Following Institutional Review Board approval (#1912819563) at West Virginia University, a total of 38 women were enrolled from March 2019 until August 2020. Inclusion criteria included women ages 18–35 years old, who were currently physically active at least three or more times per week. For inclusion, women also had to not be currently pregnant, but hoping to conceive within the next 6 months. Recruitment occurred primarily through the Reproductive Endocrinology and Infertility clinic at West Virginia University (WVU). Data were collected between March 2019 and July 2021.
Participants were given a WHOOP® Strap 2.0 (WHOOP, Inc., Boston, MA, USA) and asked to wear it continuously on their non-dominant arm from enrollment and throughout pregnancy, delivery, and postpartum. The WHOOP® devices were purchased with internal research funds through the WVU Department of Obstetrics and Gynecology. This strap transmitted continuous data to the participants smartphone and to a WHOOP cloud platform. Participants were able to see their daily physical activity information on their phones. The comprehensive data from all participants was then downloaded from the WHOOP cloud platform for analysis.
A monthly survey was sent via email asking participants about their continued participation as well as any changes in exercise (e.g. “How many days per week do you typically exercise?”), medical history (e.g. “Do you have any medical conditions?” and “Will these medical conditions affect your ability to exercise while pregnant?”), pregnancy history (e.g. “How many times have you been pregnant?”), and pregnancy status (e.g. “When do you plan to start attempting conception?”). This survey allowed for participant retention, feedback, and to gauge overall health and fitness. The monthly response rate ranged from 89 to 100%, indicating excellent retention. Demographics were gathered post-delivery about number of prior pregnancies and live births, medical pregnancy complications (e.g., gestational hypertension), medical delivery complications (e.g., postpartum hemorrhage), maternal age at delivery, weeks at delivery, infant birth weight, and method of delivery (e.g., vaginal or c-section). Although BMI was not gathered in the surveys, all participants were below a BMI of 30 to be eligible for fertility treatment.
Patient and public involvement
Throughout the process of this study and the writing of the manuscript, patient involvement was continuously solicited. The monthly surveys not only helped to maintain connection with patients, but allowed patients to provide feedback on their pregnancy, comfort and feasibility of wearing the WHOOP strap, and any barriers or facilitators they faced in exercising. Public involvement was included through the inclusion of a registered nurse, a psychologist, and those with obstetric and public health expertise. Many of the authors have lived personal experience with pregnancy, lending public voice to this piece.
Data were imported from WHOOP® in three different tables: 1) daily HRV and RHR; 2) recorded activity per participant, including time in each heart rate zone; and 3) a daily output of strain.
Cardiovascular fitness was assessed with two daily measures: HRV and RHR. Daily HRV & RHR was measured by the WHOOP® strap using reflectance photoplethysmography . RHR was measured in beats per minute (bpm) and HRV was measured in milliseconds (ms). HRV is calculated by the root-mean-square difference of successive heartbeat intervals . Improved cardiovascular fitness is indicated by higher HRV scores and lower RHR scores.
Recorded Daily Activity was measured by the WHOOP® strap by a three-axis accelerometer and processed using a proprietary algorithm to create daily activity records.
Time Spent in Heart Rate Zones was calculated by the time individuals spent in any of the six heart rate zones: Zone 0 = 0–50% heart rate reserve (HRR); zone 1 (50–60% HRR); zone 2 (60–70% HRR); zone 3 (70–80% HRR); zone 4 (80–90% HRR); and zone 5 (90–100% HRR). These zones were measured automatically by the WHOOP® strap during exercise . This maximum heart rate zone was calculated at WHOOP® strap set up based on age, sex, and anthropometric measures entered by the participant.
Daily strain was measured using a proprietary formula and provided by the WHOOP® strap. Strain is a summary metric of the cardiovascular load, or the level of strain training takes on the cardiovascular system based on calories burned, average heart rate, and max heart rate over the course of the day. Strain is scored on a scale from 0 to 21, with higher scores indicating more strenuous activity during the day that puts stress on the body.
Recorded activities were merged into a day-by-day measure of total time by day spent in each activity, and the three tables were merged by user ID and date. Daily minutes spent in a recorded activity in any zone were included as daily activity minutes, and daily minutes of zone 3 heart rate and higher were converted into a daily moderate/vigorous minutes variable. These tables were connected to the women’s pertinent pregnancy dates, including conception and delivery date. After calculating time to date variables, these dates were stripped from the data and not otherwise utilized.
Data analyses were conducted in SAS 9.4 . Descriptive statistics are reported as frequencies and valid percentages of categorical variables, and mean, standard deviation, minimum and maximum values for continuous variables. Data were summarized by participant and by week for some descriptive analysis. Linear mixed models were used to model the longitudinal data. After assumptions were checked and found satisfactory, a variety of models were tested, including random intercept, random slope, both random intercept and slope, along with continuous (e.g., day to delivery) and categorical (e.g., trimester) time effects. Time was restricted to 43 weeks prior to birth and 8 weeks post-partum for the models, as most participants had data for this time-period. Splines were fitted for non-linear patterns with the continuous time fixed effects. The best fitting models were selected via lowest Akaike information criterion (AIC). Kuder-Richardson degree of freedom correction was used for all models. All available data was used via Restricted Maximum Likelihood (REML) method. The best fitting model included a random intercept and random continuous time slope (days to delivery), with a variance components covariance matrix and two splines set at different points for the two outcomes. A series of three models are presented for each of the two outcomes: Model 1: days until delivery only; Model 2: moderators for total activity minutes per day; Model 3: moderators for moderate/vigorous activity minutes per day. Fixed effects estimates along with standard errors, df, t-value and p-value are presented for each model.
Sensitivity analysis was conducted to determine effects of specific demographic covariates on the final HRV and RHR models. Specifically, the effects of maternal age at delivery, prior live births, medical pregnancy complications, medical birth complications, and method of delivery (C-section v. vaginal) were added as covariates into the models. No significant effects were found for the covariates (all p > 0.10) and no differences were noted for the other models effects (including time, activity, and time by activity interaction terms). Thus, simplified models without covariate inclusion are presented.