Women with history of stillbirth at 'high risk of another'
“Women who suffer stillbirths are four times more likely to suffer the tragedy again,” the Daily Mirror reports. Researchers who have analysed previous data warn that women with a history of stillbirth should be regarded as being at high risk of another.
A stillbirth is when a baby is born dead after 24 completed weeks of pregnancy, and is more common than many people think. There are more than 3,600 stillbirths every year in the UK, and 1 in every 200 births ends in a stillbirth.
Researchers pooled the results of 13 previous studies. The results suggested that women who had had a previous stillbirth were more than four times more likely to have another, compared with women without a previous stillbirth. This risk reduced a little to just over three times more likely after potential contributory factors (confounders) were taken into account.
While the result appears reliable, there are small limitations to consider. The studies included in the review had very variable definitions of stillbirth and adjustment for confounders, resulting in a diverse group of studies being pooled.
Stillbirths happen for many different reasons and not all can be prevented. However, there are some things you can do to reduce your risk, such as stopping smoking and avoiding alcohol or drugs during pregnancy. Read more about preventing a stillbirth.
Where did the story come from?
The study was carried out by researchers from The University of Aberdeen and was also funded by The University of Aberdeen.
The study was published in the peer-reviewed British Medical Journal (BMJ). The study was published open-access, meaning it can be viewed online or downloaded as a PDF for free.
The Daily Mirror’s reporting of the story was accurate and contained some useful additional commentary from the lead author of the study.
What kind of research was this?
This was a systematic review and meta-analysis aiming to work out the risk of having repeated stillbirths.
A stillbirth is when a baby is born dead after 24 completed weeks of pregnancy.
If the baby dies before 24 completed weeks, it's known as a miscarriage or late foetal loss.
Stillbirth is more common than many people think. There are more than 3,600 stillbirths every year in the UK, and 1 in every 200 births ends in a stillbirth. 11 babies are stillborn every day in the UK, making it 15 times more common than sudden infant death syndrome – also known as cot death.
A systematic review and meta-analysis is one of the best ways to identify and summarise all the available evidence on a topic such as stillbirths. However, the conclusions of systematic reviews are only as good as the evidence that informs them.
What did the research involve?
The study team systematically searched the science literature for published and unpublished studies looking at links between stillbirth in an initial pregnancy and risk of stillbirth in a subsequent pregnancy. The results of included studies were combined in a meta-analysis.
Only cohort studies or case-control studies from high-income countries were included.
For the purposes of this review, and somewhat oddly, the researchers used a definition of stillbirth as foetal death occurring at more than 20 weeks’ gestation or a birth weight of at least 400g. This is not the standard definition in the UK, where stillbirth means a baby born dead after 24 completed weeks of pregnancy (notably the World Health Organization set the definition much later, at 28 weeks).
Two reviewers independently screened search results against pre-defined inclusion and exclusion criteria, and scored the studies for methodological quality.
Some of the meta-analysis made adjustment for confounders identified in the primary studies. Most primary studies adjusted for maternal age, smoking and socioeconomic status. Adjustment for other potential confounders, such as living with a partner or marital status, education, race or ethnicity, and interval between pregnancies, varied among the studies. Two studies adjusted for body mass index, six adjusted for pregnancy complications such as pre-eclampsia, placental abruption (when the placenta prematurely breaks away from the wall of the womb), or risk factors for preterm birth.
What were the basic results?
13 cohort studies and three case-control studies were included in the meta-analysis.
This included information on 3,412,079 women with pregnancies beyond 20 weeks. Of these, most (99.3%) had had a previous live birth and 24,541 (0.7%) a stillbirth.
A total of 14,283 stillbirths occurred in subsequent pregnancies; 606 out of 24,541 (2.5%) in women with a history of stillbirth and 13,677 out of 3,387,538 (0.4%) among women with no such history. This meant that women with a history of stillbirth were almost 4.8 times more likely to have a subsequent stillbirth, compared with women without (pooled odds ratio (OR) 4.83, 95% confidence interval (CI) 3.77 to 6.18). Meta-analyses are most effective when they pool the results of studies measuring the same thing in a similar way. However, this wasn’t the case in this meta-analysis. The studies varied a lot, so the pooled result represents a mixed bag of methods and measures, lessening its precision.
12 studies specifically examined the risk of stillbirth in second pregnancies. The pooled risk increase for this sub-analysis (OR 4.77, 95% CI 3.70 to 6.15) was very similar to the risk increase found in those with any history.
The pooled odds ratio using the confounder-adjusted effect measures from the primary studies was 3.38 (95% CI 2.61 to 4.38).
Four studies examined the risk of recurrent unexplained stillbirth. Methodological differences between these studies meant it wasn’t sensible to pool the results.
How did the researchers interpret the results?
The study team say they: “… have shown that women who experience a stillbirth in their initial pregnancy have a higher risk of stillbirth in a subsequent pregnancy. Even after adjusting for potential confounding factors, the increased risk remains. Risk of recurrent unexplained stillbirth is largely unstudied, and therefore evidence about this remains controversial.”
In considering the implications of their research, the team say: “Smoking and obesity are independently associated with an increased risk of stillbirth, and modification of these lifestyle factors may make a small, but important, reduction in the risk of recurrence. Current management of pregnancies should take account of pregnancy history and make use of pre-pregnancy counselling services.”
Conclusion
This systematic review and meta-analysis of 13 cohort studies and three case-control studies showed that women who had had a previous stillbirth were more than four times more likely to have another, compared to women without a previous stillbirth. The research team wanted to look at the combined risk associated with unexplained stillbirths, but were unable to do so due to lack of suitable evidence.
The review and associated BMJ editorial say that current guidance from the UK’s Royal College of Obstetricians and Gynaecologists recommend that women with a previous stillbirth are managed as high risk during a subsequent pregnancy. The results of this systematic review and meta-analysis seem consistent with this advice.
While the review conclusions can be considered reliable, there are a number of limitations to bear in mind. For example, the meta-analysis was limited by large variations in the definition of stillbirth and the extent of adjustments for confounding factors. This meant the pooled results were a bit of a mixed bag of studies, lessening confidence in the overall result a little. The researchers called for international standardisation of definitions of stillbirth, to help conduct more accurate research in the future.
The meta-analysis taking account of confounders produced a lower relative risk increase (OR 3.38) compared with the unadjusted result (OR 4.83), suggesting confounders were influencing the results.
The team were unable to explore the contribution of specific causes of stillbirth to risk in a subsequent pregnancy. The implication of this, as the BMJ Editorial pointed out, is that “if heightened surveillance is recommended for pregnant women with a history of stillbirth, it should be offered to all affected women, not just those with an identifiable and potentially recurring cause.”
Not all stillbirths can be prevented. However, there are some things you can do to reduce your risk.
These include:
- stopping smoking if you smoke
- avoiding alcohol and drugs during pregnancy – these can seriously affect your baby's development, as well as increasing the risk of miscarriage and stillbirth
- attending all your antenatal appointments, so that midwives can monitor the growth and wellbeing of your baby
- making sure you're a healthy weight before trying to get pregnant
- protecting yourself against infections (see causes of stillbirth) and avoiding certain foods
- reporting any tummy pain or vaginal bleeding that you have to your midwife on the same day
- being aware of your baby's movements and reporting any concerns you have to your midwife straight away
- reporting any itching to your midwife
