Teenage pregnancy is a major social problem in Kakamega County in Kenya that tends to have negative ramifications especially on the lives of young girls. Recent studies have documented school children lack the necessary skills to manage their sexual life due to the misconceptions they have about menstrual cycle despite having gone through Kenyan school science curriculum. This can be supported by worldwide reports that teachers can actually be one of the sources of such misconceptions in their learners. This study sought to assess teachers’ understanding of menstrual cycle in Navakholo sub-county, Kakamega County, Kenya. A case study design with a mixed-method approach was used in two secondary schools in Navakholo sub-County, Kakamega County Kenya. A mixed gender sample of 20 teachers aged 24 years and above, responded to a multiple choice questionnaire targeting the menstrual cycle concept. Four focus group discussions and 4 Key Informant Interviews were conducted. Three levels of understanding emerged in the data analysis, i.e., Full Comprehensive Basic, Full Comprehensive Intermediate and Full Comprehensive Understanding. There was a statistically significant relationship be full comprehensive understanding level and ones knowledge of menstrual cycle, (χ2=13.7620, p=0.003). Basic understanding was found to have no statistically significant effect on knowledge (t=-0.44, p=0.661) at the 5% level of significance. Intermediate understanding was found to have statistically significant effect on knowledge (t=-1.89, p=0.062) at the 10% level of significance. Full understanding had positive and statistically significant effect on knowledge (t=5.33, p=0.0000) at the 5% level of significance. As the understanding graduated from basic through intermediate to full comprehensive understanding, the fewer the number of respondents who achieved higher level of understanding. Full comprehensive understanding of the concept of menstrual cycle was not achieved by the majority meaning that teachers lack a high level understanding of the menstrual cycle concept.