WHOOP Pregnancy Data: Trends in Resting Heart Rate, HRV, Strain, Sleep & More

An Olympian’s physiological changes during pregnancy.

When I found out last winter that I was pregnant, my immediate emotions were split between thrilled and terrified. My husband and I were excited to start a family, but I was also worried about what being pregnant, recovering from pregnancy, and ultimately what being a mom would do to my athletic performance. I have been an athlete my whole life. I rowed at Princeton University before joining the National Team, winning two World Championships, breaking a world record, and finishing 4th at the London Olympics in the Women’s Pair. Living a high performance lifestyle has always been a huge part of who I am, so I didn’t know how it would feel to share my body, and my priorities, with another human being. As I began the motherhood journey, I decided to face it like I have faced all my athletic challenges--with an adaptable yet data-driven perspective. I’ve been a WHOOP user since 2017, so I felt immense comfort knowing that I had a tool to help measure and analyze these many physiological changes and my body’s new performance. Looking back at my pregnancy, I’m very grateful I had a fairly easy pregnancy that allowed me to stay active throughout. My son Maverick arrived on August 6th, and I’ve since been loving how it feels to be a Mom! Below I’ll share some of my personal experiences, as well as how WHOOP helped me gather insights about what my body was going through at different periods of my pregnancy journey.

Heart Rate During Pregnancy

For any given athlete, a decrease in resting heart rate (RHR) is associated with an improvement in fitness. An increase in heart rate variability (HRV) predicts your readiness to take on strain (cardiovascular exertion). But as a pregnant athlete, you can expect both of these metrics to trend in the opposite direction. My resting heart rate quickly increased by over an average of 15 bpm in my first trimester, and most women see these changes within the first 2-5 weeks of pregnancy. But, my RHR also returned to normal levels just as rapidly. Within my first week postpartum, my RHR had dropped about 10 bpm. This change in heart rate occurs to support the amount of blood pumped by the heart (cardiac output), which increases by 30-50% during pregnancy to match the needs of your growing baby.

Pregnancy Resting Heart Rate Charts

My body’s ability to take on strain also trended down over time, which was displayed by a decrease in heart rate variability and daily WHOOP strain scores. The six months prior to becoming pregnant, my average HRV was 64, compared to the first six months of pregnancy when it dropped to an average of 50. As you can see in the chart below, my HRV climbed back up around my normal baseline almost immediately after giving birth (August 6).

Heart Rate Variability During Pregnancy and Afterwards

While the change to my HRV didn’t surprise me, I found the decrease in strain to be very counterintuitive--I expected a harder-working body to mean higher strain numbers. However, the opposite was true. Because my body was so focused on the task of growing another human, my ability to take on strain diminished. Before pregnancy, a day of high activity for me resulted in a strain score of at least 10 or more (WHOOP quantifies it on a 0-21 scale). During pregnancy, a day of high activity was often reflected in a WHOOP strain closer to 7:

Working Out While Pregnant: Adapting Through the Trimesters

Not very long ago ago the exercise recommendation for pregnant women was to engage in low intensity cardiovascular efforts. Many obstetricians would also suggest maintaining a heart rate of under 140 beats per minute. However, new research has suggested that pregnant women are safe to continue participating in physical activities they engaged in prior to pregnancy, and high intensity exercise can be beneficial in low risk pregnancies. Throughout my pregnancy, I chose to closely listen to what my body could handle. As my body changed, so did my level of activity and the exercises that felt comfortable.

FIRST TRIMESTER (0-12 WEEKS):

In my first trimester, my aerobic capacity noticeably plummeted. It became much harder for me to maintain a comfortably elevated heart rate, which makes sense considering my increased cardiac output and initial surge of hormones. I chose to focus on strength workouts with a lot of built in rest so that I could allow my heart rate to decrease between sets. Because my bump was still very small during this phase, there weren’t many movements I felt I needed to scale yet. There were also incredible waves of fatigue that came in my first trimester, so I began to take off days whenever my body asked for one.

SECOND TRIMESTER (13-26 WEEKS):

The second trimester was my favorite period of the pregnancy. By this time, I had a cute pregnant bump and my first trimester nausea had disappeared. My aerobic capacity also felt like it had returned a bit, making it much easier to add back in more cardiovascular efforts. This period of time also coincided with the COVID-19 quarantine, so it was necessary to get creative about workouts with limited equipment. I chose to focus on 20-40 minute workouts consisting of both cardiovascular and weighted efforts.

PREGNANCY CONING

As many pregnant women experience during this phase, I began to notice coning during some exercises. Coning is when your abdomen looks like a cone, with the peak being along the middle line of your abdomen, also known as the linea alba. Coning tends to occur when you add substantial pressure to your abdomen through certain exercises, or even just when you sit up to get out of bed. If coning isn’t avoided, it can contribute to the development of diastasis recti, or the separation of the rectus abdominis muscles. To account for this, I began avoiding all core work on my back, pull-ups, and rowing. I decreased my amount of running and jumping which contributed to pelvic pressure. Finally, I adjusted my deadlift stance from a standard position to a sumo stance to accommodate my growing belly.

