Your weight has a monthly pattern as well as a weekly one
If you menstruate, your daily weight carries a second rhythm on top of the weekly one, and a 7-day average does not remove it — seven days is the wrong length to cancel something that repeats roughly monthly. It is still readable, once you know its shape. The measured shape is not quite the one people expect.
What was measured
Daily diary data covering 765 menstrual cycles in 62 healthy women were collected over a year, with ovulation confirmed by quantitative basal temperature analysis rather than assumed from dates. Self-reported fluid retention — the thing usually described as bloating — was scored daily on a 0–4 scale.
The pattern:
- Fluid retention peaked on the first day of menstrual flow (0.9 ± 0.1), not in the days before it.
- It was lowest in the mid-follicular phase — the stretch after bleeding ends.
- It rose gradually around ovulation, from 0.22 ± 0.05 to 0.50 ± 0.09 across the eleven days surrounding it.
The common belief is that the premenstrual week is the swollen one. In this data the mid-cycle rise was the more notable one, and the peak arrived with the bleeding rather than before it.
What that means for reading the scale
Compare like with like. The useful comparison is the same phase one cycle ago, not last week. A rise from the mid-follicular low to the first day of flow is a comparison between the lowest and highest points of your own cycle — it will look alarming and mean nothing.
Expect a low after bleeding ends. That dip is not the diet suddenly working, any more than the peak was it failing.
The trend still works, over a longer window. A 7-day average smooths the day-to-day noise, and comparing two averages a fortnight apart is enough for most purposes. But if you are judging a month of effort, compare the same point in two consecutive cycles.
Two rhythms, one scale
This sits on top of the weekly pattern, which is separate and applies to everyone. Analysing 4,657 daily weigh-ins from 80 adults found weight highest on Sunday and Monday, falling from Tuesday, rising again from Saturday — a real group-level rhythm the authors said should be treated as normal rather than as a sign of gain.
So a single morning reading contains: the weekly rhythm, the cycle rhythm, yesterday's salt, yesterday's carbohydrate, and — somewhere underneath all of it — the actual change in your body. Over two weeks of ordinary living, 84% of a unit change in body weight was fat-free mass. The signal you want is the smallest thing in the number.
The limits of this
This is 62 women in one prospective cohort, and what was recorded was self-reported bloating scored daily, not body weight measured on a scale. It establishes that a monthly fluid rhythm exists and roughly when it peaks. It does not tell you how many kilograms it is worth for you. That, you find out by keeping the daily record and reading the average across a couple of cycles.
Hormonal contraception, cycle irregularity, and perimenopause all change this picture, and none of them were what this study set out to measure.