Hyperemesis gravidarum is a severe form of nausea and vomiting that occurs during pregnancy. The connection between psychiatric morbidity, especially depression, and hyperemesis gravidarum has been debated from contradictory findings. Therefore, we aimed to evaluate the associations between hyperemesis gravidarum and both pre-pregnancy and new-onset post-pregnancy depression.

We conducted a nationwide register-based controlled study in Finland between Jan 1, 2004, and Dec 31, 2017. Data were collected from 2004 to assess at least 1 year of pre-pregnancy depression as deliveries were assessed from 2005 onward. Data on hyperemesis gravidarum and psychiatric diagnoses were obtained from the Finnish Hospital Discharge Register, and delivery data from the Finnish Medical Birth Register. All women with at least one pregnancy resulting in delivery with livebirth during the study period were included. Abortions (ie, spontaneous and induced), ectopic pregnancies, and stillbirths were excluded from the analyses. Women with hyperemesis gravidarum (ICD-10 diagnosis codes O21, O21.0, O21.1, O21.2, O21.8, and O21.9) in their first pregnancy resulting in delivery from Jan 1, 2005, to Dec 31, 2017, were chosen as cases, and women with no hyperemesis gravidarum as controls. The primary outcome was depression (ICD-10 diagnosis codes F32, F33, and F34.1), retrieved from the registers from Jan 1, 2004, to Dec 31, 2017. Associations between depression and hyperemesis gravidarum were analysed using binary logistic regression, adjusted for age, BMI, socioeconomic status, smoking, and psychiatric diagnoses other than depression. People with related lived experience were involved in the study design.

A total of 437 465 women had pregnancies resulting in delivery between 2005 and 2017, of whom 130 537 were excluded, 4265 were included in the hyperemesis gravidarum group, and 302 663 in the non-hyperemesis gravidarum group as only women whose first pregnancy resulted in a livebirth were included. 377 (8·8%) of 4265 in the hyperemesis gravidarum group and 2874 (1·0%) of 302 663 in the non-hyperemesis gravidarum group had a pre-pregnancy depression diagnosis. The mean age in the hyperemesis gravidarum group was 26·6 years (SD 5·2, range 15·0–46·0) and in the non-hyperemesis gravidarum group 27·9 years (5·3, 13·0–55·0). Ethnicity data were not available. Women in the hyperemesis gravidarum group were more likely to have been diagnosed with pre-pregnancy depression compared with those in the non-hyperemesis gravidarum group (adjusted odds ratio [AOR] 5·2, 95% CI 4·3–6·3; p<0·0001). 210 (4·9%) women in the hyperemesis gravidarum group and 2901 (1·0%) in the non-hyperemesis gravidarum group had a new-onset depression diagnosis after pregnancy (AOR 3·6, 95% CI 3·0–4·4; p<0·0001). Depression was diagnosed more than a year after delivery in most women (170 [81·0%] in the hyperemesis gravidarum group, 2496 [86·0%] in the non-hyperemesis gravidarum group), but women in the hyperemesis gravidarum group were diagnosed earlier (3·3 vs 4·5 years; p<0·0002).

The bidirectional association between hyperemesis gravidarum and depression suggests shared underlying biological mechanisms and highlights the long-term mental health burden associated with hyperemesis gravidarum. This finding emphasises the importance of systematic psychiatric screening during and after pregnancy, as well as the assessment of lifetime depression history in reproductive care. Improved collaboration between psychiatry, gynaecology, and primary care is essential to enhance patient outcomes.