Exposure to Mirtazapine during Pregnancy: A Study of Birth Outcomes
BackgroundMirtazapine is a new treatment for depression which is chemically unrelated to other antidepressants. Very little evidence is available on the safety of this medicine when used during pregnancy. The objective of this study was to determine whether or not mirtazapine increases the risk of major birth malformations in newborn children when used by pregnant women.
Methods
From June 2002 to August 2005 women were recruited from centres around the world which specialise in studying information about possible birth defects. These were based in Toronto, Canada; Connecticut, U.S.A.; Jerusalem, Israel; Rome, Italy; Sydney, Australia; and Southampton, United Kingdom. Pregnant women who had used mirtazapine were identified and followed up to establish the outcome of pregnancy. Comparisons were made with two other groups of pregnant women – (1) those diagnosed with depression and taking other antidepressants and (2) those who had used other medicines known not to cause birth defects. The main analysis looked at major birth malformations but we also analysed information on spontaneous and therapeutic abortions, gestational age at birth, and mean birth weights.
Results
We were able to follow-up 104 pregnancies in each of the three groups. In the mirtazapine group there were 77 live births, 1 stillbirth, 20 spontaneous abortions, 6 therapeutic abortions, and 2 major malformations. The average birth weight was 3.3 kilograms and the average gestational age at delivery was 38.9 weeks. Most (95%) of the women took mirtazapine in the first third of the pregnancy, only 25% of them took it throughout pregnancy. The rate of spontaneous abortions was higher in both the mirtazapine (19%) and other antidepressant (17%) groups than in users of drugs known not to cause birth defects (11%) but such differences could have occurred purely by chance. The rate of pre-term birth (before 37 weeks’ gestation) was higher in the mirtazapine group (10%) and in the other antidepressant group (7%) than in users of drugs known not to cause birth defects (2%). This difference between mirtazapine and users of drugs known not to cause birth defects was unlikely to have occurred by chance.
Conclusion
Mirtazapine does not appear to increase the baseline rate of major birth malformations since this was expected to be between 1% and 3%. Our findings were consistent with an increase in spontaneous abortions in women taking antidepressants which has been found in previous studies.