THIRD TRIMESTER (27 WEEKS-DELIVERY):

In the final few months of my pregnancy, my baby had grown considerably in size. This made me a lot more uncomfortable, especially near my ribs and pelvic floor. In fact, I know women who have broken ribs from the pressure their baby places on their torso. Needless to say, it became much harder to breathe toward the end of pregnancy. For these reasons, I began adding in shorter workouts with more rest. Most of my sessions lasted about 20 minutes, and I relied on walking and biking to contribute to some of my low intensity cardiovascular efforts. I also focused on preparing my pelvic floor and core by practicing core compressions in all of my workout warmups and cool-downs. Strengthening these areas is critical to a smooth vaginal delivery and recovery. If I hadn’t been in quarantine during my third trimester, this is when I would have relied on a pelvic floor specialist for a physical therapy program. However, in this digital age, there are many fantastic online resources, like the Expecting and Empowered program.

Pregnancy Sleep: Disturbances and Dehydration

When preparing for a child, you definitely hear about the lack of sleep you can expect once the baby arrives. However, what you don’t hear about as often is the sleep disruption you can expect throughout your pregnancy. My WHOOP data displayed quite a shift in my sleep patterns, especially during the first and third trimesters:

Nausea, hormones, and discomfort as you grow in size all contribute to sleep disruptions throughout the night. But I found that dehydration also played a part in my decreasing sleep quality. As your body mass increases, it becomes harder to regulate body temperature and meet your hydration requirement needs. In fact, most pregnant women do not reach these hydration needs. My best bet became monitoring my water intake throughout the day, and moving all of my workouts indoors and into air conditioning. Pregnancy alone can lead to dehydration and sleep disturbances. A pregnancy with a third trimester during the summer makes that even harder. Then add a pregnancy during a global pandemic where masks are constantly required, and you can understand why it might be difficult to maintain hydration.

The Strain of Delivery and Comparing it to Training

I was especially excited to track the physiological effect on my body during the delivery of my baby. How would the pain translate to strain? Would I have a spike in heart rate while pushing? For all the pregnant athletes out there, you’ll be happy to hear that being physically fit absolutely prepares you for delivery. In fact, strength and fitness has been shown to contribute to smoother delivery and faster delivery times (in one study, by an average of three hours)! The delivery stage of my labor lasted 50 minutes. When asked to evaluate how this physical effort compared to other training experiences, I would describe it as repeated maximal lift efforts with an even work-to-rest ratio. Each pushing effort is relatively short (30 to 60 seconds) and then you rest until your next contraction (usually about a minute), so it felt much more like a weighted workout than a cardiovascular effort. My WHOOP strain reflected this and my heart rate never raised above 110 bpm. And yes, I logged delivery as “wrestling”:

I also chose to get an epidural, which dramatically improved my body’s ability to relax during labor and delivery. This allowed me to take several naps while I was laboring in order to store up the energy required for delivery. Even with a lot of building excitement, these naps made a huge difference in keeping me feeling refreshed.

Postpartum Workouts and Building Back Fitness

Giving birth is a physically traumatic event, and it’s incredibly important to patiently allow your body to heal. After delivery, your body has permanent structural changes, often in your hips, back, and feet. If you rush your return to movement, you are not allowing yourself the required time to re-learn proper biomechanics in your new body. So while I was excited to get back to training after delivering my son, I was surprised by how taxing walking, getting up out of a chair, and even going to the bathroom were in the first few days after delivery. Instead of attempting to begin working out right away, I used the first 10 days postpartum to enjoy the time with my new baby. After my initial healing had begun, I started integrating pelvic floor exercises and core compressions. Just as these are important during pregnancy, they are also important postpartum to help avoid incontinence, diastasis recti, and prolapse. But, the most pleasant surprise was how quickly my WHOOP numbers bounced back. Even though sleeping with a newborn was challenging, my sleep was far less disrupted by my baby than it was by my pregnant body. Without the added strain of carrying a baby, my RHR and HRV quickly rebounded, leading to a week of green recoveries:

Pregnancy Insights and Understanding Your Body

I’m now 6 weeks postpartum and starting to feel more like my old self. When I look back at my pregnancy, I love that I was able to track so much of what my body was experiencing by using my WHOOP. While not everyone is an elite athlete, and each pregnancy is different, listening to your body can really help you feel connected to the process and better understand what you are going through physiologically. For me, this helped me have patience, understanding, and love for how my body was performing. I hope this can help give you some insight into what to expect if you are lucky enough to embark on a parenthood journey yourself!   Learn More: What's a Normal Heart Rate for Women?