You can do everything the same — the same food, the same training, the same discipline — and watch your body look leaner one week and softer the next. Most women blame themselves for this. The honest answer is that three hormones are shifting underneath you, and they change how you store fat, how much water you hold, and how defined your muscles look. None of it is a character flaw. All of it is physiology.

Estrogen — your fat-burning ally

Estrogen tends to support fat oxidation and lean tissue. It is the dominant hormone of the first half of your cycle, and it is broadly favourable for body composition: it supports the use of fat as fuel, helps preserve muscle, and is associated with better insulin sensitivity. This is part of why the follicular phase — the roughly two weeks from your period to ovulation — often feels like the easier window to train hard and lean out.

Estrogen also matters for muscle itself. Research shows it influences muscle protein synthesis and helps protect muscle from breakdown, which is one reason muscle loss tends to accelerate after menopause when estrogen falls.

Source: Hormonal Influences on Skeletal Muscle Function in Women across Life Stages: A Systematic Review — MDPI, 2024. Estrogen promotes muscle protein synthesis and reduces muscle damage and inflammation.

The honest caveat: the idea that estrogen meaningfully boosts fat burning during a single workout is real but inconsistent. Some studies find higher fat oxidation when estrogen is high and progesterone low; others find menstrual cycle phase makes no significant difference to peak fat oxidation during exercise. So treat estrogen as a gentle tailwind for fat loss across the month — not a switch that transforms one workout.

Source: Menstrual cycle phase does not affect whole body peak fat oxidation rate during a graded exercise test — Journal of Applied Physiology, 2020.

Does estrogen help you burn fat?
Estrogen supports fat oxidation and helps preserve muscle, and the follicular phase — when estrogen is rising and progesterone is low — is often the more favourable window for fat loss. However, the effect on fat burning within a single workout is modest and inconsistent across studies. Estrogen is best understood as a gentle, month-long influence on body composition rather than a dramatic fat-burning switch.

Progesterone — appetite, water and the softer look

Progesterone rises after ovulation and changes the picture. In the luteal phase — the roughly two weeks before your period — progesterone climbs, and for many women it brings a bigger appetite, stronger cravings, and a feeling of being puffier or softer. Crucially, progesterone can act in an anti-estrogenic way on metabolism, which partly offsets estrogen's favourable effects.

The water-retention truth. This is the single most important thing to understand about the scale. Estrogen and progesterone both influence fluid balance, and many women hold significantly more extracellular water in the mid-to-late luteal phase. A 2023 study found body weight was significantly higher around menstruation than in the first week of the cycle — by roughly half a kilogram on average — and that the increase was almost entirely extracellular water, with no significant change in fat or muscle mass. Some women retain considerably more.

Source: Kanellakis S, et al. Changes in body weight and body composition during the menstrual cycle — American Journal of Human Biology, 2023. The increase was almost entirely extracellular water, not fat.

That water is exactly why your muscles can look less defined the week before your period and sharper a few days into your cycle. The muscle has not changed in days; the fluid sitting around it has. If you have ever taken progress photos two weeks apart and despaired, the difference you saw was very likely water, not fat.

You cannot gain four pounds of fat overnight. What the scale is showing you before your period is water — and it leaves on its own.
Why does the scale jump up before my period if I haven't gained fat?
It is almost certainly water, not fat. Estrogen and progesterone both influence fluid balance, and many women retain extra extracellular water in the mid-to-late luteal phase. Research has found body weight can be significantly higher around menstruation than in the first week of the cycle, with the increase almost entirely water rather than fat or muscle, and some women retaining up to around five pounds. It is temporary and resolves within a few days of the period starting.

Testosterone — the underrated lever for tone

Women have testosterone, and it matters more than most realise. In far smaller amounts than men, yes — but testosterone supports lean mass, strength, bone, and often the energy and drive to train hard. When it is in a healthy range, training, recovering and holding onto muscle all feel a little easier. When it is low, fat loss can get harder indirectly, simply because it becomes harder to move more and push in workouts.

