I turned to our patient. “Mrs. Watson, why are you on birth control pills?”

With the tone adults use for dense 2-year-old, she explained, “Um, that would be to avoid having babies, doctor.”

Her husband, a hearty Brit, guffawed. I laughed along with him.

“Sometimes they’re used to regularize menstrual cycles or treat severe cramps,” I replied, trying to sound medical.

“No, none of that,” she replied, her eyes still twinkling. “My doctor was thinking of taking me off them soon.”

I kept my peace. Dr. Singh continued: “No other medical problems. Currently on prednisone (a steroid) for a cat allergy. Works at a financial firm downtown.”

I stopped him. “How long have you been on the prednisone?” I asked her.

“Oh, a doctor in London prescribed it for my cat allergy. I’m only taking 10 milligrams a day. All better now,” she beamed.

That’s an odd dose, I thought.

Dr. Singh continued. “On exam, the right calf is swollen and tender. The thigh appears normal. No respiratory complains, oxygen saturation is normal, and the lungs sound clear.”

I pressed gently on Mrs. Watson’s calf.

“Ow!” she cried. “That does hurt.”

“I’m so sorry. Even a little pressure like that?” I asked. She nodded.

“Diagnosis?” I asked Dr. Singh. He shrugged. “Obvious: DVT.” If there was ever a disease of modern living, deep vein thrombosis (DVT) is it. A thrombosis is a clot formation. Unlike our arboreal primate cousins, Homo sapiens is built to roam vast stretches of savanna. When it comes to endurance running, we are among the best in the animal kingdom. The only tailless bipeds on the planet, we are long-distance-running machines, with the great veins in our oversize legs thriving on brisk blood flow and the pumping action of our muscles.

Stick us behind a desk or wedge us into economy class, however, and trouble looms: Blood in the veins pools, providing a stagnant milieu for our coagulation system – which works so beautifully to stanch cuts and gorings incurred in a hunt – to go awry. Add obesity and a dose of cigarettes and the lithe runners of the Serengeti Plain turn into champion clot formers. It is no accident that two of our top three killers are heart attacks and strokes.

And then three’s the role of another modern invention: birth control pills. These hormone-laden pills (containing estrogen and progestin) pretty much defined the sexual liberation of the ’60s, but the pills’ downsize – excess clotting – wasn’t widely recognized until the ’70s. Although lowering the dose of estrogen and progestin helped, second – generation pills still triple or quadruple the risk of DVT. Third generation pills appear to be even riskier.

With 100 million women on birth control pills world wide, the odds of a blood clot can be cast with some precision. A healthy woman in her twenties has less than a 1-in-10,000 chance per year of developing a DVT. Being on the pill ups the risk to 3 or 4 in 10,000. Age dramatically alters the equation: A woman over 40 has i-in-1,000 chance, and the pill has the same multiplier effect in this group. (To be fair, the risks of pregnancy far outweigh the risks of the pill, except in high-risk women.)

DVT risk factors don’t stack, they multiply. Take plane travel. The risk for a filler on oral contraceptives soars 14-fold. In one scary study out of Charles de Gaulle Airport in Paris, researchers performing on-site ultrasound found that among travelers who had spent more than eight hours in the air, 10 percent had symptomatic clots lurking in their leg veins.

Finally, there are genetic factors. The blood coagulation system is an intricate cascade of about 20 proteins, all activating or suppressing each other. (Female hormones seem to increase proclotting proteins and decrease inhibitors.) A clotting protein variant called factor V Leiden, present in 5 percent of Caucasian women, resists inhibitors proteins. Having factor V Leiden bumps up DVT risk 35-fold when combined with taking the pill. Add air travel and age over 40 and you’re approaching a 50 percent chance of getting a clot. That’s why I was questioning the use of the pill by the pond-hopping 51-year-old Mrs. Watson, with her DVT-prone kin.

Though irritating and painful, DVTs aren’t the killers. Pulmonary emboli are. These are clots that travel and end up in the lung. (An embolus is a clot that travels.) Clots in the legs can break off from the wall of a blood vessel, course though the heart, and wedge into the great vessels of the lungs, corking the blood flow out of the right ventricle. A pulmonary embolus (PE) is treacherous because the symptoms mimic colds, rib-muscle strains, or the wheeling of an asthma attack – and there’s often a bigger one close behind. Such clots kill 200,000 people a year, often without warning.

The treatment for DVT is anticoagulation. The classic medication is heparin – a drug discovered almost a century ago – which requires continuous IV dosing. Newer versions like Lovenox can be injected subcutaneously once a day at home. In a patient with an uncomplicated leg DVT, it’s a reasonable plan.

“So can I go home now?” Mrs. Watson pleaded.

“How about we see how extensive it is first?” I counted. She lay back down.

Dr. Singh wheeled the ultrasound machine over. Pressing the probe along Mrs. Watson’s thigh, we followed the femoral vein down to the knee. Blood vessels look like black circles on the screen, but with gentle pressure, a vein should wink flat. If a clot is there, it can’t do that.

“It collapses nicely down to the knee, “I said. “No clot in the thigh.”

Mrs. Watson sat up. “Can I go now?”

“Well,” I hedged, “the clot probably goes from the knee down. That’s harder to see. The vascular technician is gone for the day. We can’t do a complete study until tomorrow.”

Dr. Singh and I stepped out of the room. “We could give her a shot of Levenox and have her follow up with her doctor tomorrow, right?” he asked.

“Yes,” I replied. “Question is, does she have a silent PE? No respiratory symptoms now, but a number of DVT patients have them, and if she did, we’d admit.”

“Should we get a CT scan?”

“It’s a fair amount of radiation. Our suspicion is low, and the treatment’s the same. I’d hold off for now.”

Maybe it was the recent case of a 30-year-old on the pill who had flown from Paris and died of a massive PE, or maybe that calf just seemed too painful, but I decided to err on side of caution.

“It’s best if you stay, “I told Mrs. Watson. “They’ll do the normal ultrasound first thing tomorrow.”

Two days later, Dr. Singh found me. “Not just a DVT,” he announced. “Remember the cat allergy? The day after she arrived to London, she started wheezing. Told the doc there about her cat allergies. They’d never involved wheezing before, but he went with that and put her on steroids. Over the next few days it got better. Then she flew back to New York and the leg flared up. After we admitted her, the attending upstairs didn’t buy the allergy story. CT scan showed a PE.”

I smacked my forehead. “Of course. Fifty-one-year-old on birth control pills with family history of DVT flies to London and develops respiratory complains.”

“Good thing we admitted her,” Dr. Singh said soothingly. “We’ll put her on a blood thinner for a few months. After that, we’ll check to see if she needs to continue. I guess her doctor will stop the pill now.”

“Yes,” I said. “It’s about time.”

Tony Dajer is acting director of the emergency department at New York Downtown Hospital. The cases described in Vital Signs are real, but patients’ names and other details have been changed.

Article printed originally by “Discover” magazine December 2007 issue.