Interpretation of the BBT chart: using the "Gap" technique compared to the Coverline technique

OBJECTIVE: The aim of the study was to compare the accuracy of the Gap and Coverline techniques of interpreting the basal body temperature chart. METHODS: We compared the proportion of menstrual cycles for which the Gap and Coverline techniques accurately identified the post-ovulatory final fertile phase (FFP) and the initial infertile phase (IIP) and the median number of days each overestimated the fertile period, using urinary LH testing as the gold standard. RESULTS: The Gap and Coverline techniques identified the FFP within +/-1 day of that identified by LH testing in 13/33 (39%) and 10/33 (30%) cycles (chi2=0.6; p=.44), respectively, and the IIP within +1 day of that of LH testing in 13/33 (55%) and 4/33 (12%) cycles, respectively (chi2=13.4; p<.001). The Gap and Coverline techniques overestimated the fertile period by 1 and 4 days, respectively (p=.0002). CONCLUSION: Based on this small study, the Gap technique appears to be more accurate than the Coverline technique in identifying the post-ovulatory IIP.