The news story could have emphasised that this study did not look at blood pressure during pregnancy (when blood pressure usually rises), but instead looked at it before and after pregnancy. Although it does have some strengths such as following the women up over many years and assessing them before and after birth, it is limited by the fact that it excluded women who either already had high blood pressure or developed high blood pressure in pregnancy.

It is also not clear whether the reduction in blood pressure recorded was large enough to have an effect on the women’s risk of events such as heart attacks or strokes.

Where did the story come from? Dr Erica P Gunderson and colleagues from Kaiser Permanente and other institutes in the US carried out this research. The work was funded by the National Heart, Lung and Blood Institute and the National Institute of Diabetes, Digestive, and Kidney Diseases.

The study was published in the peer-reviewed medical journal, Obstetrics & Gynecology.

What kind of scientific study was this? This cohort study investigated whether having a baby results in a lasting change in women’s blood pressure. Previous research has conflicting findings, with some studies suggesting that women with children have lower blood pressure, while others have not. Most of these studies were cross-sectional and so could not determine whether women who eventually had children already had lower blood pressure even before pregnancy. The researchers wanted to rectify this by measuring blood pressure in women before and after pregnancy, and comparing them with women who had not given birth.

The researchers used the data from a previous study called the Coronary Artery Risk Development In Young Adults (CARDIA) study, which recruited over 5,000 participants aged 18 to 30 years in 1985, and followed them until 2006. From that study, the researchers took the data on female participants who did not have high blood pressure or have a history of it (systolic blood pressure of 140mmHg or more, or diastolic blood pressure of 90mmHg or more). The women were also required to not have had a hysterectomy or their ovaries removed, to not be taking blood pressure medication, heart medication or asthma medication when they enrolled, and to have provided full blood pressure information at the beginning of the study and during follow-up.

The researchers identified 2,304 eligible women, 1,531 of whom did not have children at the start of the study and 773 who did. The women’s blood pressure was measured three times when they enrolled, with measurements one minute apart. The second and third readings were averaged and this blood pressure was recorded.

Blood pressure was measured at 2, 5, 7, 10, 15 and 20 years during the follow-up. Women were asked whether they were using or had used high blood pressure medication at these points, as well as details of any pregnancies since the last assessment, including whether they had high blood pressure during these pregnancies. Women were grouped into those who had one birth during follow-up, those who had two or more and those who had not given birth, as well as those who had high blood pressure or pre-eclampsia in pregnancy and those who did not.

The researchers then compared changes in blood pressure over the study period in these different groups of women. They used a method that took into account factors that might affect the results, such as whether the women already had children at the start of the study, their age, race and blood pressure at the start of the study, body mass index, use of oral contraceptives, whether they smoked, use of blood pressure medication during follow up, and weight gain over follow-up.

What were the results of the study? There were 1,373 women who did not have any children when the study began, and who did not have high blood pressure in any subsequent pregnancies. By the end of the study, the women in this group who had a baby had lower blood pressure than those who did not (about 2mmHg reduction in systolic blood pressure [SBP] and 1.5 mmHg in diastolic blood pressure [DBP]).

Having two or more babies did not seem to reduce blood pressure any further. Reduction in blood pressure was greater among white women than among black women.

Among the 707 women who had children at the start of the study and who did not have high blood pressure in any subsequent pregnancies, there was no difference in blood pressure between those who had a baby in the study period and those who did not. This supported the observation that blood pressure does not seem to reduce after a first pregnancy any further.

What interpretations did the researchers draw from these results? The researchers concluded that a woman’s first birth produces a persistent lowering of her blood pressure over the following years.

What does the NHS Knowledge Service make of this study? This study did have some strengths in that it collected data both before and after the women had been pregnant.  However, there are some limitations to note:

The study did not measure women’s blood pressure during pregnancy, only before and after. One aspect that the news story could have emphasised more is that blood pressure usually rises during pregnancy and this study does not provide evidence that the opposite is true, but only looked at blood pressure before and after pregnancy. Although most women were demonstrated to have a reduction in blood pressure after becoming mothers, the effects during pregnancy have not been clarified by this study.Although blood pressure was not measured during pregnancy, the researchers did ask women if they had been diagnosed with high blood pressure whilst pregnant (called gestational hypertension), and excluded those women who reported that they had. It is unclear what effect including these women in the analyses would have had on the results, but it may have reduced the differences between the women who gave birth and those who didn’t.The study excluded women who already had high blood pressure. It therefore provides no evidence that women who already have high blood pressure will experience any benefit to their blood pressure from having a baby.As with all studies of this type, there may be differences between the groups of women that were compared other than the factor of interest (in this case having a baby) that contribute to the differences seen. Although the researchers tried to take some of these factors (confounders) into account these adjustments may not fully remove their effect, and cannot remove the effect of unknown or unmeasured factors. In particular, the long intervals between assessments may have made it difficult to accurately assess and take into account any confounders that occurred in these periods.Use of high blood pressure medication relied on women’s reports only, and was not checked with their doctors.The reduction in blood pressure was only minimal and what effects this may or may not have on the risk of future cardiovascular events such as heart attack or stroke is unclear and has not been examined.Overall, this study indicates that after their first birth there may be some reduction in blood pressure in women who do not already have high blood pressure.

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