Hormonal Contraceptives and Body Composition: How the Pill, Injection, and IUD Affect Fat, Muscle, and Bone

If you have ever been told that the pill will make you gain weight, or worried that your IUD might be affecting your muscle tone, you are not alone. Concerns about hormonal contraceptives and body composition are among the most common reasons women seek a DEXA scan at our Harley Street clinic. The reality, as the clinical evidence shows, is more nuanced than most advice suggests. Different contraceptive methods have markedly different effects on fat mass, lean mass, and bone density, and a DEXA scan is the most precise way to measure what is actually happening in your body.

The belief that combined oral contraceptives (COCs) cause weight gain is one of the most persistent myths in women’s health. A 2014 Cochrane systematic review of 49 trials found no consistent evidence that COCs increase body weight or fat mass compared with placebo or non-hormonal methods. Placebo-controlled trials are the gold standard here because women who expect to gain weight on the pill often do, regardless of what they are actually taking.

What COCs can cause is temporary fluid retention. The oestrogen component stimulates the renin-angiotensin-aldosterone system, encouraging sodium and water reabsorption. This may add one to two kilograms in the first few months, which often resolves as the body adjusts. Newer pills containing drospirenone (a progestogen with mild diuretic properties) were designed partly to counter this effect.

DEXA scanning is particularly useful here because it distinguishes lean mass (which includes water) from fat mass. A woman who gains 1.5 kg of fluid will see her lean mass rise with no change in fat, a finding that would be invisible on bathroom scales but clearly visible on a body composition DEXA scan.

Depot medroxyprogesterone acetate (DMPA), marketed as Depo-Provera, is the contraceptive method most clearly linked to genuine fat accumulation. A study published in the American Journal of Obstetrics and Gynecology followed women for 36 months and found that DMPA users gained significantly more fat mass than those using non-hormonal methods, with increases concentrated in both upper and lower body fat stores.

The mechanism appears to involve appetite stimulation via central progestogen receptors and possible effects on insulin sensitivity. Unlike the pill’s fluid-related changes, DMPA-associated weight gain is real adipose tissue, and it tends to persist for as long as the injection continues.

A DEXA scan before starting DMPA and again at six to twelve months would show whether fat mass has genuinely increased and, importantly, where that fat is being deposited. A rise in trunk fat visible on DEXA carries different metabolic implications than a general increase spread evenly across the body.

Hormonal IUDs (such as the Mirena or Kyleena), the subdermal implant (Nexplanon), and progestogen-only pills release relatively low doses of progestogen that act primarily at the local tissue level. The systemic hormonal exposure is substantially lower than with DMPA or combined pills.

Large cohort studies, including a 2016 analysis in Obstetrics and Gynecology, found no clinically significant difference in weight or body composition between hormonal IUD users and non-users over five years. The implant shows similarly neutral results in most trials, though a small subset of users do report subjective weight gain.

For women using these methods who are concerned about body composition, a baseline DEXA body composition scan before insertion and a follow-up at 12 months provides objective data. If fat mass has not changed, the concern can be put to rest with evidence rather than guesswork.

While most discussions about contraceptives and body composition focus on fat and weight, bone density is an equally important consideration that receives far less attention. DMPA is the method most clearly associated with bone mineral density (BMD) loss. The Royal College of Obstetricians and Gynaecologists (RCOG) notes that DMPA use is associated with a small loss of BMD, which is usually recovered after discontinuation but which may be of concern for women with additional risk factors for osteoporosis.

Combined oral contraceptives, by contrast, appear to have a neutral or mildly protective effect on BMD in most age groups. However, very early use (before peak bone mass is achieved, typically around age 25) may slightly reduce peak BMD attainment, according to a study in the Journal of Clinical Endocrinology and Metabolism.

A bone density DEXA scan is the clinical standard for measuring BMD with precision. For women on long-term DMPA (more than two years), or those with a family history of osteoporosis, a DEXA scan provides a T-score and Z-score that quantify exactly where bone density stands relative to healthy reference populations.

Research into the effects of hormonal contraceptives on skeletal muscle and exercise performance is still emerging, but what exists is reassuring. A 2020 systematic review in Sports Medicine found no consistent evidence that combined oral contraceptives impair muscle hypertrophy, strength gains, or aerobic performance in women who train regularly.

Some studies have suggested that the synthetic oestrogen and progestogen in COCs may slightly blunt the acute anabolic response to resistance training, but the clinical significance of this effect appears minimal over training cycles of 10 weeks or more. Women on the pill who train consistently still build muscle and gain strength at rates comparable to non-users.

DEXA scanning is the most precise method available for tracking lean mass changes over time. Unlike bioimpedance scales (which estimate lean mass from electrical resistance and are heavily influenced by hydration), DEXA directly measures tissue density in each body region. A woman who wants to know whether her contraceptive is affecting her training outcomes can compare lean mass in her arms, legs, and trunk across two scans taken several months apart, giving her objective data rather than gym-floor speculation.

Most women considering contraceptive options receive advice based on population averages. Your GP might say “the pill doesn’t cause weight gain” or “the injection might add a few pounds,” and both statements are broadly true. But population averages tell you nothing about what is happening in your body specifically.

A DEXA scan measures three things that matter for this decision: fat mass (total and regional), lean mass (including muscle and water), and bone mineral density. Together, these measurements let you and your doctor distinguish between genuine fat gain, temporary fluid retention, muscle loss, and bone density changes, each of which has different implications and different solutions.

For example, a woman who starts DMPA and gains 3 kg over six months might assume it is all fat. A DEXA scan could reveal that 1.5 kg is fat (concentrated in the trunk), 1 kg is fluid (visible as increased lean mass with no change in muscle cross-section), and 0.5 kg is measurement variability. That breakdown changes the conversation with her doctor entirely. If you have been considering changes to your contraceptive method based on how your hormonal health is affecting your body composition, a DEXA scan provides the objective evidence to guide that decision.

Does the pill make you gain weight?
The best available evidence says no. A Cochrane review of 49 trials found no consistent link between combined oral contraceptives and weight gain. Some women experience temporary fluid retention (1 to 2 kg) in the first few months, which typically resolves. A DEXA scan can confirm whether any change is fluid or fat.

Which contraceptive is most likely to cause fat gain?
Depot medroxyprogesterone acetate (Depo-Provera) is the only method with strong evidence of genuine fat mass increase. Other hormonal methods, including the pill, hormonal IUDs, and the implant, show no consistent fat gain in clinical trials.

Can contraceptives affect bone density?
Yes. DMPA is associated with small decreases in bone mineral density during use, which usually recover after stopping. Combined pills appear neutral or mildly protective. A bone density DEXA scan is the standard way to monitor this.

Will the pill affect my gym progress?
Current evidence suggests combined oral contraceptives do not significantly impair muscle growth or strength gains in women who train regularly. DEXA scanning can objectively track lean mass changes over time if you want certainty.

How often should I get a DEXA scan if I am on hormonal contraception?
For most women, a baseline scan before starting a new method and a follow-up at 12 months is sufficient to detect any meaningful changes in fat mass, lean mass, or bone density. Women on long-term DMPA may benefit from more frequent bone density monitoring.

Whether you are starting a new contraceptive, switching methods, or simply want to know what your current method is doing to your body, a DEXA scan gives you the data to make an informed choice. At DEXA London, our Harley Street clinic provides clinical-grade body composition and bone density scanning with results reviewed by a doctor.

Book your DEXA scan today and get a clear, objective picture of your fat mass, lean mass, and bone density, so your contraceptive decisions are guided by evidence, not anxiety.