Thursday October 4 2012
High blood pressure affects around 10%-15% of pregnancies
Being born to a mother who had high blood pressure during pregnancy can have a lifelong effect on your intelligence, claims the Daily Mail.
The story is based on data collected from a group of 398 Swedish men born in the 1930s and 40s. It found that men born to women who had high blood pressure during pregnancy scored a few points less on cognitive ability tests conducted both around the age of 20 and when they were in their late 60s, compared to men born to women who had normal blood pressure during pregnancy.
High blood pressure in pregnancy is relatively common, affecting around 10-15% of women. It can be pre-existing or arise as a complication of pregnancy.
It is important to stress that the decrease in intelligence detected was very small – an average of 4.36 points as calculated by standardised IQ testing.
So even if the difference is directly linked to maternal blood pressure (which cannot be proven by this study) any adverse effects are likley to be minimal. It might take slighty longer for an affected person to complete a Sudoku puzzle, but is unlikely to have a significant impact on a person’s life chances.
Other limitations to the study include:
- it relied on health records that were over 60 years old which could have included inaccuracies
- the standard of care for pregnant women with high blood pressure is likely to be far higher today than it was in the 1930s and 40s
- the study sample was relatively small and included male offspring only
Overall, women who suffer from high blood pressure during pregnancy should not be overly concerned that their blood pressure is going to have any effect at all upon their child’s intelligence.
Where did the story come from?
The study was carried out by researchers from the University of Helsinki and several other Finnish institutions; and the University of Southampton. There is no information about external funding.
The study was published in the peer-reviewed journal: Neurology.
Neither BBC News nor the Daily Mail reported any mention of the study’s many limitations, or any comments on it from independent experts. The Mail also illustrated its story with a photo of an elderly woman, although the study’s findings are limited to men.
However, both the Mail and BBC News did point out that the difference in IQ scores was relatively low.
What kind of research was this?
This was a cohort study which looked at whether high blood pressure in pregnancy was associated with later changes in cognitive ability in offspring, up to old age. This type of study enables researchers to follow large groups of people for many years. It can be useful for looking at the relationship between early life events (such as fetal exposure to high blood pressure in the mother) and later health outcomes, but it cannot prove direct cause and effect.
The researchers say that high blood pressure (either affecting the mother in the long term or only occurring during pregnancy), including the complication of pre-eclampsia (where high blood pressure is associated with fluid retention and protein in the urine), affects about 10%-15% of all pregnancies.
This, in turn, can lead to complications affecting the pregnancy such as reduced blood flow to the placenta which may be associated with lower cognitive ability in later life.
They report that a previous study found that men born after pregnancies complicated by high blood pressure, scored lower on cognitive ability tests at the average age of 20 years.
In this study, they tested what effect high blood pressure in pregnancy has on the cognitive ability of the male offspring, and whether these effects persist into old age, which the researchers defined as being aged 65 or over.
What did the research involve?
The researchers used data from a large national study in Finland called the Helsinki Birth Cohort Study which initially included 13,345 participants (both men and women) born between 1934 and 1944.
A subsample of 931 men from this group underwent testing for their cognitive abilities twice – first at an average age of 20.1 and then, on average, 47.7 years later.
For their study, the researchers included 398 men from this subsample, for whom data on maternal blood pressure was also available.
The researchers used the measurements of the mothers’ blood pressure and measurements of their urinary protein (which can indicate pre-eclampsia), recorded during pregnancy in antenatal clinics or in hospital, to identify high blood pressure in pregnancy. From this data, they defined two groups of mothers – one in which the women had normal high blood pressure, and a second with women who had high blood pressure during pregnancy.
The researchers used results from a basic cognitive ability test taken by the men in both young life and older life, which included subtests of verbal, arithmetic and ‘visuospatial reasoning’. Visuospatial reasoning is the ability to make sense of visual information, such as looking at a diagram of a car brake and being able to determine the principles of how the brake works.
Using standard statistical methods they analysed the association between the measurement of maternal blood pressure during pregnancy and the men’s test scores on cognitive ability. They adjusted the results to take account of several confounders including:
- weight at birth
- mother’s age
- father’s occupation
- level of later education
- diagnoses of stroke and coronary heart disease
What were the basic results?
The researchers found that, compared with men born to women who had normal blood pressure during pregnancy, men born after pregnancies complicated by high blood pressure:
- scored 2.88 points (95% confidence interval 0.07 to 5.06) lower on total cognitive ability (overall score) when they were 20.1 years
- scored 4.36 points (95% confidence interval, 1.17 to 7.55) lower on total cognitive ability (overall score) at 68.5 years
Of the individual cognitive tests (that contributed to the total score), the association between high blood pressure in pregnancy and lower ability was strongest for arithmetic reasoning (essentially the ability ‘to do sums in your head’).
How did the researchers interpret the results?
The researchers suggest that lower cognitive ability in old age may have its origins in the antenatal period, when the development of brain structure and function occurs.
They suggest that the association between maternal high blood pressure in pregnancy and lower cognitive ability in later years could be due to various factors, including reduced blood flow to the placenta, effects on hormones that regulate glucose, inflammatory processes and genetic factors.
The study is of interest, but it does not show that high blood pressure in pregnancy causes significantly lower cognitive ability in the offspring in later life. Pregnant women today who suffer from high blood pressure should not be concerned.
There are many limitations to this research:
- The differences in scores between men born to women who had high blood pressure while pregnant and those born to women without high blood pressure was very small, both in young adult life (around 20), and later life (approaching 70): only 3-4 points difference to the total score. Whether these differences had any effect at all upon the person’s everyday life and functioning is highly questionable.
- There may be many (confounding) factors that were associated with both the mother’s risk of pregnancy complications and her child’s intelligence (such as socioeconomic factors), and while the authors adjusted for several of these factors, it is still possible that there are others that have affected the results. Likewise, there may be many other factors that have influenced the offspring’s cognitive ability over their lifetime.
- The research is relying on maternity data collected during the 1930s and 40s. Using these past hospital and clinic records meant that, as the researchers point out, they were not able to obtain from the data recordings of high maternal blood pressure on two separate occasions, which is needed to establish a diagnosis.
- Maternity care itself is also likely to be different today, and pregnant women with high blood pressure now may receive more vigilant care than they would have done in the 1930s and 40s. For example, from the past maternity records it is unknown which women were treated for high blood pressure and whether this would have had any effect and also, whether the women had any other conditions (treated or untreated) which might have affected the development of the fetus.
- Finally, the study relies on data from a relatively small sample of men only - it is unclear whether a similar pattern would be found in women, or in people of other ethnic groups.
High blood pressure in pregnancy can be a serious complication for both mother and child and needs careful management. Whether it causes significantly lower cognitive ability in later life remains open to doubt.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.