In the medical literature, the term “risk” is often used interchangeably with “frequency”. In other words, how often does a certain event happen? Risk can be presented in terms of percentage, rate, or incidence proportion. For example, cord prolapse occurs in approximately 0.17% of live births[1]. This is the risk expressed as a percentage. The rate of cord prolapse is 0.0017 (remember converting percentages to decimals in high school?). To express this risk as an incidence proportion, we would say cord prolapse occurs in 0.17 out of 100 live births. The proportion can also be converted to various sizes. For example, 0.17 out of 100 is equivalent to (the same as) 1.7 out of 1,000 or 17 out of 10,000.
It is important to know that, in reality, risk is a combination of both frequency and consequence. Sometimes, the consequences of an obstetric event are obvious. For example, if you are discussing the risk of stillbirth, it is understood to imply a loss of life.
However, for many obstetric events, the risk doesn’t necessarily equate to any particular consequence. For example, the risk of uterine rupture in a trial of labor after cesarean (TOLAC) for a low risk woman is less than 1%.[2] So, less than 1 out of 100 TOLACs result in a uterine rupture, but… then what? Most uterine ruptures don't carry significant long-term consequences for either mother of baby. In fact, the National Institute of Health has stated that, “There have been no reported maternal deaths due to uterine rupture.” [3] Mothers do not die from uterine ruptures, but what about their babies? Out of the less than 1% of TOLACs that result in a uterine rupture, about 6% of those result in infant death.[4] So, if you only look at the chance of uterine rupture and ignore the low rate of serious consequences, you’re missing a big part of the picture.
Another example is preterm premature rupture of membranes (PPROM). PPROM occurs in approximately 3% of pregnancies, but… then what? 3% is the frequency, but what is the consequence? PPROM increases the risk of infection, preterm labor, and infant death.[5] However, sometimes PPROM has no negative outcomes. When making decisions in pregnancy and birth, it is important to understand the risk of an initiating event (like uterine rupture or PPROM) and the risk of all the various things that could happen after that.
To come up with a number for the overall risk of a series of events, you have to multiply the individual risks of the events. For example, the risk of uterine rupture during a TOLAC is 1% (or 0.01), and of those, the risk of infant death is 6% (or 0.06). Therefore, the risk of infant death due to a uterine rupture during a TOLAC is 0.01*0.06 = 0.0006 or 0.06%. And remember, this is equivalent to (the same as) as 1 in about 1,667 or 6 in 10,000.
There are obviously many things to consider when making decisions about risk in pregnancy and birth. Learn more about those by signing up for our free Decision Making Guide!
[2] https://www.lamaze.org/Giving-Birth-with-Confidence/GBWC-Post/a-womans-guide-to-vbac-putting-uterine-rupture-into-perspective [3] https://consensus.nih.gov/2010/images/vbac/vbac_statement.pdf [4] https://vbacfacts.com/2012/04/03/confusing-fact-only-6-of-uterine-ruptures-are-catastrophic/ [5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4048519/
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