Friday March 2 2012
Being born even slightly early carried greater risk of illness
“Babies born just a few weeks early have a higher risk of poor health,” The Guardian reported today. According to the newspaper, new research has found that being born just a few weeks early can raise their risk of conditions such as asthma.
It is already known that babies born prematurely (before 37 weeks of pregnancy) may have a higher risk of immediate or longer-term health problems, and the earlier a baby is born, the higher the risk. To examine the issue, researchers followed over 14,000 children born between 2000 and 2002, and assessed their health at the ages of three and five years old. Outcomes including growth, hospital admissions, use of medication, asthma and long-standing illnesses were looked at particularly in relation to whether the children were moderately premature (32-36 weeks of pregnancy) or born at what the researchers called “early” full term (37-38 weeks). Babies born moderately prematurely or at early term were more likely to have been re-admitted to hospital in the first few months of life than babies born at 39-41 weeks. Babies born moderately prematurely also had a higher risk of asthma symptoms than full-term babies.
These findings are broadly in line with what is already known about the effects of prematurity, and do not change the UK’s current definition of full-term pregnancy as 37 weeks and over. However, the study does show how different degrees of prematurity may affect health. Further study of the issue would be valuable, to explore longer-term health outcomes that may be caused by prematurity and the factors that may influence the likelihood of these poor health outcomes.
Where did the story come from?
The study was carried out by researchers from the University of Leicester and other UK institutions. It was funded by the Bupa Foundation and published in the peer-reviewed British Medical Journal.
The media generally covered this research in a balanced way.
What kind of research was this?
In the UK, the normal length of a pregnancy is classed as 37 weeks or above. It is already known that babies born prematurely (before 37 weeks) may be at increased risk of immediate and longer-term health problems, and that the risks are higher the earlier a baby is born. However, the authors say that there has been minimal research into the longer-term health outcomes of infants specifically born moderately preterm (which this study defines as 32-36 weeks) and at what the researchers termed as "early full term" (37-38 weeks).
To investigate this, the researchers used a cohort study. This is a good way to follow up and compare health outcomes in groups of people that have been exposed to different factors. In this case, the exposure was the number of weeks of pregnancy at which the babies were born. However, a cohort study that looks at a group’s health relies on the accuracy of reported health outcomes and diagnoses. For example, one condition this study looked at was asthma, and the researchers asked parents about whether their child had wheezing symptom or asthma. However, this does not necessarily equate to a medical diagnosis of asthma.
This type of study also needs to take into account potential factors that could be related to both risk of prematurity and risk of the health outcome (confounding factors). For example, parental smoking is linked to an increase risk of prematurity, and also to an increased risk of asthma in the child.
What did the research involve?
This study involved participants of the Millennium Cohort Study (MCS), a piece of research in which the subjects were gathered by random sampling of child benefit registers. It featured 18,818 infants born in the UK between 2000 and 2002. The number of weeks of pregnancy at birth was calculated from the mother’s report of her expected due date. Births were grouped into:
- very preterm (defined by the authors as 23-31 weeks)
- moderate preterm (32-33 weeks)
- late preterm (34-36 weeks)
- early term (37-38 weeks)
- full term (39-41 weeks)
These are not the standard accepted definitions. For example, the charity BLISS, for “babies born too soon”, defines full-term pregnancy as 37 weeks or more, moderately premature as 35-37 weeks, very premature as 29-34 weeks, and extremely premature as birth before 29 weeks.
Child health outcomes were monitored over five years of follow-up. Outcomes assessed included:
- child height, weight and body mass index at three and five years
- parental reports of the number of hospital admissions (not related to accidents) since birth or the previous interview, collected at nine months and at three and five years.
- parental reports of any longstanding illness or disability of more than three months’ duration and diagnosed by a health professional, collected at three and five years (a limiting longstanding illness was defined as one which limited activities that are normal for the child’s age group)
- parental reports of wheezing within the previous 12 months, and parental reports of asthma collected at three and five years
- parental reports of the use of prescribed drugs, collected at five years
- parents’ ratings of child health, defined as excellent, very good, good, fair or poor, collected at five years
The researchers used statistical methods to look at the outcomes in groups born at different stages of pregnancy and compared them to (their definition of) full-term babies. Analyses were adjusted to account for various potential confounding factors, principally numerous social and demographic factors. The researchers also estimated “population attributable fractions” (PAFs) associated with preterm and early term birth. This is an estimate of the contribution that a particular risk factor has to a health outcome. PAF represents the reduction in the proportion of people in the population with a particular health problem that could be expected if the exposure to a risk factor were reduced to the ideal exposure. In this case, it would represent the proportion of children that would no longer have a particular health problem if all babies were born at full term rather than preterm.
