Bone marrow is found inside your bones, concentrated in flat bones like the pelvis, sternum, ribs, and vertebrae. In adults, the posterior iliac crest (the back of your hip bone) is the primary site where doctors extract marrow for both diagnostic biopsies and donor transplants. It’s the largest, most accessible reservoir of the blood-forming marrow your body produces.
Where Bone Marrow Lives in Your Body
Your skeleton contains two types of marrow. Red marrow is the active, blood-producing type that generates red blood cells, white blood cells, and platelets. Yellow marrow is mostly fat and serves as an energy reserve. The balance between them shifts dramatically over your lifetime.
In children under five, red marrow fills virtually every bone in the body. As you grow, it gradually converts to yellow (fatty) marrow in a predictable pattern: the long bones of your arms and legs lose their red marrow first, starting in the central shafts during childhood and finishing in the ends of the thighbone by around age 24. By adulthood, red marrow is limited to the central flat bones: your skull, collarbones, sternum, ribs, shoulder blades, spine, and pelvis, plus the very tops of your upper arm and thigh bones.
The pelvis holds the single largest concentration of red marrow in the adult body, which is why it’s the go-to site for collection.
The Pelvis: Primary Collection Site
For both diagnostic biopsies and transplant donations, doctors almost always draw marrow from the posterior superior iliac spine, a bony landmark at the back of your pelvis that you can feel just above your buttocks. The bone here is thick enough to access safely, rich in marrow, and far from vital organs.
The procedure starts with local anesthesia injected into the skin, tissue, and bone surface. After about 10 minutes for the numbing to take full effect, the doctor makes a tiny incision (often just a nick from a small blade or a large-gauge needle) and inserts a hollow needle, typically 13 to 15 gauge, through the hard outer shell of bone and into the spongy interior. Using a rotating motion, they advance the needle until it sits just inside the bone’s inner surface. A syringe then draws out about 4 milliliters of marrow at a time. The needle is repositioned and the process repeats until enough marrow has been collected.
For a diagnostic biopsy, only a small sample is needed. For a transplant donation, the procedure is done under general or regional anesthesia, and the surgeon may collect a much larger volume from multiple puncture sites along both sides of the pelvis.
The Sternum: A Less Common Alternative
The breastbone (sternum) is occasionally used for marrow aspiration, particularly in some European countries. It offers easy access for a quick diagnostic sample, but it carries higher stakes. The sternum is thin, and the heart and major blood vessels sit directly behind it. Serious complications are rare: in an analysis of nearly 55,000 sternal biopsies, complications occurred in only 26 cases, with bleeding accounting for about half. No heart or aortic injuries were recorded in that series. Still, the overall complication rate of roughly 0.8% and the theoretical risk of puncturing through to the chest cavity make the pelvis the preferred site in most medical centers.
Collecting Stem Cells Without Surgery
Not all bone marrow donation requires a needle in your hip. The most common method for transplant donation today is peripheral blood stem cell (PBSC) collection, which pulls stem cells from your bloodstream instead of your bones. For several days before the procedure, you take injections of a medication that coaxes stem cells out of the marrow and into circulating blood. On collection day, blood is drawn from one arm, passed through a machine that filters out stem cells (a process called apheresis), and returned through the other arm.
The trade-off is different side effects. PBSC donors typically experience bone pain and fatigue from the pre-collection medication, while surgical marrow donors deal with soreness at the hip puncture sites. Median recovery for PBSC donation is about one week. Surgical marrow donors report pain and discomfort for roughly three weeks, though complications lasting beyond a month are unusual. In a series of over 1,100 surgical collections at one major center, only seven donors experienced pain past one month, and all resolved within six months.
What Happens After Surgical Donation
If you donate marrow surgically, you’ll typically avoid lifting anything over 20 pounds for two weeks. Standard anti-inflammatory painkillers like ibuprofen are off-limits for the first three days because they can promote bleeding at the harvest sites. Most donors take an iron supplement for about a month to help rebuild their red blood cell supply. The soreness at the collection site feels like a deep bruise in the lower back or hip area, and it fades gradually.
Older donors, women, and those with preexisting health conditions may take longer to bounce back. Related donors (those giving marrow for a family member) sometimes report a slower return to their baseline quality of life, possibly influenced by the emotional weight of the situation.
Why Doctors Need Bone Marrow Samples
Beyond transplant donation, bone marrow biopsies are a critical diagnostic tool. A blood test can flag abnormal cell counts, but only a marrow sample reveals what’s happening at the source. Conditions that often require a biopsy for definitive diagnosis include leukemia, lymphoma (both Hodgkin and non-Hodgkin), multiple myeloma, aplastic anemia, myelodysplastic syndromes, myelofibrosis, and polycythemia vera. Doctors also use biopsies to investigate unexplained low blood counts, certain systemic fungal infections, and a protein-deposit disorder called amyloidosis.
In each case, examining the marrow under a microscope shows whether the body is producing blood cells normally, overproducing abnormal cells, or failing to produce enough. No imaging scan or blood draw can provide that level of detail.
Finding a Donor Match
When someone needs a bone marrow or stem cell transplant and no family member is a match, doctors search international registries that now include more than 42 million potential donors and over 760,000 stored cord blood units. Matching depends on specific immune system markers, and the odds of finding a close match vary significantly by ethnicity. Patients of non-European ancestry have historically faced much lower chances of finding a suitable donor, a gap that registries are actively working to close by broadening acceptable match criteria and recruiting more diverse volunteers.