The four phases and what happens to weight in each
Understanding why weight fluctuates requires mapping it against the hormonal architecture of the cycle across its four phases.
The menstrual phase (approximately days 1 to 5) is when most women see their lowest weight reading. As progesterone drops at the end of the previous cycle, the fluid it helped retain in the tissues begins to be released. Prostaglandins, the inflammatory compounds that drive uterine contractions and menstrual flow, also accelerate gut transit in many women, producing looser bowel movements in the first one to two days that clear gut-lumen weight. The estrogenic fluid-retention effect is at its monthly minimum because estrogen is also at its lowest point in this phase. The net result is a weight reading that typically reflects the lowest point in the cycle, not because the body has genuinely become lighter in terms of body composition, but because fluid and transit conditions are at their least retentive point.
The follicular phase (approximately days 6 to 13) sees a gradual, gentle rise in weight as estrogen increases. Rising estrogen begins to promote mild fluid retention through its aldosterone-stimulating effect, and blood volume expands slightly. This is typically modest in a well-regulated hormonal pattern: a kilogram or less above the menstrual-phase baseline.
Around ovulation (approximately day 14), there is often a secondary weight spike. The ovulatory estrogen surge is the highest estrogen point in the cycle, producing the highest estrogenic aldosterone stimulation and a corresponding brief increase in fluid retention. Some women also experience ovulation-related bloating from the follicular rupture and localised inflammatory response. This mid-cycle weight increase typically resolves within two to three days as the ovulatory estrogen peak subsides.
The luteal phase (approximately days 15 to 28) is where the most significant weight fluctuation occurs for most women. Progesterone's influence on smooth muscle slows gut motility, reducing bowel frequency and allowing more transit weight to accumulate. Blood volume continues to expand in the luteal phase. And as the luteal phase progresses toward its end, progesterone begins to decline before its protective diuretic effect fully counterbalances estrogen, producing the classic premenstrual fluid retention that peaks in the final three to five days before menstruation. The combined effect of slower gut transit, blood volume expansion, and reduced diuretic counterbalance typically produces the highest weight reading of the cycle in this final premenstrual window.
How estrogen dominance amplifies normal fluctuation
The cyclical fluctuation described above is a feature of any menstrual cycle. In women with estrogen dominance, specifically the low progesterone relative to estrogen pattern that is common in populations under sustained occupational stress, the range of that fluctuation widens.
In a well-regulated cycle, progesterone's diuretic effect in the mid to late luteal phase counterbalances a significant proportion of the fluid retention that estrogen is driving. This limits how much fluid accumulates premenstrually. In the estrogen dominance pattern, progesterone is insufficient to perform this counterbalancing function, meaning estrogen's aldosterone-driven fluid retention proceeds with less opposition, producing more fluid accumulation in the luteal phase and a higher premenstrual weight peak.
The follicular-phase weight nadir also tends to be less complete in estrogen dominance. Because the progesterone that would normally drive a clear diuretic response is insufficient, the fluid release that should occur after menstruation is less pronounced. The result is a higher baseline throughout the cycle, with an amplified premenstrual spike and a less complete post-menstrual drop, producing the feeling that weight is consistently elevated rather than fluctuating clearly around a stable baseline.
Cortisol's contribution compounds this further. Chronic cortisol elevation promotes aldosterone-driven fluid retention through the RAAS pathway independently of the estrogen-progesterone dynamic, adding a non-cyclical fluid retention layer that sits underneath the hormonal cyclical pattern and makes the fluctuations feel both larger and less predictable.
Gut transit and the scale
A portion of the weight fluctuation that women attribute to fluid retention is actually driven by changes in gut transit speed, which is directly regulated by progesterone and varies across the cycle.
Progesterone relaxes smooth muscle, including the smooth muscle of the gastrointestinal tract. In the luteal phase, when progesterone is highest, gut transit slows. Slower transit means more undigested and partially digested food mass remaining in the gut lumen at any given time, more gas from the extended fermentation window, and less frequent bowel movements. This transit mass has weight: the difference between high-transit and low-transit states can account for one to two kilograms of scale weight, independently of any fluid or fat change.
When progesterone drops at menstruation and prostaglandins accelerate transit, gut mass reduces and scale weight drops. Many women interpret this as evidence that their diet or efforts produced results when in fact it reflects a gut-transit change driven by the hormonal shift.
This mechanism is relevant to how crew on board interpret their weight in the context of irregular eating schedules and highly variable meal composition. The slow-transit luteal phase combined with chef-controlled meals high in refined carbohydrates and low in fibre compounds the gut-mass contribution to weight fluctuation, making premenstrual scale increases more pronounced than they would be in a diet with consistently higher fibre and more regular eating timing.
How to interpret weight data in context
The practical question is how to extract useful information from weight measurements when normal cyclical fluctuation accounts for one to four kilograms of variation, estrogen dominance amplifies that range, and non-cyclical factors (food composition, transit, hydration, time of weighing) add additional noise.
The most useful approach for women tracking weight is to compare the same cycle point across months rather than individual readings across days. A weight taken at the same cycle phase, same time of day, same conditions, compared month to month gives a picture that controls for the cyclical hormonal variation and reflects genuine changes in body composition more accurately than any within-cycle comparison.
If the goal is understanding whether the hormonal fluctuation is normal or amplified, tracking weight daily for two to three complete cycles while noting cycle phase creates a pattern map. A normal cyclical pattern shows a recognisable curve: low around menstruation, gentle rise through the follicular phase, a brief mid-cycle peak around ovulation, a clearer peak premenstrually, then a drop at menstruation. An amplified estrogen-dominance pattern shows the same shape with a wider range, a less complete follicular-phase drop, and a more dramatic premenstrual peak. Seeing the pattern visually often clarifies what is hormonal fluctuation and what, if anything, reflects other contributors.
For crew, the additional variability introduced by charter seasons (higher sodium in guest meals, eating at unpredictable times, reduced sleep, elevated cortisol) means that within-season weight tracking is particularly noisy. The most informative comparison is often the weight pattern from one inter-season rest period to the next, which gives a better picture of whether the baseline is shifting over time than any within-charter tracking can provide.
Belinda's Perspective
What tracking taught me about numbers that have nothing to do with food
When I started tracking properly around 2020, one of the first things I noticed was how much the scale moved without anything obviously accounting for it. Not large amounts, but consistent, patterned movement that didn't correlate with what I was eating. I hadn't been paying close attention to this before because I wasn't tracking anything methodically, so the pattern had been happening without me noticing it as a pattern.
What the data eventually made clear was that the fluctuation was following a hormonal rhythm, not a dietary one. The timing of the heavier readings was consistent with the luteal phase and the premenstrual window. The lighter readings came predictably around the early days of menstruation. Once I could see that, the individual numbers became much less meaningful than the pattern they were tracing.
This matters particularly in the context of the estrogen dominance picture I've documented across four consecutive panels. A consistently low Pg/E2 ratio means the natural diuretic counterbalance of adequate progesterone is reduced, which mechanistically produces exactly the kind of amplified premenstrual fluid retention I was seeing. The data from the hormonal panels and the weight tracking pattern were telling the same story from different angles.
What I take from this, and what I think is most useful for crew, is that weight is a piece of data with context requirements. Without knowing where you are in your cycle, what phase the hormonal panel reflects, what the sodium content of the last two days of meals has been, and what your gut transit has been doing, a single scale reading tells you almost nothing. What tells you something is the pattern over time, measured consistently enough to separate the signal from the noise. And what the pattern usually reveals, once you have enough data to see it, is that most of the fluctuation is hormonal and structural rather than something requiring correction.
