The Lowdown:

Blood is pretty nifty. When we are injured it can become sticky and clump together, forming a clot to plug the wound and prevent excessive bleeding1. This is a vital process, but if clots form when they are not supposed to, the results can be very dangerous

Doctors use the blanket term venous thromboembolism to refer to blood clots that form in deep veins (deep vein thrombosis) – often in the leg or arm – and for blood clots that dislodge and travel to arteries elsewhere in the body – often the lungs (pulmonary embolism)2.

If a blood clot blocks an artery it can prevent blood reaching important organs. In worst case scenarios this can result in long term, irreversible damage, or death.

Which all sounds pretty scary.

That’s why, when we read headlines linking blood clots to the pill we take every day, it’s important to know what that risk really means.

What causes blood clots in contraception?

Most forms of contraception use the hormones oestrogen and progestin. Hormones are chemicals produced by the body to regulate different processes, and researchers have known since the 1970s that there is a relationship between increased oestrogen levels and blood clots.

While the exact mechanism isn’t known, research suggests that oestrogen reduces the body’s ability to regulate clotting, making the blood less able to stop forming clots even when it’s not the right time or place.

More specifically, oestrogen appears to dull the body’s sensitivity to an enzyme (a protein that speeds up a biochemical process) called activated protein C (APC). APC downregulates clotting, which means it acts to slow or stop the blood clotting process in response to the body’s needs. With the body less sensitive to APC, the blood is less able to stop clotting.

Research studies have shown higher levels of APC resistance in people taking the Combined Pill compared to those who are not taking the pill3,4,5.

Before we continue, let’s take a moment to talk risk.

What do we mean by increased risk?

Whenever we read about increased risk, it’s important to know what the baseline risk is. In this case, the baseline risk is the risk of developing a blood clot if you are premenopausal, not pregnant, and not taking any form of contraceptive. If we tracked 10,000 such women for a whole year, we would expect between two and six of them to develop blood clots6. Put another way, that’s less than 0.1% per year. This baseline risk is so low that increasing it several times over will still result in a very small overall risk for most people.

However, even a small risk can be a cause of concern if you have other risk factors for blood clots, such as a family history. That’s why it’s so important to talk to your GP about your medical and family history before making any decisions.

Armed with knowledge of the baseline risk, let’s look at different types of contraception.

What is the increased risk of blood clots for different contraceptive methods?

Combined oral contraceptives

Different types of Combined Pill and blood clot risk

Progestogen-only Pill and the Implant (Nexplanon)

The Patch and the Vaginal Ring

The IUS and IUD

This table summarises the risk of blood clots for every method of contraception

Statistics taken from the following studies  1) Venous thrombosis in users of non-oral hormonal contraception: follow-up study, Denmark 2001-10 & 2) The epidemiology of venous thromboembolism)

Should you be worried about blood clots on the pill?

What are the signs of a blood clot?

Find out more about symptoms and what to do if you think you have a blood clot on the NHS website: https://www.nhs.uk/conditions/blood-clots/

In summary:

References

  1. A question of clotting, www.bhf.org.uk/what-we-do/news-from-the-bhf/news-archive/2015/may/national-thrombosis-week

2. Venous thromboembolism in adults: diagnosis and management, www.nice.org.uk/guidance/qs29/chapter/Introduction-and-overview

3. Svetlana N Tchaikovski , Jan Rosing, 2010, Mechanisms of estrogen-induced venous thromboembolism www.thrombosisresearch.com/article/S0049-3848(10)00100-3/fulltext

4. Abou-Ismail MY , Sridha DC, Nayak L , 2020, Estrogen and thrombosis: A bench to bedside review, www.sciencedirect.com/science/article/abs/pii/S0049384820301808

5. Raps M, Curvers J, Helmerhorst FM, et al. 2014, Thyroid function, activated protein C resistance and the risk of venous thrombosis in users of hormonal contraceptives, www.thrombosisresearch.com/article/S0049-3848(14)00005-X/fulltext

6. Næss, I.A., Christiansen, S.C., Romundstat, P., Cannegieter, S.C., Rosendaal, F.R. and Hammerstom, J. 2007, Incidence and mortality of venous thrombosis: a population‐based study, https://onlinelibrary.wiley.com/doi/full/10.1111/j.1538-7836.2007.02450.x

7. Marcos de Bastos, Bernardine H Stegeman, Frits R Rosendaal, Astrid Van Hylckama Vlieg, Frans M Helmerhorst, Theo Stijnen, Olaf M Dekkers, 2014, Combined oral contraceptives: venous thrombosis, https://www.cochrane.org/CD010813/FERTILREG_contraceptive-pills-and-venous-thrombosis

8. Gray B, Floyd S, James AH, 2018, Contraceptive Management for Women Who Are at High Risk of Thrombosis. https://pubmed.ncbi.nlm.nih.gov/29521659/

9. Vinogradova Yana, Coupland Carol, Hippisley-Cox Julia, 2015, Use of combined oral contraceptives and risk of venous thromboembolism: nested case-control studies using the QResearch and CPRD databases, www.bmj.com/content/350/bmj.h2135 

10. NHS, Combined pills increase risks of blood clots, https://www.nhs.uk/news/medication/combined-contraceptive-pills-increase-risk-of-blood-clots/

11. Tepper NK, Whiteman MK, Marchbanks PA, James AH, Curtis KM, 2016, Progestogen-only contraception and thromboembolism: A systematic review. Contraception, www.sciencedirect.com/science/article/abs/pii/S0010782416300476 

12. Mantha S, Karp R, Raghavan V, Terrin N, Bauer K A, Zwicker J I et al, 2012, Assessing the risk of venous thromboembolic events in women taking progestogen-only contraception: a meta-analysis, www.bmj.com/content/345/bmj.e4944

13. Blanco-Molina MA, Lozano M, Cano A, Cristobal I, Pallardo LP, Lete I, 2012, Progestin-only contraception and venous thromboembolism, www.thrombosisresearch.com/article/S0049-3848(12)00090-4/fulltext 

14. Lidegaard O, Nielsen LH, Skovlund CW, Løkkegaard E, 2012, Venous thrombosis in users of non-oral hormonal contraception: follow-up study, Denmark 2001-10, www.bmj.com/content/344/bmj.e2990 

15. Tepper NK, Dragoman MV, Gaffield ME, Curtis KM, 2017, Nonoral combined hormonal contraceptives and thromboembolism: a systematic review, www.contraceptionjournal.org/article/S0010-7824(16)30455-3/fulltext 

16. NHS, Vaginal ring, www.nhs.uk/conditions/contraception/vaginal-ring/

17. NHS, Contraceptive patch, www.nhs.uk/conditions/contraception/contraceptive-patch

18. van Hylckama Vlieg A, Helmerhorst FM, Rosendaal FR, 2010, The risk of deep venous thrombosis associated with injectable depot-medroxyprogesterone acetate contraceptives or a levonorgestrel intrauterine device, www.ahajournals.org/doi/10.1161/ATVBAHA.110.211482

19. NICE, 2019, Contraception Assessment, https://cks.nice.org.uk/topics/contraception-assessment/

20. Pabinger I, Grafenhofer H, 2002, Thrombosis during pregnancy: risk factors, diagnosis and treatment, www.karger.com/Article/Abstract/73590 

21. Konkle BA, 2015, Diagnosis and management of thrombosis in pregnancy, https://onlinelibrary.wiley.com/doi/abs/10.1002/bdrc.21104 

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