Most birth control pills contain 2 hormones: estrogen and progestin (a synthetic form of the hormone progesterone). These hormones prevent ovulation thereby providing highly effective birth control. However, the estrogen in the pill increases the risk of blood clots. Blood clots (venous thrombosis) can cause painful leg swelling and sometime, a clot can travel to the lungs. This is called a pulmonary embolism, and if it is big enough, it can be fatal. Some clots can travel to the brain, causing a stroke.
If 100,000 women take a pill with estrogen for 1 year, approximately 10 will develop a blood clot. The risk of blood clots in pregnancy is also elevated, in fact for every 1,000 pregnancies approximately 2-3 women will have a blood clot (the risk of a blood clot increases 3-5 times during pregnancy compared with not being pregnant). So while the risk of blood clots with the pill exist, that risk needs to be balanced against the risks of blood clots during pregnancy and the desire not to be pregnant.
However, some pills have an even greater risk of blood clots. These higher risk pills contain the progestin drospirenone (Yasmin and Yaz are the two pills in the US that contain drospirenone). For several years doctors have suspected a possible increased risk of clots with drospirenone pills, and two large studies published in April in the BMJ have confirmed those suspicions. These are large, well-done studies looking at a lot of women and controlling for other factors that can increase the risk of clots.
What is boils down to is the risk of a blood clot with a drospirenone pill are 2.4-3.3 times higher than a pill containing levonorgesterel (another type of progestin).
Advertising for Yaz and Yasmin targets women concerned about weight gain, acne, and PMS, but there are no studies to say these drospirenone pills are superior to the pills with a lower risk of blood clots. The makers of Yaz/Yasmin were simply smart in designing their studies so they could target that demographic with their direct-to-consumer advertising. All pills help acne and PMS. Birth control pills do not cause weight gain.
Bottom line? It is always best to take the medication with the lowest risk. If you are an otherwise healthy women looking to start the pill, there is no medical reason to say Yaz/Yasmin is better and some convincing evidence to say it is slightly riskier than other options.