Question: How does the birth control pill cause thrombotic problems?
One of the rare side effects of taking the oral contraceptive pill is the development of a blood clot (thrombus) in a vein. This is now a very rare complication with the low dose pills that doctors have available.
If a blood clot does occur, it is usually in the veins in the calf. This causes considerable pain and tenderness of that part of the leg, and most patients would present to doctors with these symptoms for treatment. If the clot is not treated early, it is possible for the clot to break away from the wall of the vein, and travel up the vein, through the heart and lodge in the lung. The effect this will have depends on the size of the clot, but it will block an artery in the lung, and the part of the lung affected will then die. If this is a very small area, the patient will have no serious problem, but if a major artery is blocked by a large clot, the result may rarely be fatal.
The thrombus (clot) forms because the hormones in the pill slightly alter the clotting factors in the blood, so that a minor injury to the vein (eg. repeatedly pushing back a chair with your calf as you stand) may be enough to start a clot in the blood.
Treatment involves medication to prevent the spread of the clot or further clot formation, and women who have such a problem should not use the contraceptive pill.

Question: What is the difference between the mini-pill and other oral contraceptives?
Most oral contraceptives (known as combined pills) contain two different hormones that roughly correspond to the two types of hormone produced by your ovaries. They may be given as a stead}’ dose of hormone throughout the month, or may have two or three different levels of hormone to correspond with the natural changes in your body. After 21 days, you stop the pill or start taking sugar pills. This drop in hormone levels allows you to have a period.
The mini-pill has only one hormone in it, and this in a very low dose. It is taken constantly, with no break for periods, and must be taken at the same time every day. Periods will still occur, but they may be irregular and are usually light. It is not as reliable as the combined form of pill, and is normally only given to older women, those who cannot tolerate the combined pill, smokers, or those who are breast feeding.