Cycle day 22, and the scale reads 70.4 kg (155.2 lb). A week ago it said 69.4 kg (153.0 lb). You ate the same and trained the same. If you menstruate, you probably saw this number in the week before your period last month too.
The short answer: in the second half of the cycle, the luteal phase, body water rises. Takano et al. 2026 measured an increase of water in the trunk and arm during the mid and late luteal phase. The scale shows more because of water, while your body fat has not changed. The rise belongs to the cycle and comes back in a similar form the next month.
What the measurement shows
The luteal phase starts after ovulation and ends when your period begins, so it covers the days in which many people notice the gain. The study measured body water in several body regions across the cycle. The result concerns water, not tissue. A gain before your period is therefore no sign that you are building fat.
How big the rise is for you cannot be stated as a number. The study gives no value you could transfer to yourself, and people differ a lot. When your weight drops again after the period starts also varies from person to person. Your own curve over several cycles tells you more than any average.
If you take medication, could be pregnant or have severe symptoms, ask a doctor before you explain swings by the cycle alone.
Why a single reading misleads in your cycle
On top of the cycle come the usual swings: salt, carbohydrates, digestion, sleep, training. What they are and how they work is covered in Why your weight fluctuates daily. The cycle puts a slow wave over this daily noise.
That leads to two typical wrong conclusions. First: in the late luteal phase you see a plus and cut your food, although your fat loss is running fine. Second: after the period the number falls, you credit the last few days of dieting and give your plan the credit for something the scale would have shown anyway. Both happen when you compare two single readings from different cycle phases. The scale measures correctly in both cases. It gives you the mass of that day, and in the second half of the cycle that mass contains more water.
Compare the same cycle phase
The way out is a comparison that cancels the wave: same cycle day, consecutive months. Pick day 1 of your period, or the day before, as a fixed point and note your trend weight on that day. An example with invented values:
| Cycle | Trend weight on day 1 |
|---|---|
| July | 70.0 kg (154.3 lb) |
| August | 69.7 kg (153.7 lb) |
| September | 69.5 kg (153.2 lb) |
| October | 69.2 kg (152.6 lb) |
From July to October that is 0.8 kg (1.8 lb) less, about 0.27 kg (0.6 lb) per cycle. The daily readings in the week before each period were probably higher, but that does not change this line, because it compares day 1 with day 1. If you instead compare day 22 in September with day 8 in August, you compare a water high with a water low.
The comparison works the same way for gaining or holding: if the value on the same cycle day stays stable, you hold your weight, even when the curve rises and falls in between. If it climbs from cycle to cycle, you are gaining, however the week before your period feels.
A cycle lasts roughly a month, so this comparison is slow. That is the price of accuracy: you get one clean data point every four weeks. How fast your trend should fall on average and what a flat stretch means is covered in How fast should you lose weight?.
How to read the cycle in Trendcurve
Trendcurve smooths daily readings into a trend weight. A wave that builds over about a week moves this line only a little. In the example above, with daily readings from 69.4 to 70.4 kg over six days, the trend weight moves from 69.5 to about 69.7 kg (153.2 to 153.7 lb). The average over several days dampens a short swing, and that damping is what you need here. When the wave fades, the line continues without a jump, and you do not have to delete or correct any value. How the method calculates is covered in Trend weight instead of daily weight.
Two features help you make the cycle visible:
- Markers: Under "Settings & more" → "Experiments and markers" you create a marker without a duration, for example with the type "Other" and the label "Period". It appears as a line in the chart and in the history. Creating markers is included in the free version. You see in the curve itself where a cycle starts, and you can put the trend values at the markers side by side.
- Ignore a reading: A daily value you consider implausible stays visible but does not feed into the trend. This is free too. For normal cycle swings you rarely need it, because the trend absorbs them.
The before and after analysis of experiments, meaning the weekly rate before and after a marker, is Pro. For the cycle comparison, looking at the curve with its markers is enough.
Trendcurve is not a medical device and does not detect cycle phases. You enter the cycle day yourself through the markers.
A routine for the next cycle
Keep weighing yourself at your usual time, including the week before your period. Anyone who avoids the scale on those days is missing from the curve exactly where the answer would be. Leave those readings in place, they are measurements and belong in your history. Judge your progress by the trend weight, not by one number.
Set a marker on the first day of every period. After three cycles, look at how the trend weight developed on those days: does it fall, rise or stay the same? That line is your answer. If you wait three months before deciding whether to adjust your calories, the cycle swing is no longer in the way.
If you have cycle pain, irregular periods or a weight gain that does not reverse over several cycles, talk to your doctor about it.