Trends in haemoglobin levels from 1968 to 2017 and association with hormonal contraceptives: observations from the population study of women in Gothenburg, Sweden

AIM: To investigate trends in the haemoglobin (Hb) level in middle-aged Swedish women from 1968 to 2017 and to examine the potential association between Hb and the use of hormonal contraceptives (HCs). DESIGN: A prospective observational population study of representative 38- and 50-year-old women of Gothenburg, Sweden. SETTING: The population study of women in Gothenburg started in 1968–1969 and has continued since then with new examinations every 12 years, including both follow-ups and new recruited cohorts. The study consists of both physical examinations and questionnaires. SUBJECTS: Two thousand four hundred eighty-eight women aged 38 and 50 participated in the study from 1968 to 2017. STATISTICAL METHODS: Linear regression model analyses were used to analyse linear and non-linear trends in the level of Hb. Linear and logistic regression models were used to analyse possible associations between HC and Hb and possible associations between the use of HC and anaemia, respectively. MAIN OUTCOME MEASURES AND COVARIATES: Hb was measured in g/L. HC included any ongoing use of HC therapy. Covariates were smoking, body mass index (BMI), alcohol consumption and education. RESULTS: A non-linear U-shaped trend in mean Hb was seen in the two age groups, 38- and 50-years old. After adjusting for covariates, a significantly higher mean Hb was seen in the 2016–2017 examination compared to 1980–1981, 1992–1993 and 2004–2005. In 38-year-olds, using HC was associated with a reduced risk of anaemia (OR 0.35, 95% CI 0.13–0.75). In both age groups, the use of HC was significantly associated with having a higher Hb. CONCLUSIONS: Mean levels of Hb in middle-aged women of the general population seem to be increasing again after lower levels in the 1980s and 1990s. The use of HC was associated with having a higher Hb and a lower risk of anaemia in 38-year-old women.