Dose response relationship between lithium serum levels during pregnancy and birth outcomes.

INTRODUCTION: Lithium use during pregnancy reduces the risk of mood episodes in the perinatal period for women with bipolar disorder. Some previous studies found deleterious effects of intrauterine lithium exposure on birth outcomes, yet little is known about a dose response relationship. The current study investigated the influence of maternal lithium serum levels on birth outcomes.

METHODS: This retrospective observational cohort study included women with a bipolar spectrum disorder who were referred to a specialized psychiatric and obstetric outpatient clinic from 2003 to 2019 and used lithium during the entire pregnancy. For 101 pregnancies at least one lithium level during pregnancy was available. A weighted average lithium level was calculated for the entire pregnancy, as well as for each trimester. Detailed information on maternal, obstetric and neonatal outcomes were retrieved from the medical records. Linear and logistic regression models were used to investigate the association between weighted average lithium level and pregnancy duration, birth weight percentiles, preterm birth and large for gestational age births (LGA). In subsequent exploratory analyses, we studied the role of thyroid-stimulating hormone (TSH) and thyroxine (T4) as a mediator in the found associations.

RESULTS: The weighted average lithium serum level during pregnancy was negatively associated with pregnancy duration and positively with preterm birth, but not with birth weight percentile or LGA. In exploratory analyses, TSH and T4 did not mediate the association between average lithium serum level and pregnancy duration.

CONCLUSION: The results of this cohort study during pregnancy indicate a dose response relationship between maternal lithium serum levels and pregnancy duration.