Heart Rate in pregnancy: Why the "keep it under 150" advice stinks

Ever heard the old advice "just keep your heart rate under 150 and you'll be fine!" as a reference for exercising while pregnant? This is old, antiquated advice with tons of evidence as to why it's no longer relevant, but STILL, we're hearing women get this advice at their OB. (And this doesn't make them a bad OB - they may be amazing at delivering babies, but still dole out this frustrating piece of advice.) 

On a basic level, here's why that advice stinks. Heart rate is personal, to start. Athletes tend to have a lower resting HR than others, but there are some people who naturally "run higher" even as athletes. Our heart rate *zones* are also highly personal. My threshhold is on the higher end from what I know anecdotally, and there are friends of mine who are better/stronger/faster athletes than me who have lower threshholds than me. There are also friends of mine who are better/stronger/faster who run even higher than I do. Your body's own efficiency is unique. 

Pregnancy complicates this. 

In pregnancy, on average, a healthy woman with an average sized fetus will have an increase in blood volume of just under 50%. Wow. Yes. But hold that thought.  

In pregnancy, one of the first signs a lot of women have is a jump in HR. This is because your circulatory system is expanding to accommodate that blood increase, but your blood volume has yet to catch up. (This happens somewhere in the range of 16-20 weeks, initially.) Now, while you may have a "false jump" because of this - don't push it off - you're probably breathing heavily too and struggling to maintain the effort. This is where RPE is QUEEN. 

Focus on keeping your effort level at a "fast conversational" pace at the max. Think - going on a run and chatting with your friend the entire time, not needing to take any breaks to catch your breath. This is full sentence pace we're talking - now, that sentence can be 4-5 words, but you need to be a bit lighter than yes or no answer territory. Still, I PROMISE, you are getting a good workout. Just because you're not gasping for air doesn't mean you're not getting quality work done. (this should be a benchmark, generally, but that's another post.) 

Once the blood volume evens out, you may notice some HR "trends" and this is where the gray area of this space can be navigated. If it makes you feel better to keep wearing your HR/you're genuinely interested and not obsessed with the data, then keep it on, and you may see a trend in what your HR goes to when you regularly hit that 7/10 effort level. Don't live by this, but if you like having that point of reference, go for it! But if you’re someone who will obsess over the numbers and do too much googling, please, leave that HR monitor aside for the rest of your pregnancy.

 All this said, it'll take a while for your HR to settle back in postpartum. People seem to think a magic wand is waved once that baby comes out and you just go back to normal. But normal is new, now - and your heart rate may not just go right back.  Treat this time like you would a reverse taper (but much, much more conservatively) and take cues from your heart rate when it starts to level out.

In general, keep going off RPE well into that postpartum phase until you get into some real consistency and a point where it makes sense to track metrics again. And when that time comes - take a LTHR test on the run and bike to see what your new zones are as you start to get back into zone based work. And then follow it - again, treating this time more like medical recovery than a break means respecting your body and its cues!