On “Downton Abbey,” Lady Sybil and Tom Branson celebrate the birth of their child. Soon after, the mother dies. (Joss Barratt/JOSS BARRATT)

Most of the viewers of “Downton Abbey” who saw Lady Sybil die in childbirth Sunday night were left with a long list of questions accompanying their shock and grief.

What did she die of? Was the diagnosis clear? Could she have been saved? In a show with punctilious art direction, how realistic was this death?

Lady Sybil died of eclampsia, a condition of unknown cause that used to be called “toxemia of pregnancy.” (Dr. Clarkson, the family doc pushed aside in favor of silk-stocking-trade physician Sir Philip, used the term at one point.) It is most common in the late stage of first pregnancies. Sybil Branson, 24, was nearing the due date for her first child, so that part’s right.

Eclampsia, strictly speaking, isn’t present until the woman has a seizure. By that time, the patient, the doctors (and the baby, if not yet born) are in deep trouble. The job is to diagnose what’s happening before then, when the condition is known as preeclampsia.

In Sybil’s case, there were a lot of clues.

The hallmark of preeclampsia is elevated blood pressure. Taking the blood pressure with stethoscope and inflatable cuff was about the only test a doctor could perform on a woman delivering at home. Sybil’s pressure appears to have been up.

Equally important, however, were the other signs and symptoms she showed. As the blood pressure rises, headache and nausea are common. As it gets worse, a woman often gets delirious. As it gets even worse, it can lead to seizures — eclampsia — stroke, coma and death.

Long before then — when there was still time to diagnose her condition correctly — Sybil had “peripheral edema” — the fancy term for swelling of the legs.

Preeclampsia damages the cells lining veins and arteries; they leak fluid into the tissues. Because of gravity, the most common place to notice this is the ankles. Dr. Clarkson saw it early. Sir Philip dismissed the finding, saying the youngest of the Grantham daughters might just have thick ankles. (Not likely!) Message: Have a family doctor who knows you.

Preeclampsia also causes a protein called albumin to spill into the urine rather than stay in the blood. Dr. Clarkson suggests testing Sybil’s urine. He may have had a chemistry set in his bag. Or he could have gotten some idea just by shaking up a jar of it. Urine full of protein foams. In any case, he reports elevated protein.

The treatment of preeclampsia is delivery. If Lord Grantham had listened to the country doc and sent his daughter to the hospital for a Caesarean section, would she have lived? Well, of course we’ll never know.

One thing is clear, however. Having seizures and dying after delivery is unusual. In a study published last year of 39,000 births in a hospital in India, 1 percent of the women had eclampsia, and only one-quarter of those occurred after delivery.

The mechanism of Sybil’s death was a little unclear.

Seizures can cause a pause in breathing. But people rarely suffocate from them. The doomed young woman may have had a massive bleed in her brain. Before she relaxed in the dusky pallor of death, she had what looked like “decerebrate posturing” — an arching of the neck that occurs as the brain is squeezed through the base of the skull.

Nobody knows what caused preeclampsia in the early 1920s or causes it now. It appears to be an out-of-control state of inflammation. There is some evidence it may be a reaction against “paternal factors.” That, in turn, has led to the idea of the “dangerous father” as the cause of the condition. Sybil’s husband and the father of their daughter is Tom Branson, the former Grantham chauffeur and Irish revolutionary who can’t go home. For purposes of drama, it was a fitting way for her to die.

There was another message, too. Beware the “VIP syndrome.” Among Sir Philip’s mistakes was his apparent desire not to discomfit the Grantham family by sending Sybil to the hospital. However, doctors who treat you differently because you are a very important person can be dangerous to your health.