You've probably already come across this kind of guide: under 30, you're pointed toward a small size; over 30, a large one. Never given birth vaginally, small size; given birth, large size. You might be wondering whether, at 32 or after giving birth, you're automatically stuck with size L, even if your flow is light.

These two criteria, age and childbirth, still shape a huge number of size choices even though they have no physiological basis. This article breaks down these two myths point by point, and shows you the one criterion that actually determines the right size.

1- The Facts: two rules you hear everywhere

These two rules come back in very similar forms, in guides, videos, or pharmacy advice. You might recognize yourself in one of these situations:

• You're 35 and you're automatically pointed toward size L, even though your flow is light.
• You gave birth vaginally two years ago, and you're advised to get the largest size "to make sure it stays in place".
• You're 22 with a very heavy flow, but the guide puts you in size S because you're "young".
• You've never given birth, and you keep hearing that the small size is "logical" for you, without anyone ever asking about your actual flow.

In all four cases, the criterion used is neither flow nor insertion comfort: it's a demographic data point (age, having given birth) that gets wrongly equated with vaginal diameter. That's the confusion we need to unpack.

2- The Science: what age and childbirth don't change

The clearest comparison is the tampon. No one tells someone who's given birth that they can no longer use a mini tampon because it might fall out: if her flow is light, she uses a small tampon, without ever questioning her age or obstetric history. For a cup, the logic is exactly the same.

👉 Bottom line: age and childbirth change neither the size of the vagina nor how a cup stays in place. The only number that should guide your choice is your flow.

3- Luneale's Advice: flow, the only real criterion

At Luneale, the criterion we use to choose a size is flow: a cup is first and foremost a reservoir, and its capacity should match the volume of menstrual blood to collect, no more, no less. That's actually why the difference in diameter between two sizes of La Cup Luneale is only around 3 mm: it's not that detail that "locks" the cup in place, it's the volume matched to your flow.

• Light to moderate flow: size S
• Moderate to heavy flow: size M
• Very heavy flow: size L

A second guideline complements this one: aim for as little air as possible inside the cup once it's in place. Blood that sits stagnant in contact with air oxidizes faster, which encourages odor. A cup that's well filled relative to your actual flow, neither oversized nor too small, naturally limits this.

For a full table based on your current flow (tampon or pad), our menstrual cup size guide covers every case. La Cup Luneale comes in three sizes designed around this logic, without assuming anything about your age or your history.

4- Special Cases: when comfort comes before flow

Flow remains the main criterion, but two situations justify putting insertion comfort first, at least to start:

• A very heavy flow with no prior vaginal penetration: it's better to start with size M, even if it means emptying it more often, rather than going straight for size L.
• Dyspareunia, vaginismus, or significant apprehension about insertion: same logic, size M limits discomfort, at the cost of slightly more frequent emptying.
• Your very first cycles with a cup, whatever your flow: inserting and removing it is a skill you learn, and an intermediate size can be reassuring while you find your footing.

Age and childbirth change neither the size of the vagina nor how a cup stays in place. Flow remains the only reliable criterion for choosing your size, with insertion comfort as a possible adjustment in certain situations.