It is usually caused by an excessively thick endometrium (uterine lining) or by hormonal effects of ovulation.
Though breakthrough bleeding can happen at any time it usually occurs when a woman starts taking oral contraceptives or changes between two types of contraceptive. It Is the most common side effect of oral contraceptives. Breakthrough bleeding typically lasts for two or three cycles.
Smoking affects breakthrough bleeding in a number of ways, smokers are more likely to experience it and it tends to last for much longer than with non-smokers.
The bleeding is caused by a change in the level of oestrogen in the body. Oestrogen is a hormone which controls the development and shedding of the endometrium, contraceptive pills contain oestrogen and so when women first use they pill they have an increased level of oestrogen.
For many the solution is to change to another type of contraceptive pill or to take a non hormonal contraceptive, but it is always advised to see a doctor before changing your contraceptive pill.
Non-hormonal contraceptives refer to any contraceptives which do not rely on a pill. The most common type of non-hormonal contraceptive is the condom which is a thin rubber sheath placed over the penis before sex and acts a barrier to the sperm. Condoms are the most common non-hormonal contraception as they protect against STIs as well as pregnancy.