Babies are twice as likely to be born bottom first if either or both parents were themselves born in that position, reports The Times . A study suggests that “there are genetic factors, passed on by fathers and mothers, that create a predisposition to breech birth”, the newspaper adds. Breech births, which can occur in one in 20 deliveries, carry greater health risks for babies than the usual head-first position.
The report is based on a large, reliable Norwegian study that seems applicable to pregnancy care in the UK. Despite the suggestion of a genetic trait for breech births, some of the results indicate an environmental factor or interaction. This means that there are a number of factors that could explain the position babies adopt for birth. On the whole, women should not be too concerned about the possibility of a breech baby, as long as they are receiving proper antenatal care, which should include enquiries about both parents’ birth history.
Where did the story come from?
Irene Nordtveit and colleagues from the University of Bergen, the Norwegian Institute of Public Health and the University Hospital of Bergen in Norway carried out this research. The study was funded by the Norwegian Medical Research Council. It was published in the peer-reviewed British Medical Journal .
What kind of scientific study was this?
This was a population-based cohort study which looked at data on all 2.2 million babies born in Norway between 1967 and 2004. In Norway, everyone has a national identification number. Midwives complete a standard notification form for all births occurring after four months of pregnancy. The form includes background data on both parents, including their own national numbers as well as details of the mother’s health before and during pregnancy, and any complications and procedures during delivery.
The researchers matched the babies who were delivered in the breech position with the birth details of both mothers and fathers using the national identification numbers. They collected over 450,000 mother/baby pairs and almost 300,000 father/baby pairs for further assessment. They excluded all twins and multiple births and restricted their analysis to first-born babies only. This left 232,704 mother/baby pairs and 154,851 father/baby pairs to analyse in depth. All the mothers and fathers were born during 1967–86. In the second generation, more than 98% of the offspring were born during 1987–2004.
In their analysis, the researchers compared the number of breech births in the second generation between parents who were themselves delivered in a breech position and parents who had been delivered in the usual head-first position. They adjusted the results for other factors that they thought would modify the effect, such as gestational age, birth order, type of delivery (caesarean or natural), birth weight by gestational age, period of birth (one of four year blocks), mother’s age and education. They also stratified the results, that is, they reported the chances in groups by presentation at birth (breech or not) of the mother and father, but also grouped by the mode of delivery (vaginal, emergency or non-emergency caesarean section) and gestational age of the baby.
What were the results of the study?
Mothers and fathers who were breech babies had more than twice the risk of a breech delivery in their own first pregnancies compared with men and women who had been not been breech babies. The difference was statistically significant and similar for fathers and mothers (odds ratios 2.2, i.e. just over double the risk). When the results were stratified by the type of delivery, the researchers found that the strongest risks of recurrence were for vaginally delivered, full-term offspring. For this group, there was just over three times the risk of breech delivery when either the mother or father had been breech themselves. However, the increased risk was less (odds ratio 1.5) for parents who delivered before 37 weeks of pregnancy (prematurely) and for those born by planned caesarean section (odds ratio 1.2).
The authors also calculated the “attributable risk” for the babies. This statistic measures the proportion of breach babies in parents who were breech births minus the proportion in those parents who were not. It can be interpreted as the reduction in breech rate that could be achieved if the influence of the parent was removed. The researchers found that 3% of the cases of breech delivery were attributable to breech delivery in the father and 3% were attributable to breech delivery in the mother. Therefore, 6% of the breech deliveries in the population were accounted for by parental influence. This leaves a large proportion of breech deliveries accounted for by other factors, for example, the environment.
What interpretations did the researchers draw from these results?
The researchers conclude that the two-fold increase in breech babies associated with a father’s breech delivery was as strong as the recurrence associated with a mother’s breech delivery. As a result, they infer that “foetal genes from either the mother or the father are strongly related to breech delivery in the next generation”. They go on to suggest that “men, delivered in breech presentation, seem to carry genes predisposing to breech delivery that are then transferred to their offspring, increasing their partner’s risk of breech deliveries.”
What does the NHS Knowledge Service make of this study?
was a very large registry-based cohort study, which made use of mandatory reporting over 37 years. This reduces the possibility that the babies that took part in the study were selected unevenly. Also, the size of this study has ensured that there were enough breech deliveries to allow analysis of the data by predetermined sub-groups. These factors both add to the reliability of the results.
Norway has very similar midwifery and obstetric practices to the United Kingdom, though these are not described in detail in this paper. It is likely that these results apply to the UK.
The authors mention an unexplained finding whereby the overall rates of breech delivery increased over the 37 years of the study from 2.5% in the first generation to 3–4% in the offspring generation. This may be due to differences in reporting practices and is unlikely to have affected the overall conclusions to the study.
The “attributable risk” calculation that 6% of the breech deliveries in the population were accounted for by parental influence leaves a large proportion of breech deliveries accounted for by other environmental factors.
The somewhat surprising conclusion that fathers contribute to the risk of having a breech baby through their genes reinforces the need for health professionals to ask about both the mother’s and father’s own births during antenatal care. In theory, this could mean that some undiagnosed breech deliveries might be avoided; however, in practice, it is unknown to what extent such enquiries will help.