When it comes to determining the duration of your pregnancy, it is generally more accurate to rely on the information provided by an ultrasound rather than your menstrual cycle. Ultrasounds are able to provide a more precise estimation of gestational age by measuring the size and development of the fetus. Menstrual cycles can vary in length and ovulation timing, which can lead to inconsistencies in calculating pregnancy duration based solely on this information. However, if there is a significant discrepancy between your estimated due date based on your last menstrual period and the ultrasound measurement, it is recommended to consult with your healthcare provider for further evaluation and clarification. They may consider additional factors such as irregular cycles or potential complications that could affect the accuracy of the ultrasound measurement. Ultimately, your healthcare provider will be able to guide you in determining the most reliable estimate for your pregnancy duration.