Abstract
The WHO aims to reduce iron deficiency anaemia globally. Use of modern hormonal contraception (HC) could offer protection against anaemia in premenopausal women, but population-level effectiveness is unclear. We aim to quantify the effect of HC on anaemia. This nested case-control study includes over half the fertile-aged female population of Finland in 2017. Data on HC use from the national Prescription Center were combined with data on anaemia diagnosis from national care registries. Cases (anaemia diagnosis during follow-up, 2019-2020) were matched with up to five controls by age and municipality. After calculating the minimally sufficient adjustment set, adjusted odds ratios were derived in a conditional multivariable regression model accounting for matching. 3 100 cases of anaemia were matched with 13 143 controls. The minimally sufficient adjustment set included age, education level, obesity, abnormal uterine bleeding, and cancer. Compared to non-users of HC, risk of anaemia was lower in users of combined oral contraceptives containing ethinylestradiol (0·74 [0·66-0·83]) or oestradiol (0·49 [0·35-0·68]), progestin-only oral contraceptives (0·42 [0·35-0·51]), LNG-IUDs (0·64 [0·43-0·94]), and contraceptive vaginal rings (0·68 [0·49-0·94]). Individual product effects ranged from 0·77 [0·66-0·90] for drospirenone and ethinylestradiol to 0·40 [0·32-0·48] for desogestrel-only. Benefits of HC use extend to anaemia protection on population level. Anaemia protection should be included in guidelines on HC to support clinical decision making.