An intrauterine device (IUD) is a small, T-shaped form of long-acting reversible contraception inserted into the uterus. This highly effective device comes in two primary types: the hormone-free copper IUD and the hormonal IUD, which releases a progestin hormone. Concerns about whether an IUD causes weight gain are common among people considering this method. Examining the clinical evidence and biological mechanisms clarifies the relationship between IUD use and changes in body weight.
Scientific Evidence on Weight Gain and IUDs
The medical consensus from large-scale clinical trials indicates that IUDs do not cause significant weight gain. Studies comparing hormonal IUD users with non-hormonal copper IUD users consistently find that both groups gain a similar amount of weight over time. For instance, users of both devices gained an average of approximately 8.8 pounds over periods up to 10 years.
Since the copper IUD contains no hormones, it has no plausible biological mechanism for increasing body mass. Therefore, weight gain observed in copper IUD users is attributed to factors unrelated to the device itself. For hormonal IUDs, such as Mirena or Kyleena, weight change is listed as a possible side effect in a small fraction of users; one clinical trial found less than 6% of participants reported an increase in weight.
When weight gain is reported with a hormonal IUD, it is frequently linked to temporary water retention or bloating caused by the progestin. This perceived weight gain is minimal, often around five pounds, and tends to subside within the first few months as the body adjusts. Overall, weight changes experienced by IUD users are comparable to the natural weight changes seen in people not using contraception.
Hormones, Localized Action, and Metabolic Effects
Hormonal IUDs release a synthetic progestin called levonorgestrel, the same type of hormone found in some birth control pills. The delivery method is highly localized, which minimizes the IUD’s effect on metabolism. The hormone is released directly into the uterine cavity, where it primarily acts to thicken cervical mucus and thin the uterine lining to prevent pregnancy.
This targeted action means that only a very low dose of levonorgestrel is absorbed systemically into the bloodstream. The resulting blood concentration is substantially lower than the levels produced by daily oral contraceptive pills. Oral contraceptives deliver a higher, systemic dose that must circulate throughout the entire body, making them more likely to affect appetite regulation or fat deposition.
Because the hormonal IUD maintains low systemic exposure, its influence on body-wide metabolic processes is negligible. This localized mechanism is a distinction from other hormonal methods, such as the contraceptive shot, which delivers a much higher hormone dose that circulates throughout the body and has a stronger association with weight change. The minimal systemic absorption limits the likelihood of the IUD causing a change in body fat or an increase in weight.
Identifying Other Causes of Weight Fluctuation
Weight gain is a common life event for many adults during their reproductive years, irrespective of their chosen contraceptive method. The time frame during which people use an IUD naturally coincides with a period of life where weight tends to increase due to various factors.
One common factor is the natural slowing of metabolism that occurs with age. As people move through their twenties and thirties, their caloric needs gradually decrease, and maintaining a stable weight often requires conscious changes to diet or exercise routines. Weight gain can also be a result of lifestyle shifts, such as changes in diet, physical activity levels, or increased stress that accompanies professional and family life.
For those switching from a systemic hormonal method, such as the pill, any previous fluid retention masked by that method may resolve. Conversely, a change in lifestyle that coincided with the IUD insertion could be the actual cause of a weight change. The fact that both hormonal and copper IUD users gain weight at similar rates strongly suggests that the weight change is due to non-contraceptive factors.