It's Hot and I'm Pregnant. What is Safe to Do in This Heat?



Benefits of exercise while pregnant have been proven time and again. Wait, you don’t know the benefits of exercising while pregnant? Well here is a list coming at ya! Benefits include: improved blood glucose; decreased total gestational weight gain; decreased severity of depressive symptoms; decreased lumbopelvic pain; decreased risk of pre-eclampsia, decreased risk of gestational diabetes and hypertension; decreased need for caesarean section; and decreased prevalence of urinary incontinence. That all sounds great right!? However, an overwhelming majority of pregnant women still do not meet the minimum recommended exercise guidelines. And one major barrier to exercise while pregnant is concern for overheating, especially now that the hotter months are upon us. Unfortunately, like a lot of misconceptions surrounding pregnancy, many women are told that going for a walk during a hot day or taking a hot bath to relax that aching back could spike their core temperature and potentially harm their baby. So these women, who are already wonderful mothers and only want the best for their babies, avoid these things that are completely safe and could actually be helping them! So what is the real deal here?


A systematic review published in 2019 looked at limits for exercise and heat stress during pregnancy and was able to give specific recommendations. Finally! Real numbers based on real evidence on how to handle the heat while pregnant. This systematic review looked at 12 different studies that focused on heat stress (either active or passive stress) and its effect on the mother’s core temperature. The critical threshold for core temperature for increased teratogenic risk is 102.2° Fahrenheit and in all 12 of these studies, women were monitored to avoid reaching this threshold. And thanks to COVID-19 many of us now have thermometers handy to quickly check and make sure we are not reaching that threshold during our activities.


Let’s dive in a little, shall we? The principal result of this article is that pregnant women can safely exercise for up to 35 minutes at 80%-90% of their maximum heart rate in 25° C and 45% relative humidity. Okay so let’s digest that statement by using an example. Let’s take a 35 y/o female who is 24 weeks pregnant. We can find her max heart rate by a simple equation (220-Age). So a 35 year old woman would have a max HR of 185 bpm; 80-90% of that is roughly 145-165 bpm. And she can consistently exercise at that intensity for 35 minutes on a 77° Fahrenheit day (yeah, I don’t speak Celsius either) with humidity at 45% or less without reaching a dangerous core temperature. Also 80-90% of our max HR is usually only achieved during vigorous activity, not moderate cardiovascular activity such as walking, jogging, hiking, rowing, swimming, etc. For swimmers, the recommendation is even a little more lax. As long as the water is 92° or less, this same woman can exercise for a longer 45 minute bout at 145-165 bpm (which is a pretty darn vigorous swim). Now on those hotter or more humid days, we would adjust these numbers. So say it is 90° with 75% humidity, maybe she exercises only for 15-20 minute at 60-70% max HR. Then she goes inside, cools down, checks her heart rate, and if she is feeling up to it, can resume another bout of exercise.


Benefits of exercise while pregnant have been proven time and again. Wait, you don’t know the benefits of exercising while pregnant? Well here is a list coming at ya! Benefits include: improved blood glucose; decreased total gestational weight gain; decreased severity of depressive symptoms; decreased lumbopelvic pain; decreased risk of pre-eclampsia, decreased risk of gestational diabetes and hypertension; decreased need for caesarean section; and decreased prevalence of urinary incontinence. That all sounds great right!? However, an overwhelming majority of pregnant women still do not meet the minimum recommended exercise guidelines. And one major barrier to exercise while pregnant is concern for overheating, especially now that the hotter months are upon us. Unfortunately, like a lot of misconceptions surrounding pregnancy, many women are told that going for a walk during a hot day or taking a hot bath to relax that aching back could spike their core temperature and potentially harm their baby. So these women, who are already wonderful mothers and only want the best for their babies, avoid these things that are completely safe and could actually be helping them! So what is the real deal here?


So if you are a mom-to-be and you are worried about exercising on these hot days, just make sure you have a reliable way to track your heart rate and if you want to be extra careful, pack a thermometer into your SPI belt during your jog. Also, remember, no amount of heart-rate tracking and temperature monitoring is a substitute for proper hydration. So please, drink, drink, drink that water!


Shannon Hall, PT, DPT

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