Obese women are more than two and half times more likely to develop a potentially fatal blood clot in the lung while giving birth, according to new research.
The findings follow a study of the UK's 229 consultant-led maternity units.
The study used data from the UK Obstetric Surveillance System (UKOSS) on more than a million pregnancies over a 19-month period from February 2005 to August 2006.
All together, 143 women suffered a pulmonary embolism, representing an estimated incidence of 1.3 per 10,000 pregnancies.
Of these, 70% of women had one or more "classic" risk factors for the condition, as set out in UK guidelines, such as a history of blood clots, being 35 or over, recent bedrest, five or more previous births, obesity, recent long-haul travel, surgery during pregnancy, or known abnormal blood clotting.
But 30% of the women who suffered a clot had no known risk factors, and only 18% of women affected had one or more, suggesting that not all women receive appropriate treatment, the research found.
Nine women who had a pulmonary embolism should have received thromboprophylaxis as set out in UK guidelines, but only three did.