Heart and lungs

Asthma 'linked to premature babies'

“Women with poorly managed asthma have a higher chance of giving birth early or having a small baby,” reported BBC News.

This news is based on a systematic review that combined data from 26 cohort studies looking at whether having asthma increases a pregnant woman’s risk of having complications at around the time of giving birth. The researchers specifically looked at pre-eclampsia during pregnancy, baby’s birthweight and size for gestational age, as well as timing of delivery, i.e. whether the baby was born full term or premature.
The review indicated that maternal asthma was associated with an increased risk for all of these outcomes. However, when the reviewers separately analysed five studies which clearly described that the asthma was being appropriately managed with medications, there was no longer an increased risk of prematurity in these studies. This suggests that any risks of asthma during pregnancy may be mitigated by active asthma management.

Prior research is reported to have given conflicting results over whether asthma has any effect on pregnancy outcomes. Most importantly, these findings demonstrate the importance of appropriate control of asthma symptoms. The study’s authors call for further research to determine optimal asthma management techniques for pregnant women.

Pregnant women with asthma should continue to take their asthma medications as prescribed. They should consult their doctor if they find that their symptoms are worsening during pregnancy.

Where did the story come from?

The study was carried out by researchers from the University of Newcastle and Hunter Medical Research Institute and the John Hunter Hospital in Australia, the Scripps Clinic, Kaiser Permanente Medical Center and University of California-San Diego in the US. The study was funded by the Kaiser Permanente Southern California Regional Research Committee and the Australian National Health and Medical Research Council.

The research was published in the peer-reviewed British Journal of Obstetrics and Gynaecology.

The findings were reported accurately by BBC News.

What kind of research was this?

The aim of this study was to establish whether maternal asthma is associated with an increased risk of adverse perinatal outcomes (during the weeks preceding and immediately following birth) and to determine the size of these effects.

The research involved a systematic review and meta-analysis of cohort studies, published between 1975 and 2009, which had examined this association, including maternal pre-eclampsia (high blood pressure and protein in the urine during pregnancy, which can be associated with other complications), birthweight and size for gestational age, premature labour and delivery.

A systematic review is a way of collecting as much of the research evidence on a particular question as possible. High-quality systematic reviews use rigorous methods to find, collate and assess the quality of the studies that are included.

A meta-analysis pools the findings of the included studies and analyses the data as a large set. Looking at the data in this way increases the ‘power’ (ability) of the analysis to detect an effect. The power of the analysis increases with the number of participants that are included. For instance, this systematic review included 40 publications involving 1,637,180 participants, far more than any single study could examine on its own.

What did the research involve?

The researchers conducted a literature search, and included studies for analysis if:

  • the design was a cohort study
  • the study involved a group of pregnant women who had a clear definition of asthma
  • the study compared them to a control group of pregnant women without asthma
  • the study reported at least one perinatal outcome
  • the study was carried out between 1975 and 2009

Researchers extracted data on various perinatal outcomes, and compared the risk of seeing these outcomes in women with and without asthma.

The researchers assessed the quality (risk of bias) in each of the selected studies, and analysed the data by pooling the results in several different ways.

First, researchers assessed the risk of developing each outcome in women with asthma compared to women without asthma. They then conducted a subgroup analysis, where they looked at five studies which specifically described that women with asthma were being actively managed with appropriate medications. They also looked at 10 studies where no active management was described. They then examined the risks in women who were receiving active asthma management, and looked at the risks in women who were not described as receiving active management.

What were the basic results?

The analysis included 26 studies involving 1,637,180 individuals. These studies had been reported in 33 published research papers.

The researchers found that, compared to women without asthma, women with asthma had:

  • a 54% increased risk of pre-eclampsia (RR 1.54, 95% CI 1.32–1.81)
  • a 46% increased risk of having a baby of low birthweight (less than 2500 grams) (RR 1.46, 95% CI 1.22–1.75). On average, the babies were 93 grams lighter at birth than babies of women without asthma
  • a 22% increased risk of the baby being small for gestational age (RR 1.22, 95% CI 1.14–1.31)
  • a 71% increased risk of preterm labour (contractions before 37 weeks) (RR 1.71, 95% CI 1.14–2.57)
  • a 41% increased risk of preterm delivery (birth before 37 weeks) (RR 1.41, 95% CI 1.22–1.61).

When the researchers analysed the studies separately according to degree of active asthma management, they found that in the studies where women with asthma were appropriately managed there was:

  • no significant increased risk for low birthweight (RR 1.55, 95% CI 0.69–3.46; combined results of three studies)
  • no significant increased risk for preterm labour (RR 0.96, 95% CI 0.73–1.26; combined results of five studies)
  • no significant increased risk for preterm delivery (RR 1.07, 95% CI 0.91–1.26; combined results of five studies)

The 10 studies reporting no active management had shown a significantly increased risk of these outcomes.

How did the researchers interpret the results?

The researchers conclude that pregnant women with asthma are at increased risk for a variety of perinatal conditions, including having pre-eclampsia, pre-term delivery, having a baby of low birthweight and small size for gestational age. They also conclude that active management of asthma mitigates these risks, especially that of pre-term delivery.


Prior research is reported to have found conflicting results on whether asthma has any effect on pregnancy outcomes. This study aimed to review the available literature to see whether maternal asthma is associated with an increased risk of complications around the time of late pregnancy and birth.

Performing a systematic review and meta-analysis can increase the statistical power of research on a topic, helping to detect differences in outcomes. This review was large, and its authors report high statistical power for the analyses. This may be the case, but there are still some important points to note when considering the results:

  • Cohort studies are observational, not experimental. This limits their ability to produce causative explanations. The observed increased risks do not prove that maternal asthma was the cause of these adverse perinatal outcomes. There may be confounding factors linked to both maternal asthma and the outcomes which explain the association. The researchers admit that socioeconomic status could potentially explain the observed association (lower socioeconomic status is associated with both increased incidence of asthma, and independently with increased risk of these birth outcomes). However, they also point out that the likelihood of this is limited by the fact that each of the included individual studies obtained their control group of mothers without asthma from a similar population group.
  • The risks presented in this research are relative and not absolute, i.e. they show how much of a higher risk a woman with asthma has of experiencing these outcomes compared to a woman without asthma. Absolute rates of these outcomes in each group (women with and without asthma) are presented for the individual studies, but no pooled results are presented to give the average rate of these outcomes in each group. However, these assessed perinatal outcomes are all relatively common, e.g. prematurity is not an uncommon in women with or without asthma. What this review tells us is that the risk may be slightly higher in women with asthma than without.
  • Finally, and most importantly, the researchers suggest that active asthma management can reduce much of the observed increased risk. They recommend that asthmatic women have their disease regularly monitored during pregnancy. The researchers suggest that further studies are conducted to establish optimal asthma management strategies during pregnancy.

As the authors of this review have noted, further research into optimal asthma management techniques during pregnancy is warranted. Pregnant women with asthma should continue to take their asthma medications as prescribed, and should consult their doctor if they find that their symptoms are worsening during pregnancy.

NHS Attribution