How do contraceptives work - and why you may want to know.
In this series of guest blogs, Dr Grace Petkovic takes us through some of the basics of reproduction and reproductive health.
Grace is a junior doctor in Liverpool, England with an interest in reproductive endrocrinology.
How do contraceptives work?
Contraceptives are what people use when they wish to have sex, but to avoid an unwanted pregnancy. There are several different types of contraceptive.
Different contraceptives have different rates of effectiveness at preventing pregnancy.
Why would you want to know when contraceptives act?
Some women would not want to use a ‘contraceptive’ that acts after fertilisation. For ‘what is fertilisation?’ see blog the first blog in this series.
Some contraceptives may act post-fertilisation.
But if contraceptives act post-fertilisation, wouldn’t that make them abortifacients?
Abortion is when a pregnancy is intentionally ended so that it does not result in the birth of the child [1].
Remember that different people use the term pregnancy to mean different things (see the second blog in this series). Some people use it to mean when a new organism is formed, at fertilisation. Others, use it to mean when implantation completes.
The Royal College of Obstetricians and Gynaecologists define pregnancy as beginning at implantation. The College’s definition affects what language is used in the UK, to describe some drug types. In the case of drugs which could act pre-implantation, they may be advertised as being ‘contraceptives.’
ONLY drugs that act post-implantation are called “abortive,” because the RCOG defines pregnancy as beginning at implantation. A drug intended to end a pregnancy after implantation is therefore “abortive.”
But what about the period in between the fertilisation of the egg and its implantation? Some drugs may act to stop the stop development of the zygote or to stop the zygote from implanting. This means that the life of a human zygote is ended.
The language used for contraception can confuse many people who think that the word , ”contraception,” implies something that acts “against” conception by stopping fertilisation. As we have seen, this is not always the case.
Additionally, some medical professionals use the terms “fertilisation” and “conception” interchangeably. Others use the terms “conception” and “implantation” interchangeably.
For this reason, to avoid confusion, for the rest of this post we will use the term ‘birth-control’ rather than ‘contraception’.
How could birth control work?
To stop sperm getting into the vagina
pre-fertilisationTo stop the egg getting released
pre-fertilisationBy stopping the zygote/embryo implanting
post-fertilisation
So for each type of birth-control, we can ask how it works. .
Bear in mind that even if contraception could theoretically stop an embryo implanting, it may in practice never actually have this effect if its pre-fertilisation mechanisms are very effective. Just because a drug could act this way does not mean that it will act this way.
Furthermore, those who use birth control differ in their interest in the mechanisms of action. Some people may want to be 100% certain that the birth control they use, or plan to use, never stops implantation. Others would be satisfied with 70% certainty. Others may not be interested in the question.
It is important to see a doctor if you are considering birth control. They can advise you more regarding other factors such as side-effects, risks and effectiveness.
Below is a summary of how different forms of birth control may act. This data is subject to change and correction but was the author’s best attempt to summarise information as of April 2020:
Are there any other ways to avoid pregnancy that do not involve birth control?
Some couples choose not to use birth control but would still like to avoid pregnancy.
They may use a ‘fertility awareness method’.
What is the fertility awareness method?
The basic principle is that the natural rhythm of a woman’s reproductive monthly cycle means that there will be times when, following sex, she is likely to conceive, and a few days when she is almost certain not to conceive, and some days where she may be less likely to conceive.
By abstaining on certain days, a couple may be unlikely to conceive.
Do fertility-awareness methods actually work?
Yes, trials show they work, but they work differently, depending on the method used.
For example, sympto-thermal methods combine daily cervical mucus observations (to see when a women is likely to be fertile) with daily temperature measurements (to see when she has ovulated). Sympto- refers to the ‘symptom’ of ‘cervical mucus observation. ‘Thermal refers to ‘temperature’ i.e. the daily temperature measurements.
One-year-pregnancy rates with one year of perfect use of the sympto-thermal method is noted to be comparable to one year of perfect use of oral contraceptives. In one year, for every 1000 couples using the symptom-thermal method perfectly, 4 would become pregnant. For every 1000 couples using an oral contraceptive perfectly, 3 would become pregnant [13] [14].
However, it is difficult to use either method perfectly, so the number of pregnancies is likely to be higher.
Where can I go to find out more?
Your doctor should be able to provide you with more information. But if you want to do more research, some useful websites include:
A summary document of when different birth control methods act (this has provided many of the sources for this blog!)
International Federation of Gynaecology and Obstetrics (IFGO) has up to date information about female health globally
Taking Charge Of Your Fertility is a website by Toni Weschler who has written about the fertility awareness method
References
[1] https://www.bpas.org/abortion-care/considering-abortion/what-is-abortion/
[2] NHS UK - most women continue to ovulate on mirena https://www.nhs.uk/conditions/contraception/ius-intrauterine-system/
[3] May be less reliable at stopping ovulation when used for longer
[4] Remember just because a drug CAN act this way does not mean it has a chance to. See Studies on implanon. https://www.ncbi.nlm.nih.gov/pubmed/9548163 and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2778430/
[5] Depo-Provera usually profoundly suppresses ovulation although ovulation returns before ‘fertility’ (measured pregnancies) suggesting a potential for stopping embryo implantation. Given regularly, this is reassuring but if one wished to avoid effects on the embryo it would be important not to extend the period between injections.
[6] Remember just because a drug CAN act this way does not mean it has a chance to.
[7] we think in women who take them regularly they ovulate only 3% of the time. But in women who miss pills, the ovulation rate can rise to about 35%. We know that on average around 1/2 of women miss pills during a cycle. The take home message? These pills work mainly through pre-fertilisation mechanisms if used perfectly but it is difficult to use them perfectly
[8] Depends when in the cycle it is taken.
[9] When taken after LH hormone surge in cycle then egg release still occurs but the LH surge is decreased. This will result in the womb lining not having enough progesterone to allow the zygote/embryo to implant.
[10] This is an area ongoing debate. This article argues AGAINST post-fertilization effects. https://www.sciencedirect.com/science/article/abs/pii/S0010782412007500
This article argues that the science DOES suggest post-fertilization effects https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4313438/
Currently, international bodies, such as FIGO argue for main action being pre-fertilizaton effects. https://www.figo.org/sites/default/files/uploads/MOA_FINAL_2011_ENG.pdf
[11] Depends when in the cycle it is taken.
[12] Post-fertilization effects for Ella could include delaying the zygote/embryo reaching the womb in time to implant and thinning the womb lining to make the embryo unable to implant. There is also some concern that Ella could have effects on the embryo AFTER implantation. This is a controversial area that needs more discussion. See footnote 14.
[13] Trussell J. Contraceptive failure in the United States. Contraception. 2011;83(5):397–404. doi:10.1016/j.contraception.2011.01.021
[14] https://academic.oup.com/humrep/article/22/5/1310/2914315#56161869 Human Reproduction, Volume 22, Issue 5, May 2007, Pages 1310–1319,