Back pain is more common in women than men, and the reasons go beyond posture or heavy lifting. About 61% of people reporting low back pain in a given week are women, compared to 39% men. Your girlfriend’s back pain could stem from her menstrual cycle, how her body carries weight, stress on her muscles and joints, or something internal that needs medical attention. The cause depends a lot on where it hurts, when it started, and whether it comes and goes or stays constant.
Menstrual Cycle Pain Is the Most Common Culprit
If her back pain shows up around her period and fades afterward, it’s almost certainly related to menstruation. During a period, the body releases chemicals called prostaglandins that force the uterus to contract and shed its lining. When the body produces too many of these chemicals, the contractions become stronger and more painful, and that pain radiates into the lower back. This is one of the most common prostaglandin-related conditions, and it affects a huge number of women every month.
This kind of back pain sits low, right around the belt line or below, and often comes with cramping in the front of the abdomen. It tends to be worst on the first day or two of a period and then eases up. Over-the-counter anti-inflammatory pain relievers work well here because they reduce prostaglandin production directly. A heating pad on her lower back can also help, though the research on heat versus cold for back pain is mixed. Both seem to provide some relief, but heat is generally what most people prefer for period-related discomfort.
Pregnancy Changes the Spine Itself
If your girlfriend is pregnant, or recently was, her back pain has a clear biological explanation. During pregnancy, the body ramps up production of a hormone called relaxin, which loosens the ligaments in the pelvis and spine to make room for delivery. Relaxin levels rise sharply by the end of the first trimester and stay elevated until birth. The trade-off is that looser ligaments mean less joint stability, and that instability in the pelvis and lower spine creates pain.
On top of the hormonal changes, the growing weight of the baby shifts her center of gravity forward, forcing the muscles along her spine to work harder just to keep her upright. Estrogen and progesterone levels also climb during pregnancy and may contribute to ligament looseness. This combination of unstable joints and overworked muscles is why somewhere between half and three-quarters of pregnant women report significant back pain at some point. The pain usually centers in the lower back or deep in the pelvis, and it gets worse with standing, walking, or sitting in one position too long.
Muscle Strain and Posture Issues
Outside of hormonal causes, the most straightforward explanation is muscle strain. Sitting at a desk all day, sleeping in an awkward position, carrying a heavy bag on one shoulder, or starting a new workout routine can all trigger back pain that lasts days or even weeks. This kind of pain usually feels like stiffness or a dull ache that worsens with certain movements and improves with rest.
There’s a persistent belief that breast size plays a major role in back pain, and it’s worth addressing directly. Larger breasts do shift the center of gravity forward and can increase the curve of the upper back, putting extra strain on the muscles between the shoulder blades. Bra straps act like pulleys over the shoulders, effectively doubling the downward pull on the upper back muscles. But when researchers actually studied this in young women, they found negligible relationships between breast size and pain intensity, and between bra fit and pain. That doesn’t mean it’s never a factor, but it’s less of a universal cause than many people assume. Tight bra straps on any body size can cause shoulder and upper back discomfort.
Kidney Infections and Internal Causes
Not all back pain comes from muscles or joints. If your girlfriend’s pain is on one side, sits higher up near her ribs in the back, and comes with fever, nausea, or painful urination, it could be a kidney infection. Women are far more prone to urinary tract infections than men, and when a UTI travels up to the kidneys, it causes a distinctive pain in the lower back or flank area. This is different from muscle pain because it doesn’t change with movement. Bending or twisting won’t make it better or worse. It’s constant, deep, and often accompanied by feeling genuinely sick.
Endometriosis is another internal cause worth knowing about. Tissue similar to the uterine lining grows outside the uterus, sometimes on structures near the spine, and causes chronic pain that can be debilitating. This pain often overlaps with periods but may also persist throughout the month. Ovarian cysts can produce similar one-sided back pain that comes on suddenly.
Stress and Emotional Tension
Chronic stress causes real, physical back pain. When you’re anxious or under pressure, the muscles along the spine tighten and stay tight for hours or days. Over time, this sustained tension creates knots, stiffness, and aching, particularly in the upper back and between the shoulder blades. Women report higher rates of stress-related musculoskeletal pain than men, partly because of hormonal fluctuations that influence how the body processes pain signals. If her back pain seems to flare during stressful weeks and ease during vacations or calm periods, this connection is worth paying attention to.
Signs That Need Medical Attention
Most back pain resolves on its own within a few weeks. But certain symptoms signal something more serious. Pain combined with fever suggests infection. Pain that wakes her from sleep, comes with unexplained weight loss, or doesn’t respond to any pain relief could indicate something beyond a muscle issue.
The symptoms that warrant urgent care are more specific: numbness or tingling in the groin area (sometimes called saddle numbness), sudden difficulty controlling the bladder or bowels, or progressive weakness in both legs. These are signs of nerve compression in the lower spine, and they require immediate evaluation. Back pain that started after a fall, accident, or injury also deserves a medical look, especially if it’s severe or getting worse rather than better over the first few days.
What You Can Actually Do to Help
If her pain is muscular or period-related, the basics work well: a heating pad, gentle stretching, and anti-inflammatory pain relief. Encourage movement rather than bed rest. Lying still for days actually makes most back pain worse because the muscles stiffen further. Walking, even short distances, keeps things loose.
For period-related pain specifically, applying heat to the lower back or abdomen is a go-to approach. If the pain is in the upper back between the shoulder blades, gentle massage and posture checks (is her desk setup forcing her to hunch?) are more useful. For pain that recurs monthly, tracking when it shows up relative to her cycle helps distinguish hormonal pain from something structural.
If the pain has lasted more than four to six weeks without improvement, keeps coming back in the same spot, or is intense enough to limit daily activities, that’s the point where imaging or a professional evaluation adds real value. Most back pain doesn’t need an X-ray or MRI early on, but persistent or worsening pain does benefit from a closer look.