breast and thyroid cancer were less consistent across type of drug exposure and parity. Impact Continued monitoring of fertility treatments is warranted.
- Received October 13, 2016.
- Revision received December 21, 2016.
- Accepted December 22, 2016.
- Copyright ©2017, American Association for Cancer Research.