Pregnant women who are admitted to the hospital for reasons other than delivery are at an increased risk of serious blood clots, according to new research published by the BMJ.

Venous thromboembolism (VTE) is one of the leading causes of maternal death in developed countries, affecting 1-2 pregnancies in every 1,000. The new study suggests that risks are higher for women aged 35 and over who are hospitalized for 3 days or more, especially if they are in late pregnancy.

According to the Centers for Disease Control and Prevention (CDC) VTE is a blanket term covering deep vein thrombosis and pulmonary embolism. Sufferers may have one form or both, and an estimated 300,000 to 600,000 people are affected in the US alone.

The CDC calculates that between 60,000 and 100,000 Americans will die from VTE each year but does not record figures for pregnant women.

According to March of Dimes, pregnant women are more at risk of VTE because normal hormonal changes affect the blood clotting to limit blood loss during delivery and protect against miscarriage. These hormones are present as early as the first trimester.

Researchers from the University of Nottingham and Guy’s and St. Thomas’ Foundation Trust in London, both in the UK, conducted a cohort study of 206,785 women aged between 15 and 44 who had one or more pregnancies between 1997 and 2010.

They found that hospitalization during pregnancy carried a 17.5-fold increased risk of developing VTE. The study showed that the risk of VTE was also high in the 28 days following discharge – recording a six-fold increased risk.

The researchers hope the study will influence how pharmacological thromboprophylaxis is administered. They call for “prudent consideration of all pregnant women” in the 28 days following discharge from hospital.

They also found that the risk of VTE increased four-fold for women whose hospital stay was less than 3 days, adding prophylaxis “might also require careful consideration.”

Anticoagulants, such as heparin, are traditionally used to prevent blood clots, but they carry an increased risk of major hemorrhage. The authors call for additional research, such as a risk-benefit analysis, as hemorrhage is believed to occur in 1% of pregnant women.

Heparin also increases the risk of some potentially dangerous changes to the blood, as well as bone loss.

The authors acknowlege that diagnozing VTE during pregnancy can be difficult, as leg swelling and calf pain – symptoms of the condition – are also common in women without the condition.

The study concludes:

The overall risk of first venous thromboembolism in pregnant women increased during admissions to hospital not related to delivery and remained significantly higher in the 28 days after discharge. During these periods, need for thromboprophylaxis should receive careful consideration.”