BJOG Release: Further risks highlighted for obese pregnant women

Blood clotting in the lungs is the most important cause of death directly related to pregnancy in women in the UK today, and obese women were found to have an increased risk of 165% compared with non-obese women.

The study was conducted by the National Perinatal Epidemiology Unit (NPEU), in collaboration with doctors and midwives throughout the UK, using information collected from the UK Obstetric Surveillance System (UKOSS). The study addressed management, outcomes and the extent to which preventive and management guidelines are followed in the United Kingdom. All 229 consultant-led maternity units throughout the UK took part over a 19 month period from February 2005 until August 2006.

During this time there were a total of 143 antenatal pulmonary embolisms in an estimated 1 132 964 maternities, giving an incidence rate of 1.3 per 10 000 maternities or 1 per 7700 maternities. Of these cases, there were five maternal deaths.

The study suggested that 62% of pulmonary embolism cases could be attributed to a number of births of one or more, and 21% to obesity (a BMI of 30kg/m2 or greater).

However, while 70% of women had a documented “classical” risk factor (as listed in current UK guidelines), such as a history of blood clots, age of 35 or older, recent bedrest, five or more previous births, obesity (BMI ³ 30 kg/m2), recent long haul travel, surgery during pregnancy, or known thrombophilia (abnormal blood clotting) worryingly, nearly one-third of women had no classical risk factors at all, and only 18% had more than one. This meant that only nine women with classical risk factors were eligible for preventive treatment under current guidelines. Preventive treatment (usually subcutaneous injections of low molecular weight heparin) has been shown in large studies to be remarkably safe, and so in the light of these results, there may be scope to revisit national guidelines, in particular whether the threshold for preventive treatment should be lower.

The study also found a need for further work on guideline implementation. Only two of the nine women with an antenatal pulmonary embolism received the appropriate dose of preventive medication as recommended in the guidelines. Half the women who received antenatal treatment were given lower than the recommended dosage.

Dr Marian Knight, Senior Clinical Research Fellow and Honorary Consultant in Public Health at the National Perinatal Epidemiology Unit, University of Oxford, said “Pulmonary embolism remains an important cause of death in pregnant women in the UK.

“This national study shows that for every woman who dies, more than thirty have a blood clot but survive. Overweight and obese pregnant women were at particular risk, with a more than two and a half times increased risk compared with women of normal weight. This is an especially important observation given the increasing prevalence of obesity in our population.

“Both women and their doctors and midwives should be aware of the risk factors for blood clotting in order that preventive measures can be undertaken where appropriate.”

Professor Philip Steer, BJOG editor in chief, said “Antenatal pulmonary embolism is a rare but very serious pregnancy complication and this study has produced some striking results. One of the most interesting is the relatively weak association between pulmonary embolism and many of the classical risk factors for the condition, alerting clinicians to remain ever vigilant when assessing the women in their care.

“Because of the rarity of the condition and thus fairly small numbers of women affected, further multi-national studies are required to ascertain the need for guidelines to be updated regarding diagnosis and treatment.”

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  • BJOG: An International Journal of Obstetrics and Gynaecology is owned by the Royal College of Obstetricians and Gynaecologists (RCOG) but is editorially independent and published monthly by Blackwell Publishing. The journal features original, peer-reviewed, high-quality medical research in all areas of obstetrics and gynaecology worldwide. Please quote ‘ BJOG' or ‘ BJOG: An International Journal of Obstetrics and Gynaecology ' when referring to the journal.
  • The National Perinatal Epidemiology Unit (NPEU) is a research unit at Oxford University established in 1978 by the Department of Health. The unit has expanded considerably in recent years and now has well over 40 staff including epidemiologists, obstetricians, midwives, nurses, paediatricians, social scientists, and information specialists. The NPEU is funded by the Department of Health in England. The views expressed in this paper are those of the authors and do not necessarily reflect the views of the Department of Health.
  • UKOSS is a joint research initiative between the NPEU and the Royal College of Obstetricians and Gynaecologists, in collaboration with the Royal College of Midwives, the Obstetric Anaesthetists Association, the National Childbirth Trust, the Faculty of Public Health of the Royal College of Physicians, and the Confidential Enquiry into Maternal and Child Health, and is supported by the Department of Health.
  • UKOSS is a UK-wide obstetric surveillance system to describe the epidemiology of a variety of uncommon disorders of pregnancy. The aim is to use this system to lessen the burden on reporting clinicians of multiple requests for information from different sources.
  • The recent report “Saving Mothers Lives: Reviewing maternal deaths to make motherhood safer - 2003-2005” from the Confidential Enquiry into Maternal and Child Health showed that 27% of women who died from causes directly or indirectly related to pregnancy were obese.


Knight M on behalf of UKOSS. Antenatal pulmonary embolism: risk factors, management and outcomes. BJOG 2008; DOI: 10.1111/j.1471-0528.2007.01622.x. Please include a link to the paper in your online article:

Date published: 20/02/2008
Published by: Website Manager
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