Swimsuit Body
Your Body in a Swimsuit
Is Not a Problem to Solve.
The hormonal science of why your body literally changes size, shape, and feeling across the month — and what that means for how you show up at the pool.
There is a specific version of summer dread I have felt every year since I was a teenager. It is not the heat or the logistics of packing a bag or finding the right sunscreen. It is the swimsuit. More precisely: it is the morning of, standing in front of a mirror, and having the number on the scale from three days ago still running in the background of my brain like an unwanted tab that won't close.
What I was doing, without knowing it, was comparing my body on a Tuesday in week three of my cycle to an imagined ideal that existed nowhere in particular — and finding the Tuesday version wanting. The mirror was the same. The swimsuit was the same. The only thing that had changed was when in the month it was. And that, as it turns out, changes quite a lot.
I have spent the past several years learning the hormonal mechanics of my own body. What I now understand — what I wish someone had told me at fifteen, at twenty-three, at thirty — is that my body is not bigger or smaller because of anything I did or failed to do. It is bigger or smaller because it is cycling. Because estrogen and progesterone and aldosterone and vasopressin are orchestrating a monthly physiological programme that includes, among many other things, measurable changes in fluid distribution, tissue density, and the way I feel in my own skin.
This is not a story about learning to love yourself despite your body. It is a story about learning to understand your body well enough to stop treating it like a problem in the first place.
Your body's fluid balance is not fixed. It is actively regulated by a cascade of hormones — estrogen, progesterone, aldosterone, and arginine vasopressin (AVP) — that shift across the 28-day cycle in ways that directly affect how much water your tissues hold, where they hold it, and how you feel as a result.
Estrogen is the primary driver of fluid retention in the first half of the cycle and around ovulation. It works by stimulating the liver to produce angiotensinogen, which triggers the renin-angiotensin-aldosterone system — a hormonal cascade that ultimately instructs the kidneys to retain sodium. And where sodium goes, water follows. Estrogen also lowers the osmotic threshold for arginine vasopressin, the hormone responsible for telling the kidneys to hold onto free water. The result is that in high-estrogen states, your body retains more fluid — measurably [3].
Progesterone, which rises in the luteal phase after ovulation, has a more complex relationship with fluid. It competes with aldosterone for the mineralocorticoid receptor in the kidney's distal tubule, which gives it a natriuretic — or sodium-excreting — effect. In theory, progesterone should counterbalance estrogen's water-retaining tendency. In practice, this is where individual variation is wide, and where the progesterone withdrawal that occurs in the premenstrual window removes that counterbalance precisely when estrogen-driven retention is still present [2].
Your body across the cycle — click any woman to explore her phase
How much does the body actually change? A 2023 study in the American Journal of Human Biology by Kanellakis et al. measured body weight and body composition twice per week across a full menstrual cycle in 42 women. Body weight was significantly higher during menstruation compared to the first week of the cycle by an average of 0.45 kg — almost entirely attributable to a 0.47 kg increase in extracellular water. No significant changes in fat mass were found at any point across the cycle [1]. The body is not storing fat differently. It is distributing fluid differently.
The hormonal mechanics of fluid retention. A foundational review published in Exercise and Sport Sciences Reviews by Stachenfeld mapped the specific mechanisms through which estradiol and progesterone alter fluid regulation in premenopausal women. Estrogen lowers the osmotic threshold for AVP release and thirst onset, increases plasma volume, and interacts with the renin-angiotensin-aldosterone system to promote sodium and water retention. Progesterone exerts a competing natriuretic effect through the mineralocorticoid receptor, partially counterbalancing estrogen's retentive tendency — but this balance shifts predictably across the cycle, with its greatest imbalance in the premenstrual window when progesterone withdraws before estrogen does [2].
Fluid retention in real time. A one-year prospective cohort study published in Obstetrics & Gynecology International by Prior et al. tracked fluid retention across 765 menstrual cycles in 62 women. Fluid retention scores peaked on the first day of menstrual flow — not in the days before, as is commonly assumed — and were lowest during the mid-follicular window, gradually rising around ovulation. The study found that neither estradiol nor progesterone levels alone were significantly associated with fluid retention scores, suggesting the mechanism involves the interplay between the two hormones rather than either in isolation [3].
Body dissatisfaction follows the hormonal pattern. A study by Carr-Nangle et al. published in the International Journal of Eating Disorders measured body image across three menstrual phases in 26 normally cycling women. Body dissatisfaction — measured as the frequency of negative body-related thoughts and anxiety about appearance — was significantly highest during the perimenstrual phase. Critically, measures of actual body size perception remained stable across the cycle. What changed was not the body. What changed was the relationship to it — and that change was strongly associated with the physical symptoms of fluid retention, negative affect, and reduced sense of control that characterise the premenstrual window [4].
Breast tissue changes. A 2025 review in Advances in General Practice of Medicine documented that breast volume, density, and moisture content change measurably across the menstrual cycle, with the most pronounced changes occurring in the luteal phase. Progesterone drives stromal oedema and glandular development, resulting in increased breast volume and density mediated through the RAAS and vasopressin pathways. This is one of the reasons the body can feel genuinely different in a swimsuit at different points in the month — because it is [5].
The body is not bigger because of anything you ate or failed to do. It is bigger because it is cycling. Estrogen, progesterone, and aldosterone are running a physiological programme. The swimsuit is just catching you mid-programme.