Testosterone also contributes directly to muscle protein synthesis — the process that builds and repairs the muscle tissue underneath the fat. And it is the muscle underneath that creates what most women mean by "tone" and "definition." There is no separate "toning" hormone or "toning" exercise; visible definition is muscle made visible, supported by testosterone and revealed when body fat and water are lower.

Source: The Impact of Female Hormones on Muscle Strength — Hormone Health Clinic, Minter et al. Testosterone contributes to muscle protein synthesis, promoting growth and repair of muscle tissue.

Does testosterone affect muscle tone in women?
Yes. Testosterone, although present in much smaller amounts in women than men, supports lean mass, strength and muscle protein synthesis — the building and repair of muscle tissue. Since visible "tone" is really muscle revealed when body fat and water are lower, healthy testosterone supports definition indirectly by helping you build and maintain that muscle and by supporting the energy to train.

What muscle definition actually is — and why it moves

Put the three hormones together and the mirror makes sense. Definition is the product of three things: the muscle you have built (supported by testosterone and estrogen), the body fat sitting over it, and the water held in and around the tissue (driven heavily by estrogen and progesterone across your cycle).

Of those three, only body fat changes slowly. Muscle changes over weeks and months. Water changes within days. So when your definition seems to vanish before your period and return after it, you are not losing and regaining muscle or fat — you are watching the fluid layer rise and fall with your hormones. This is why the most useful habit is to judge your progress at the same phase each cycle, not day to day.

Does muscle definition really change across the menstrual cycle?
Visible definition can change, mostly because of water retention rather than actual changes in muscle or fat. When the body holds more extracellular fluid in the luteal phase, muscles can look softer or less defined; the same body can look leaner in the follicular phase when fluid is lower. The underlying muscle has not changed in days — the water around it has.

Working with it — phase by phase

Follicular phase (after your period). Estrogen is rising, water is lower, insulin sensitivity is good. This is your strongest window for a moderate deficit and harder training. Push here. Take your progress photos and measurements here, when fluid is lowest and definition is at its truest.

Ovulation. Estrogen and testosterone are around their peak. Energy and drive are high — a good time for your most demanding sessions.

Luteal phase (before your period). Progesterone is high, appetite and water are up. Expect the softer look and the scale creep, and do not panic-restrict — that only raises cortisol and worsens both water retention and cravings. Keep protein high, lean on complex carbohydrates, reduce sodium if bloating is heavy, and stay hydrated (counter-intuitively, drinking enough helps your body hold less). Maintain rather than aggressively cut.

Menstrual phase. Water usually starts to drop within a few days of bleeding beginning, and the leaner look returns. This is a recovery window, not a punishment window.

For the complete phase-by-phase system — exactly how to eat and train through all four phases for fat loss and definition — The Women's Hormone Blueprint maps all of it. And to see your own water and definition patterns clearly across six cycles, The Aligned Woman Journal gives you the daily tracking to stop guessing.

Should I stop weighing myself during the luteal phase?
Many women find it more useful to compare weight at the same phase each cycle rather than day to day, because luteal-phase water retention can mask real fat loss and cause unnecessary discouragement. The trend across cycles is far more meaningful than any single luteal-phase reading.
About the author

Andreea Mighiu is a women's hormonal health educator and the founder of Zōē. She works alongside medical doctors to translate peer-reviewed research into clear, practical cycle education. She is an educator, not a physician — Zōē's content is designed to inform, not to replace personalised medical advice.

References

1. Hormonal Influences on Skeletal Muscle Function in Women across Life Stages: A Systematic Review. MDPI, 2024. mdpi.com
2. Kanellakis S, et al. Changes in body weight and body composition during the menstrual cycle. American Journal of Human Biology, 2023. onlinelibrary.wiley.com
3. Menstrual cycle phase does not affect whole body peak fat oxidation rate during a graded exercise test. Journal of Applied Physiology, 2020. journals.physiology.org
4. The Impact of Female Hormones on Muscle Strength. Hormone Health Clinic, Minter et al. minteretal.com

This article is educational and not a substitute for medical advice. Speak with a qualified clinician before making significant changes to diet, training, supplementation or medication.