Newer contraceptive pills have been linked to higher risk of serious blood clots (known as venous thromboembolism or VTE), in a new study published in the British Medical Journal BMJ.

The research, by the University of Nottingham, indicates pills containing one of the newer types of progestogen hormone (drospirenone, desogestrel, gestodene, and cyproterone) are associated with an increased risk of VTE than pills containing older progestogens (levonorgestrel and norethisterone), Medical Xpress reports.

Past studies have found increased clot risks associated with combined oral contraceptives, but the new research is "an important clarifying study" that "has sufficient power to provide reliable comparative findings for different formulations of combined oral contraceptives," the investigators said.

About 9 percent of women of reproductive age worldwide use oral contraceptives. Twice as many — 18 percent — of women in developed countries use them.

The new findings were based on an analysis of prescription data from two large British general practice databases and VTE records in women aged 15-49 years. They showed women using older pills were more than twice as likely to develop VTE while those on newer contraceptives were four times as likely to suffer blood clots, compared to women who never used them.