What happens in the second half of the cycle
After ovulation, the luteal phase begins. Progesterone rises and shapes the second half of the cycle until, shortly before the period, both hormones fall again. This phase brings the symptoms many know as premenstrual syndrome: tension, bloating, breast tenderness, mood swings and a change in body weight.
During this time the body holds on to more water. On the scales, that looks like weight gain. Your body fat hasn't changed that week. That would take a considerable energy surplus, and a few days simply can't produce one.
How much it is and when
Measured, it's usually less than it feels. In a 2023 study, 42 women were weighed twice a week over one cycle and measured with bioimpedance. None took medication that affects the cycle or fluid balance. The women were lightest in the week after their period started, on average 0.45 kg lighter than on the first day of the period. After that, weight slowly rose again. The difference came from water outside the cells. Skinfolds and almost all circumferences stayed the same.
Three further studies from 2021 to 2026 found no measurable difference across the cycle at all. In 52 trained women, neither weight nor body water changed. In 36 women with and without hormonal contraception, body composition stayed the same with three different measuring methods. And in 29 students, weight and total body water stayed the same as well. Only the trunk and the right arm held slightly more water in the second half of the cycle.
The last of these studies also explains why it often feels stronger than the scales show. Where and when the women felt swollen didn't match what was measured. They named their lower legs most often.
Why it costs more than a kilo
The real problem is the interpretation. If you take the number as a verdict on your training, you draw the wrong conclusions in exactly that week: calories get cut, training gets harder, or the plan is dropped altogether.
Measuring more sensibly
- Always weigh yourself on the same cycle day. In the 2023 study, weight was lowest about a week after the period started. A fixed day in that week makes the numbers comparable from cycle to cycle.
- Around the start of your period, the scales tell you the least. That's where weight was highest in the same study. A number from those days costs motivation more than it shows anything.
- Use several measures. Strength, measurements, sleep and energy together give a more reliable picture than a single number.
- Look over months. Body composition changes slowly. A trend over three cycles says something; a swing over three days doesn't.
When it isn't cycle water
Water from the cycle comes and goes. In the measuring study, weight was back down a week after the period started. If your number on the same cycle day keeps rising over several cycles, it's no longer cycle water. Comparing across cycles shows that sooner and more clearly than any number from a single morning.
What Rhythmus does with this
The app brings your entries together across cycles and doesn't set daily values against each other. The daily energy check-in is deliberately not a weight field: how you feel steers training. What the scales show on a single morning doesn't.