It’s all relative. Or is it? Have you ever had someone tell you that the risk triples if you do Option 1 instead of Option 2? Or maybe you’ve heard the risk increases by 200% if you choose Option 2. These are all ways to communicate relative risk. (Curious about what risk even is? Read our article about What is Risk?)
Relative risk tells you how much a risk changes. Relative risk does not actually tell you what the risk is. The problem with relative risk is that, if you’re comparing two very small risks, the difference between them may appear very large. For example, if the absolute risk of Option 1 is 0.002, and the absolute risk of Option 2 is 0.006, the risk triples between Option 1 and Option 2. It is also mathematically true to say that the risk between Option 1 and Option 2 increases by 200%. However, when you look at the absolute risk of Option 2, it is only 0.004 greater. Is that a lot? That’s actually up to you to decide. Remember, you can’t make a truly informed decision unless you know the absolute risk and not just the relative risk.
Here’s a real-life example to show how this works: The risk of stillbirth at 39 weeks gestation is 0.000335 (0.335 out of 1,000). The risk of stillbirth at 42 weeks gestation is 0.00108 (1.08 out of 1,000). 0.00108 is approximately 3 x 0.000335. You could say that the risk of stillbirth triples at 42 weeks, and technically that is true. You could also say the risk of stillbirth increases 200%, which is also technically true. However, the absolute risk is still very small. Maybe an increase of 0.000745 is enough to make you want to have your baby no later than 39 weeks. Maybe it’s not.[1]
Another thing that can help you when comparing two different risks is to see a picture that illustrates the risks. For example, look at the graphic below. Each dot represents a birth. There are 1,000 dots. The green dots represent live births, and the red dot represents a stillbirth. The first graph represents the risk of stillbirth at 39 weeks. In this image, one of the dots is only partially filled in because only 0.335 births out of 1,000 result in a stillbirth at 39 weeks. The second graph represents the risk of stillbirth at 42 weeks gestation. In this image, only one dot is red because only 1.008 births out of 1,000 results in a stillbirth. When you can visualize the difference in these two risks, how do you feel about the risks of going to 42 weeks gestation?
One thing is for sure, hearing that your risk of stillbirth triples is really scary. Hearing that your risk increases by 0.000745 isn’t near as scary. Before you make a decision in pregnancy or birth, make sure you find out what the absolute risks are, not the relative risks. (To find out what other things you should consider when making decisions, sign up for our free Decision Making Guide.)
To find out more about due dates, how stillbirth rates are calculated, and the studies that have been done regarding going past your due date, check out the Evidence Based Birth article on due dates.
[1] About this reference: This data comes from Risk of stillbirth and infant death stratified by gestational age, Rosenstein, MG et al, Obstet Gynecol. 2012 Jul;120(1):76-82. This is the largest study ever done on stillbirth rates and includes data from the U.S. between 1997 and 2006. Stillbirth numbers can vary dramatically from study to study based on how stillbirth is defined and the parameters of the study.