Obesity and blood clot risk

“Obese women are 2½ times more likely to suffer a potentially fatal blood clot in the lung during pregnancy”, The Sun reports. The Daily Mail said that “these women are more likely to have a sedentary lifestyle, which leads to problems with their circulation which are exacerbated when they conceive”.

The newspaper stories are based on a study of women who experienced a blood clot during pregnancy. The research found that obesity was linked to an increased likelihood of pulmonary embolism: a blood clot that travels to the lung. Although this study is relatively small, it provides useful information about the current practice of diagnosis and management of pulmonary embolism during pregnancy in the UK.

Pregnancy is recognised as a time when a woman is at increased risk of developing blood clots in the legs (DVT), which can travel to the lungs (pulmonary embolism), although the risk is small. Likewise, obesity and reduced mobility are known risk factors for this occurrence. The findings from this study that more of the cases of blood clot occurred in obese pregnant women fits in with findings from previous research.

Where did the story come from?

Dr Marian Knight carried out this research on behalf of the UK Obstetric Surveillance System (UKOSS). The research was supported by grants to individuals and to the Perinatal Epidemiology Unit by the Department for Health. It was published in the peer-reviewed medical journal: British Journal of Obstetrics and Gynaecology.

What kind of scientific study was this?

The study behind these stories is a case–control study. Women who’d had a pulmonary embolism during pregnancy between February 2005 and August 2006 (defined as the cases) were recruited through 229 UK hospitals. This provided a representative sample of all births in the UK during this time span. Pulmonary embolism was defined as a confirmed embolism through imaging, at surgery or post-mortem, or if the woman had a diagnosis of embolism from their clinician and had received anticoagulation therapy for more than a week.

The doctors who referred a case to the study also provided information about other potential risk factors, patient care and outcomes. Potential risk factors included age, ethnicity, socio-economic group, marital status, smoking status, BMI, history of thrombosis, family history of thrombosis, recent bedrest, recent long-haul travel, DVT in pregnancy, surgery in pregnancy, number of previous pregnancies and carrying twins.

To provide a control group of women for comparison, the doctors who referred the cases each identified two women who had not suffered from pulmonary embolism during their pregnancy and who had given birth immediately before the selected case. As with the cases, the clinicians provided information about risk factors for each woman.

The reports about embolisms covered the entire cohort of UK births, so the researcher was able to work out the incidence (the number of new cases over time) of pulmonary embolism in the UK. To make sure cases weren’t missed, the researchers contacted all radiology departments and asked them to report any cases of pulmonary embolism in pregnant women with their year of birth and date of diagnosis. They also checked data from the confidential enquiry into maternal and child health (CEMACH). If additional cases were identified in this way, the researcher contacted clinicians for more information on the case. In other parts of the study, the researchers described the diagnosis, prophylaxis, treatment and outcomes for the women who had pulmonary embolism.

What were the results of the study?

The researchers found that there were 143 cases of pulmonary embolism out of a total 1,132,964 pregnancies between February 2005 and August 2006. This result shows that pulmonary embolism is extremely rare, with about 1.3 occurrences per 10,000 women. 

The analysis included 141 of women with a pulmonary embolism and 259 women in the control group. Of the risk factors taken into account, only two were significantly linked to the experience of pulmonary embolism: high BMI, and higher parity (i.e. having had more than one child). Women who had a pulmonary embolism were more than 2.5 times more likely to have a BMI over 30 than women who didn’t have an embolism. They were also 5.6 times more likely to have had one previous childbirth.

For other risk factors, e.g. long haul travel, history of thrombosis, twin pregnancy and history of bedrest, the study was underpowered; in other words, there were not enough people in the groups to assess whether these affected the risk.

What interpretations did the researchers draw from these results?

The researchers conclude that the main risk factors for antenatal pulmonary embolism are having had one or more previous children and obesity. They say that even though their study is large (covering all UK births in a particular time period), there was not enough information to detect other significant differences. They say that this highlights the need for extensive, multinational studies of rare conditions such as pulmonary embolism.

What does the NHS Knowledge Service make of this study?

This large study represents, within reason, the picture of pulmonary embolism (both fatal and non-fatal) in pregnant UK women. It is subject to some weaknesses, which the researcher discusses:

• Data on cases and controls was selected through obstetricians and midwives; as such it doesn’t capture cases of non-fatal antenatal pulmonary embolism occurring in early pregnancy which led to miscarriage or termination. The researchers did try to ensure no cases were missed by contacting radiologists, so that any under-counting of cases would not substantially weaken the study.
• Some major risk factors for pulmonary embolism, e.g. thrombophilia (an increased tendency to develop blood clots in veins and arteries), may be underdiagnosed in the UK. If this is the case, results may be biased.

The most important message is that pulmonary embolism is extremely rare. It occurs in about one in every 7,700 pregnancies.

The use of a relative measure of risk, that is, saying that obese women are 2.5 times more likely to experience a pulmonary embolism, doesn’t communicate just how rare it is. The study has some weaknesses, but until larger multinational studies are conducted, it will be hard to understand fully the relationship between weight and the risk of pulmonary embolism.

Women of all weights should not be overly alarmed at the results of this study. Pulmonary embolism is extremely rare and health professionals involved in childbirth are aware of the risk factors; however, sensible eating and gentle exercise are beneficial for both mother and baby.

Sir Muir Gray adds...

Still another reason to lose weight; 3,000 extra steps a day would help.