A study was published in the British Journal of Obstetrics and Gynaecology yesterday that led to headlines in many of the national newspapers. The message was that perhaps up to 25% of miscarriages could be avoided. The reference for the paper is: Risk factors for miscarriage from a prevention perspective: a nationwide follow-up study. Feodor Nilsson S, Andersen P, Strandberg-Larsen K, Nybo Andersen AM. BJOG. 2014 Feb 19. doi: 10.1111/1471-0528.12694.
This study from Denmark was based on telephone interviews, and at first glance seems huge with over 91,000 patients included and so appears to offer convincing evidence. The authors identified associations between the following risk factors and an increased miscarriage risk: age of 30 years or more at conception, being underweight, and obesity prior to pregnancy. During pregnancy the modifiable risk factors were: alcohol consumption, lifting of >20 kg daily, and night work. Maternal age at conception and alcohol consumption were the most important risk factors. The authors hypothesise that 25.2% of the miscarriages in their study might have been prevented by reduction of all the risk factors to low risk levels.
Now much of this is not new. We know that increased age is a risk factor for miscarriage, and it is no surprise that alcohol may be linked to an increased risk of miscarriage. The same authors have also published before on a possible association between working habits and lifting with miscarriage risk.
However before anyone becomes alarmed it is important to be clear that this study does not establish a causal relationship between the risk factors it has identified and miscarriage. The study identifies potential candidate risk factors that may be associated with miscarriages, it does not demonstrate causation, and does not show any intervention or alteration in behaviour actually reduces the number of miscarriages.
When looking at this study two issues jump out. First is the issue of association rather than causation. As the authors note, “we cannot tell from this observational study whether the associations are causal”. The second issue is that the results may be sensitive to recall bias (because participants were interviewed after their pregnancy ended in a miscarriage). A final potential problem with the study is that the majority of their participants were recruited “after the gestational age where miscarriage is most common”. It is difficult to know how this might impact on results.
So what are we supposed to think? Well – first thing is someone needs to carry out a prospective study looking at these risk factors in pregnancy. I imagine the Danish group may well already be doing this. There is a great deal of discussion already in society about age and pregnancy outcome as well as fertility. Most couples know this is an issue and this information will probably add anxiety rather than tell people anything they do not already know. The majority of women in my experience stop drinking as soon as they know they are pregnant – but inevitably many or indeed most do not know they are pregnant until after they have missed a period. This information will make people more cautious if they are planning a pregnancy about alcohol intake in the later stage of their cycle. Being very under or over weight is another high profile societal issue. Again this information will add to the argument that either end of the spectrum is not healthy.
The real areas of controversy relate to night work and lifting which are rather out of left field in terms of miscarriage risk – and clearly if causation were proven – could have a significant impact on policy and the workplace. We would then have to ask at what stage of pregnancy these factors may play a role. If very early – any women trying to conceive could argue that night shifts or lifting are putting them at risk. For the moment the study does not put forward sufficient evidence to know one way or the other. However it is clear that work needs to be carried out in this specific area to know if causation exists.