A uniboob is what it sounds like: the two breasts appear to merge into one continuous mound across the chest, with no visible gap or cleavage between them. The medical term is symmastia, from the Greek words for “together” and “breast.” Instead of two distinct breast shapes separated by a flat strip of skin over the breastbone, a bridge of skin, fat, or breast tissue stretches across the midline, creating a single connected shape.
How It Actually Looks
The defining visual feature is the loss of the intermammary cleft, the natural valley between the breasts where skin normally lies flat against the sternum. In symmastia, that space fills in. The skin over the breastbone lifts away from the chest wall, creating a web or hammock of tissue that connects one breast to the other. From the front, the breasts form a figure-eight or a single wide mound rather than two separate rounds.
The severity varies widely. In mild cases, there’s just a thin web of skin stretching across the midline, and the two breasts are still mostly distinguishable. In more pronounced cases, actual breast tissue extends across the sternum, and the breasts look fully merged. The overall shape has been described clinically as an “eight-shaped footprint” when viewed from below, because the two breast bases overlap at the center.
Another hallmark is what surgeons call “tenting.” When a person with symmastia lies on their back, the skin over the breastbone may lift up like a tent rather than settling flat against the chest. This happens because the tissue bridge connecting the breasts pulls upward as gravity draws the breasts to either side.
Congenital vs. Post-Surgical Uniboob
Symmastia comes in two forms, and they look somewhat different.
The congenital form is something a person is born with (or develops during puberty). It results from breast tissue growing across the midline during development. The appearance ranges from an empty skin web to a full confluence of breast tissue over the sternum. The cause isn’t well understood, though genetic factors have been theorized. Congenital symmastia is rare, and the breasts on either side are usually symmetric.
The acquired form happens after breast augmentation and is more common. When implants are placed behind the chest muscle, the surgeon creates a pocket for each implant. If the inner edge of that pocket is dissected too aggressively toward the sternum, the muscle fibers that normally anchor to the breastbone can detach. This allows one or both implants to drift toward the center, and in some cases the two implant pockets merge into a single space. The result is implants that sit too close together or appear to touch, with skin tenting off the breastbone between them. A survey of plastic surgeons found that about 85% of symmastia consultations involved patients who had previously undergone submuscular (behind-the-muscle) breast augmentation.
In implant-related cases, there are actually two subtypes. In one, the two implant capsules physically connect across the sternum, so the implants can shift freely between sides. In the other, there’s still a thin tissue bridge between the capsules, but the skin has still lifted off the breastbone, creating the uniboob appearance even though the implants remain in separate pockets.
What Causes It After Surgery
Several surgical factors contribute to acquired symmastia. The most commonly cited cause is over-dissection of the inner attachments of the chest muscle to the sternum. When those fibers release, there’s nothing holding the implant’s inner edge in place. Other contributors include choosing an implant with a base width too large for the patient’s chest, placing oversized implants, and disrupting the thin but important layer of connective tissue (the midsternal fascia) that helps keep the skin adhered to the breastbone.
Prevention comes down to precise surgical technique. Surgeons aim to maintain at least 3 centimeters of space between the inner edges of the two implant pockets, choosing implants whose base diameter fits the patient’s anatomy rather than maximizing size. Gentle dissection of the inner pocket, done under direct vision, helps preserve the midline attachments that prevent implant migration.
How It’s Corrected
Symmastia doesn’t resolve on its own, and correction requires surgery. For congenital cases, simply reducing breast tissue isn’t enough because the two core structural problems, the web between the breasts and the blunted crease beneath them, need to be addressed separately. Surgical repair typically involves removing the web tissue, re-establishing the natural fold under each breast, and reattaching the skin to the breastbone.
For implant-related symmastia, correction involves rebuilding the inner wall of each implant pocket. The surgeon closes off the over-dissected space with internal sutures, essentially recreating the boundary that should have been left intact. In many cases, the original implants are swapped for ones with a smaller base width, sometimes just 1 centimeter narrower, to reduce the inward pressure that contributed to the problem.
Recovery after repair involves wearing a specialized compression bra that presses the skin firmly against the sternum. This keeps the tissue in contact with the breastbone while it heals and reattaches. Counterintuitively, push-up style bras are sometimes recommended because they press the breasts inward, creating a zone of zero tension over the midline repair, which promotes stronger healing. The bra also prevents the breasts from falling to the sides when lying down, which would pull the repaired skin away from the chest wall like a tent and risk the tissue bridge reforming.
How Common It Is
Congenital symmastia is genuinely rare. The acquired form after breast augmentation is more frequent, though still uncommon relative to the total number of augmentation procedures performed each year. In a survey of board-certified plastic surgeons, respondents reported seeing an average of about 2 consultations per year for acquired symmastia. Most of those cases originated from prior surgeries performed by other surgeons, suggesting it’s more a consequence of technique than an inherent risk of augmentation itself.