Monday July 12 2010
It is not known why teen mums have higher risks
Teenage mothers are “more likely to give birth prematurely and have underweight babies”, says The Daily Telegraph.
This news is based on research that looked at records of babies born to mothers aged between 14 and 29 in the North West of England. The study found that teenage mothers aged 14 to 17 were more likely to have preterm babies than older mothers, with the risk being greater for teenagers who had their second child before the age of 17. Teenagers’ babies were also smaller on average than those of older mothers, with first babies being on average 24g lighter and second babies being on average 80g lighter.
Associations between teenage pregnancy and the adverse outcomes of premature birth and lower birthweight have been observed for some time. However, even with the evidence from this study, the reasons for these associations are unclear and theories explaining them remain unproven. Further research is now needed to assess whether this effect is due to the physical immaturity of teenage mothers or differences in their lifestyle and diet that affect the pregnancy.
Where did the story come from?
The study was carried out by researchers from the University of Cork and the University of Manchester, and was funded by the Health Research Board of Ireland. It was published in the peer-reviewed medical journal BMC Pregnancy and Childbirth.
The research was covered accurately by The Daily Telegraph. The newspaper focused on the increased risk of preterm birth with a second teenage pregnancy, but did not report the risks of preterm birth associated with first teenage pregnancy. The newspaper is also likely to give the impression that this observation has been made for the first time when, in fact, several previous studies have also noticed this, and it is quite well known in the medical profession.
What kind of research was this?
This research was a cohort study designed to address whether babies born to teenage mothers were more likely to be born early or have a low birthweight. The researchers suggest that some previous studies have found that teenage pregnancy was associated with both an increased risk of preterm birth and low birthweight, although some other studies have found no association.
What did the research involve?
The researchers used a database generated from the Northwestern Perinatal Survey, undertaken at St Mary’s Hospital in Manchester between 2004 and 2006. From this database they found records of all children born to women aged between 14 and 29 years from their first or second pregnancies. The women were classified into three groups according to their age at the time of giving birth: 14-17 years, 18-19 years and 20-29 years of age.
Normal-term pregnancies are generally considered to last 37-40 weeks. In this study the researchers defined preterm delivery as greater than 33 weeks but less than 37 gestation weeks, and very preterm delivery was defined as between 23 and 33 weeks.
They assessed whether the infants had a normal birthweight or were small for gestational age (SGA) using individualised birthweight ratios. These ratios corrected birthweight for gestational age and took into account ethnic origin, gender of the baby, whether the baby was a first or second child and the height and weight of the mother. The babies were considered SGA if their individualised birthweight ratios were in the bottom 5%, and very SGA if they were in the bottom 3%.
They estimated the odds ratios (whether there was an association) between the age of the women and birth outcome of their children using a recognised statistical technique called ‘multiple logistic regression’. In their statistical analyses they adjusted for social deprivation (estimated using the mother’s postcode) and also for the mother’s ethnicity, BMI and whether it was the mother’s first or second child.
Additionally, from 2007 onwards the database contained information on whether mothers smoked at the time of their first antenatal visit. They looked at the data from births in 2007 to assess whether there was an association between smoking, young maternal age, preterm birth and birthweight.
What were the basic results?
There were records of 56,353 births. Of these:
- 3,636 were born to women aged between 14 and 16 years
- 7,506 were born to mothers between 18 and 19 years
- 45,211 babies were born to mothers between 20 and 29 years of age
The rates of teenage pregnancy were associated with increasing social deprivation, with more than one third of teenage mothers coming from the most socially deprived areas. There was an even stronger association between social deprivation score and having a second baby before 17 years of age. Teenage mothers were more likely to be underweight and to be of white ethnicity.
In first- or second-time mothers aged between 14 and 17 years the risk of preterm birth was increased relative to the older mothers (20-29 years). The risk was 21% greater during first births and 93% greater during second births (OR 1.21, 95% CI 1.01 to 1.45 and OR 1.93, 95% CI 1.38 to 2.69, respectively).
The risk of having a lower birthweight baby was also greater in mothers under 17 than in older mothers. The mean weight difference was 24g for a first child and 80g for a second child. However, the risk of having a small for gestational age baby was similar in old and young mothers once the researchers applied individualised birthweight ratios to their analyses. (In this study small for gestational age was defined as an individualised birth ratio within the bottom 5% of birthweights. Other studies consider it to be below the lowest 10% or weight below 2,500g at full-term.)
The researchers found that smoking did not seem to have an influence on preterm birth in young mothers, but say that the association between young maternal age and birthweight could be partly related to the confounding effect of smoking.
How did the researchers interpret the results?
The researchers suggest that there is an “association between second teenage delivery and preterm birth and birthweight independent of maternal social deprivation, ethnicity, BMI and smoking”. But they suggest that, unlike in previous studies, there was little evidence for an association between teenage pregnancy and risk of delivering a small for gestational age infant. They recommend that it is appropriate to encourage postnatal health education and the promotion of contraception for teenage mothers to “prevent a second teenage pregnancy with potentially higher risks of adverse outcomes”.
This study has provided evidence that there is an increased risk of teenage mothers having a premature baby, and that the risk further increased for teenage girls having their second child before the age of 17. However, although there are numerous theories behind these associations, this particular study did not address why this may be the case.
Some points to note:
- Although the study adjusted for social deprivation, this adjustment was based on the mother’s postcode, which may not give a true representation of the mother’s living conditions and lifestyle.
- The researchers also noted that there were some missing data on the potential confounding factors. However, the missing data seemed to be spread equally across the maternal age groups and so they suggest that they were unlikely to have affected their estimates.
- The study only had data on maternal smoking from 2007. However, much of the analysis was conducted on data gathered between 2004 and 2006, which means it may not have been fully adjusted to account for the influence of smoking.
- The researchers highlighted that maternal smoking data are often liable to miscalculation as mothers misreport their smoking status, and that many quitters are reported to resume smoking during pregnancy. It is, therefore, possible that the confounding effect of smoking in younger mothers may need further investigation. Smoking during pregnancy has been associated with both prematurity and low birthweight, so is an important confounder in a study such as this.
This study had many strengths, including the use of data from a large population and the fact that the researchers made detailed adjustments for factors influencing birthweight. Further investigation is now needed to assess whether the increased likelihood of preterm babies is due to environmental influences and the teenager’s lifestyle, or to the physical immaturity of the teenage mothers.
Overall, this study highlights the association between preterm births and maternal age, as well as the need for further research into why this is the case. This type of research might aid healthy pregnancies among younger mothers.