While serious risks are extremely rare in healthy women taking the Pill, it is important to note the following: Thrombosis is a rare but very serious complication which occurs when blood clots form in major blood vessels. This can occur spontaneously, after an operation or accident, on the COCP, during and after pregnancy. Warning signs of a thrombosis are severe sudden chest pain, shortness of breath, severe pain or swelling in one leg, sudden blurred vision or loss of sight, or sudden severe headache. If you have any of these symptoms contact your doctor or go to your nearest emergency department immediately.

It is important to understand how big the risk is for you. The information below shows the risk in women in different situations, from women who are not pregnant and not taking the pill (two women in every 10,000 in one year) to the highest level of risk, for women after they have had a baby (200 times more than the lowest level, and 60 times more than the lowest risk level for a woman on the pill).

For every 10,000 women in one year, thrombosis occurs in:

  • 2 women not using the Pill/ring and who are not pregnant
  • 6-12 women using the vaginal ring
  • 29 women during pregnancy
  • 300-400 women soon after having a baby

For the COCP, the risk of thrombosis varies with the type of progesterone:

  • 5-7 women using 1st generation pills (the pills that have been in use for the longest time – containing levonorgestrel or norethisterone
  • 9-12 women using other pills

If blood clots are a specific concern for you, visit your doctor who can assess your particular risk based on your personal and family health history, as there are specific blood disorders that might increase the risk of thrombosis.