Menstrual cups offer a comfortable, sustainable, and long-lasting alternative to traditional period products. These reusable, bell-shaped devices collect menstrual fluid rather than absorbing it, sitting low in the vaginal canal. Finding the correct fit is necessary for comfort and to prevent leaks, but cup sizing is complex. The right cup is determined by a combination of factors, including your anatomy, physiological history, and the volume of your flow. A systematic assessment of your body is the most reliable path to a successful fit.
Determining Cervical Position (The Length Factor)
The most important anatomical measurement for cup selection is the height of your cervix, which dictates the necessary cup length. The cervix is the lowest part of the uterus, and its position changes throughout the menstrual cycle, usually sitting lower during menstruation. Measure the height on your heaviest day of flow, as this is when the cervix is at its lowest point.
Begin by washing your hands thoroughly and adopting a comfortable position, such as squatting or standing with one leg elevated. Gently insert a clean finger into the vagina until you feel the cervix, which feels firm and round, similar to the tip of your nose. Note how much of your finger was inserted before reaching it.
If you can only insert your finger up to the first knuckle, you have a low cervix and need a shorter cup to prevent the stem from protruding and causing irritation. Reaching your cervix at the second knuckle indicates an average height, meaning most standard-length cups are appropriate. If your entire finger is inserted and you still cannot reach your cervix, you have a high cervix, requiring a longer cup or one with a longer stem for easy removal.
Physiological Factors Affecting Diameter and Firmness
Beyond the length required by your cervix height, the cup’s diameter and firmness are determined by the tone and elasticity of the vaginal walls and pelvic floor. These muscles change over time due to age and childbirth history. Manufacturers often recommend a smaller diameter cup for individuals under 30 who have not given birth vaginally, as their pelvic floor muscles retain greater tone.
A stronger pelvic floor provides more internal pressure on the cup, meaning a smaller diameter cup is often sufficient to maintain the seal and prevent leaks. Individuals over 30 or those who have delivered a baby vaginally may require a slightly wider cup diameter. This wider size compensates for reduced pelvic floor muscle tone, ensuring a secure seal against the vaginal walls.
The firmness of the cup’s silicone is also related to muscle tone. People with strong pelvic floor muscles, such as athletes, may inadvertently compress a soft cup, causing the seal to break and leading to leaks. A firmer cup is more suitable in this scenario because it resists collapse from strong muscle contractions. A softer cup is recommended for those with a sensitive bladder or history of cramping, as it exerts less pressure on internal structures.
Matching Capacity to Menstrual Flow
The third dimension of cup sizing involves matching the cup’s capacity to the volume of your menstrual flow, which dictates how often the cup must be emptied. A menstrual cup capacity is measured in milliliters (ml). The difference between a small (often 15 to 20 ml) and a large (25 ml to over 30 ml) cup significantly impacts wear time.
If you have a light flow, requiring few changes daily, a standard small-capacity cup will likely provide twelve hours of wear. A moderate flow, requiring product changes every four to six hours, is accommodated by a medium-sized cup. For a heavy flow, where traditional products saturate rapidly, selecting a large-capacity cup maximizes the time between changes.
A heavy flow means needing to change a high-absorbency product every two to three hours. Individuals with this flow should prioritize the largest cup capacity available that still fits their anatomical measurements. Choosing a cup with adequate capacity allows for longer wear times, reducing the need for frequent emptying throughout the day or overnight.
Troubleshooting Common Fit Issues
Even after careful measurement, initial cup use may present challenges requiring adjustment. Leaking is the most frequent issue, often resulting from an improper seal, the cup not fully opening, or incorrect length. If the cup leaks when only half full, the cervix may be sitting inside the cup, displacing fluid and causing overflow around the sides.
Discomfort or pressure usually signals that the cup is too firm or too wide. If you experience persistent cramping or bladder pressure, switching to a softer silicone material or a slightly smaller diameter cup can alleviate the sensation. If the cup feels too low and the stem irritates the vaginal opening, the cup is too long for your cervical height, and a shorter model is required.
Difficulty with removal, especially if the cup seems to have traveled too high, suggests the cup is too short for your high or average cervix. The cup can migrate up the vaginal canal, making the stem difficult to reach. If this occurs, switching to a longer cup ensures the base remains accessible. When attempting removal, always break the seal by pinching the base of the cup before pulling, which prevents discomfort and maintains the integrity of the vaginal tissue.