What were the basic results?
After the researchers excluded participants in the MCS study with incomplete data on time in the womb at birth, they interviewed the parents of 14,273 children at 3 years of age and 14,056 at 5 years. They found certain sociodemographic factors, such as lower maternal educational status and maternal smoking, to be associated with prematurity, as is already known.
The researchers generally found a “dose response” effect of prematurity, meaning that the more premature a baby was, the higher the likelihood of general health problems, hospital admissions and longstanding illnesses. They calculated the odds of each outcome compared to children born at 39-41 weeks. The full details of these outcomes are as follows:
The odds for three or more hospital admissions by five years of age were:
- 6.0 times higher for children born at 23-31 weeks
- 3.0 times higher for children born at 32-33 weeks
- 1.9 times higher for children born at 34-36 weeks
- 1.4 times higher for children born at 37-38 weeks
The odds for any longstanding illness at five years of age were:
- 2.4 times higher for children born at 23-31 weeks
- 2.0 times higher for children born at 32-33 weeks
- 1.5 times higher for children born at 34-36 weeks
- 1.1 times higher for children born at 37-38 weeks
The odds for the child’s health being rated as only fair or poor by parents at five years of age were:
- 2.3 times higher for children born at 23-31 weeks
- 2.8 times higher for children born at 32-33 weeks
- 1.5 times higher for children born at 34-36 weeks
- 1.3 times higher for children born at 37-38 weeks
The odds for asthma and wheezing at five years of age were:
- 2.9 times higher for children born at 23-31 weeks
- 1.7 times higher for children born at 32-33 weeks
- 1.5 times higher for children born at 34-36 weeks
- 1.2 times higher for children born at 37-38 weeks
The greatest contribution to the burden of disease at three and five years was among children born at late/moderate preterm or early term. The calculated PAFs for being admitted to hospital at least three times between the ages of 9 months and 5 years were:
- 5.7% for children born at 32-36 weeks (i.e. you would expect a 5.7% reduction in the number of young children admitted three or more times if babies were born at full term rather than moderate preterm)
- 7.2% for children born at 37-38 weeks (you would expect a 7.2% reduction in the number of young children being admitted if babies were born at full term rather than early term)
- 3.8% for children born before 37 weeks (you would expect a 3.8% reduction in the number of young children being admitted if babies were born at full term rather than very preterm)
Similarly, PAFs for longstanding illnesses were:
- 5.4% for early term births
- 5.4% for moderate or late preterm births
- 2.7% for very preterm births
How did the researchers interpret the results?
The researchers concluded that “the health outcomes of moderate/late preterm and early term babies are worse than those of full term babies.” They say that it would be useful for further research to look into how much of the effect is due to prematurity itself, and how much is due to other factors such as maternal or foetal complications.
This valuable research examined childhood health outcomes in a large group of children born at different stages of pregnancy.
Important points to consider when interpreting this research include:
- The authors generally found that the likelihood of poorer health outcomes was higher with increasing prematurity (a dose response effect). This is in line with what is already known about the generally poor immediate and longer-term health outcomes among babies born increasingly prematurely.
- The greatest contribution to overall burden of disease at ages three and five years was calculated to be among children born at 32-36 weeks or at 37-38 weeks. Though a gestation of less than 32 weeks might be expected to have a greater influence on the burden of disease, it must be remembered that many more babies are born above 32 weeks of gestation than below it. Therefore, in the population as a whole, the greater number of babies born within the 32-38 week range would have a greater effect than the small number of babies born extremely early.
- The definitions that the authors used for the purposes of this study are not standard definitions. For example, the standard definition of full-term pregnancy is birth at 37 weeks or more, and it is not split into “early term” at 37-38 weeks and “full term” only at 39-41 weeks. Similarly, definitions of prematurity differ from those used by other UK health organisations.
- There is a possibility of inaccuracy as both age at birth and health outcomes were reported by parents, rather than assessed through medical records. For example, a parental report of wheezing or asthma does not necessarily constitute a confirmed medical diagnosis of asthma.
Overall, the study found that the more premature a baby is, the greater the likelihood of health problems in childhood, and that some effect of prematurity may even be seen in pregnancies approaching full term. Further study in this area would be valuable, both to explore the wider range of longer-term health outcomes that may be caused by prematurity, and to look into associated factors (medical or sociodemographic, for example) that may influence the likelihood of these outcomes.
Analysis by Bazian