Selective serotonin reuptake inhibitors and venlafaxine in early pregnancy and risk of birth defects: population based cohort study and sibling designBMJ 2015; 350 doi: https://doi.org/10.1136/bmj.h1798 (Published 17 April 2015) Cite this as: BMJ 2015;350:h1798
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Re: Selective serotonin reuptake inhibitors and venlafaxine in early pregnancy and risk of birth defects: population based cohort study and sibling design
I presented this article at a local journal club and it generated some interesting discussions.
A major component of this was how I had tried to explain the results. One way to make cohort study results easier to understand is to express the results as number needed to harm. So, first I constructed a 2x2 table:
Birth defect No defect
Not exposed C
• RR is probability of event occurring/total number of events possible
• = EER/CER
• = (a/a+b)
• = (1357/36,772)/ (71,374/2266875) = 1.18
• Absolute risk reduction or increase as well as number needed to treat/harm (NNT/NNH)
• ARR = CER – EER
= 0.0369 – 0.0314
= -0.0055 (negative value = increased risk, therefore risk of harm)
NNH = 1/ARR
So 181 mothers need to take SSRI/Venlafaxine for 1 additional birth defect to occur
Our critical appraisal discussion concluded that overall there were several strengths of the study:
- It was a large, relatively well conducted study
- Mandatory health registers takes away several biases
- Very detailed study with lots of analyses based on previous evidence – but hard to understand in places
Some limitations in the study included:
- Lack of consideration for some confounding factors, including depression diagnosis alone
- Sibling analyses lacked statistical power
The overall results indicate that from this study, there was a very small absolute risk (NNH 181) of birth defects from taking SSRIs or Venlafaxine during pregnancy and overall a small relative increase compared to the control group, which some people found very reassuring whilst others considered it a risk too high when the stakes are as important as birth defects.
Competing interests: No competing interests