<p>Self-reports and returned pill count as measures of adherence to the daily oral HIV preexposure prophylaxis (PrEP) regimen in clinical trials have led to misleading outcomes. There is limited adherence data for pregnant and breastfeeding women who initiated daily oral PrEP in clinical trials. In an open-label PrEP clinical trial, adherence to daily oral tenofovir disoproxil fumarate/emtricitabine (TDF/FTC) was measured using returned pill counts during pregnancy and post-delivery, up to 54 weeks. Intracellular tenofovir diphosphate (TFV-DP) was measured in stored dried blood spot (DBS) twice during pregnancy and at 6, 26, and 54 weeks post-delivery. True adherence was estimated based on TFV-DP levels corresponding to the number of doses taken per week. Of 271 women, 244 (90.0%), 116 (42.8%), and 84 (31.0%) received PrEP during pregnancy, or until 26 or 54 weeks post-delivery, respectively. Based on pill counts, 80–95% of women took &gt; 80% of the dispensed PrEP pills during pregnancy and postpartum. TFV-DP was undetectable in 13.8–32.9% of women during pregnancy, and 32.9–40.5% of women postpartum. Furthermore, only 21.6% and &lt; 10% of women had TFV-DP levels equivalent to 7 doses/week during pregnancy and postpartum, respectively. Real-time pill count data overestimated adherence to a daily oral PrEP regimen among pregnant and postpartum women. Based on TFV-DP levels, only 20% of pregnant women and 10% of postpartum women had levels suggestive of adherence greater than 80%. Lack of transparency in adherence disclosure underscores the need for real-time, objective measures of adherence to inform targeted adherence counseling.</p><p><?qj left?><?noindent??><i>Clinical Trial Number</i> CAP016 is on ClinicalTrials.gov (<a href="https://clinicaltrials.gov/ct2/show/NCT03227731">https://clinicaltrials.gov/ct2/show/NCT03227731</a>).</p>