Body weight fluctuation across the 28-day cycle — schematic
Hover any day for details · Based on Kanellakis et al. (2023) — average 0.45 kg extracellular fluid gain at menstruation
Average fluctuation ~0.45 kg, entirely extracellular fluid. No fat mass changes detected at any cycle phase. Range across individuals: 0–2.3 kg.
The feminist dimension of this matters. A significant portion of the negative body image that women experience — the specific, acute dissatisfaction that spikes in the premenstrual window, the feeling of being somehow worse than you were a week ago — is not a character flaw or a failure of self-love work. It is a physiological state, amplified by the same hormonal environment that reduces your sense of control and increases negative affect more broadly.
Knowing this does not make the feeling disappear. But it gives you something to put next to it. The body you are looking at in the mirror on day 26 of your cycle is not a truer body than the one you saw on day 10. It is a different hormonal moment of the same body. And the dissatisfaction you feel about it is partly coming from the same hormonal environment that is making your jeans feel tight. The two are related. Neither is a verdict.
None of this requires you to stop noticing how your body feels. It requires you to contextualise what you notice. Here is what that looks like in practice.
This is the single most useful reframe available. When you wake up feeling bloated, puffy, or like your waist has expanded since Tuesday, check what day of your cycle it is before you draw any conclusions. If you are in days 20–28, you are in the window of maximum progesterone withdrawal and estrogen-driven fluid retention. What you are feeling is real and physiological. It is also temporary and not fat.
Progesterone has a natriuretic effect — it promotes sodium excretion. When progesterone drops in the premenstrual window, you temporarily lose that counterbalancing effect, and the body tends to retain sodium (and therefore water) more readily. Eating to support electrolyte balance in the week before your period — adequate potassium, moderate sodium, sufficient hydration — can reduce the amplitude of retention. This is not restriction. It is working with the biology rather than against it.
If you track weight at all, the only meaningful comparison is the same day of your cycle, month to month — because the fluid fluctuation of up to 0.5 kg (and sometimes significantly more) makes within-cycle comparisons almost meaningless. Comparing your weight on day 26 to your weight on day 10 is comparing two different physiological states, not two different dietary outcomes. Day 8 to day 8, month to month, gives you signal. Everything else is noise.
This is not about avoiding the pool in week four of your cycle. It is about going in informed rather than ambushed. If you know that days 8–14 are when your body feels most like itself — when estrogen is rising but progesterone has not yet accumulated, when fluid retention is lowest — and that days 22–28 are when it feels most foreign, you can stop interpreting that foreignness as failure. You can also, if a holiday or beach day falls in week four, simply know in advance: this is not the mirror's verdict on your lifestyle. This is aldosterone doing its job.
The research on body dissatisfaction and the menstrual cycle is consistent: what changes premenstrually is not the body's actual size or shape perception — it is the emotional relationship to it. The body that you are reluctant to put in a swimsuit is not objectively larger. It is the same body, viewed through a hormonal lens that amplifies self-critical processing and reduces the sense of control. Getting in the water is, among other things, a refusal to let a hormonal state make decisions about your summer. The water does not care what day of your cycle it is.
I think about the mornings I postponed the pool. The summers I waited to feel ready — to feel like my body had earned the swimsuit, had achieved some kind of metabolic stability that would make me acceptable to myself in public. The hours I spent on scales that were measuring fluid, not failure.
What the research says, quietly and clearly, is that the body I was waiting to achieve was already there, underneath the 0.45 kilograms of extracellular fluid that aldosterone and progesterone were cycling through me as part of a monthly programme I had no knowledge of. I was waiting for a moment in the cycle that I had not learned to recognise. I was solving a problem that was never mine to solve.
Your body in a swimsuit is not a statement about your worth or your discipline or your relationship with food. It is a body, on a particular day, in a particular hormonal phase, doing what bodies do: cycling. The swimsuit is just fabric. The water is just water. The month keeps moving regardless.
Know your cycle. Trust your body. Get in.
The water has been waiting for you all along. ❤References
- Kanellakis, S., Skoufas, E., Khidirova, L., Chourdakis, M., & Aggelousis, N. (2023). Changes in body weight and body composition during the menstrual cycle. American Journal of Human Biology, 35(11), e23951. https://doi.org/10.1002/ajhb.23951
- Stachenfeld, N. S. (2008). Sex hormone effects on body fluid regulation. Exercise and Sport Sciences Reviews, 36(3), 152–159. https://doi.org/10.1097/JES.0b013e31817be928
- Prior, J. C., Hitchcock, C. L., & Harvey, A. T. (2011). Fluid retention over the menstrual cycle: 1-year data from the prospective ovulation cohort. Obstetrics and Gynecology International, 2011, 138451. https://doi.org/10.1155/2011/138451
- Carr-Nangle, R. E., Johnson, W. G., Bergeron, K. C., & Nangle, D. W. (1994). Body image changes over the menstrual cycle in normal women. International Journal of Eating Disorders, 16(3), 267–273. https://doi.org/10.1002/eat.2260160307
- Dillard, T., Jones, J., Bernard, B., Rudd, R., Anderson, S., Foster, C., & Goswami, T. (2025). Breast density and volume changes during the menstrual cycle. Advances in General Practice of Medicine, 6(1), 120–133. https://doi.org/10.25082/AGPM.2024.01.003