Obesity and high body mass index (BMI) are known to be risks for anovulation and infertility. Little is known about how BMI levels affect parameters of the menstrual cycle. The purpose of this study was to determine the influence of BMI on parameters of the menstrual cycle and the likelihood for ovulation. The participants in this study were 244 women between the ages of twenty and fifty-four (mean thirty years) who charted from one to thirty-six menstrual cycles (mean seven cycles) for a total of 2,035 cycles. Urinary luteinizing hormone (LH) threshold tests were used to estimate the day of ovulation and the lengths of the follicular and luteal phases. The 244 participants were classified as normal weight with a BMI of 18.5–24.9 kg/m2 (N = 141), overweight with a BMI of 25–29.9 kg/m2 (N = 67), and obese with a BMI of 30 kg/m2 or greater (N = 36). One-way ANOVA indicated that there was a significant difference between groups in length of the luteal phase (F = 4.62, p < 0.01) and length of menses (F = 3.03, p < 0.05). Odds ratio indicated that the combined obese and overweight group was 34 percent less likely to have a positive detected urinary LH surge. We concluded that obesity might contribute to infertility by shortening the luteal phase and decreasing the probability of ovulatory menstrual cycles.