TORONTO - Spoiler alert: If you haven't seen Episode 4 of the current season of Downton Abby and you plan to, you don't want to continue reading.

If you did see the heart-wrenching hour of TV's favourite costume drama, you may be wondering about the cruel condition that killed the youngest and sweetest of the trio of Crawley sisters, Lady Sybil.

Sybil — who defied tradition and her family to become a nurse in the First World War and marry the family's chauffeur — died shortly after giving birth to her first child.

The mounting warnings of Dr. Richard Clarkson, the local physician under whom Sybil received her nursing training, are ignored by the high society obstetrician brought to Downton for the birth, Sir Philip Tapsell.

Despite clear signs Sybil is becoming mentally altered, Tapsell insists all is going well with Sybil's delivery — until her condition takes a dramatic and irreversible turn for the worse. She dies in a series of painful paroxysms.

So what killed her?

During the episode, Clarkson warns Sybil may be suffering from pre-eclampsia, a condition which can lead to eclampsia, a potentially fatal complication of pregnancy.

What causes the condition is still not entirely understood, says Dr. Jon Barrett, a high risk pregnancy specialist at Toronto's Sunnybrook Health Sciences Centre.

The word, which is pronounced Ee-CLAMP-see-ah, comes from the Greek word for "flash," Barrett says. That's because it develops quickly, and without warning in the days before blood pressure readings and the other trappings of modern medicine. Women in labour would suffer seizures and often die.

With advances in medicine, science now knows there is an early phase to the disease (hence pre-eclampsia) which is marked by a trio of classic symptoms: fluid retention (Sybil had swollen ankles); headaches and high blood pressure.

The condition is not rare and doctors who care for pregnant women know to be on the lookout for it, Barrett says.

"Anybody who deals with pregnancy enough has this disease in the forefront of their mind, because it's common and it's very dangerous if you don't recognize it and treat it."

Scientists are still trying to figure out why it develops.

Barrett says it's thought that the roots of the condition are set down early in a pregnancy, when the placenta attaches to the uterus. For some unknown reason, toxins are released into the blood of the pregnant woman, triggering problems with numerous organs.

Some people are more likely to develop it than others. The risk runs in families. First-time mothers are at higher risk. If a woman has pre-eclampsia with her first pregnancy, her risk will drop with the next pregnancy — if the father of her second child is the father of her first. But if she has children by different partners, the risk doesn't fall to the same degree, Barrett says.

Rates vary in different parts of the world. In Africa, the risk of it developing is about three or four times higher than in North American. And it is more common in births that take place in the spring and fall, a feature that is currently unexplained. "It's really a bizarre disease," Barrett says.

The only surefire cure for eclampsia is ending the pregnancy — delivering the child. "If you deliver the baby, the condition goes away. Usually quite rapidly, actually," Barrett says.

In the days of "Downton" — the episode set in about 1920 or so — that would have been an option, says Dr. Tom Baskett, a professor emeritus of obstetrics at Dalhousie University in Halifax and an obstetrical historian.

Baskett, an avid "Downton" watcher, says in the early 1900s about one in four women who developed eclampsia died from the disease, often from a massive bleed in the brain caused by high blood pressure.

The episode was a relatively accurate portrayal of the condition, Baskett says, though he has some bones to pick with the script writers.

The conflict between the two doctors essentially represented the two schools of thought at the time about how to treat eclampsia.

Some clinicians believed inducing labour or delivering by Cesarean section — which Clarkson advocated — was the way to go. C-sections were still rare at that time but were performed, Baskett says. And births were induced by forcing the cervix — the opening to the uterus — to expand.

But others clinicians would have espoused the approach of Tapsell, insisting that C-sections were too dangerous and that the mother needed quiet and rest to recover. Doctors in this camp might have used bloodletting or enemas to purge women, or have sedated them with morphine, Baskett says.

Where Baskett questions the script relates to the timing of Sybil's convulsions and the passivity of the doctors once she progressed to that state.

"They (the convulsions) can occur for the first time post-partum, as they did with her, but it is unusual," Baskett says.

It would be more common to convulse during the birth. But the birth was not shown and Sybil only lost consciousness after a touching scene when she and her husband, Tom, discuss their family's future.

"That really doesn't make sense," Baskett says. "But you've got to keep the punters happy in the stall." (Punters is British slang for customers, in this case viewers.)

Baskett also argues doctors would not have done nothing to help a patient in the grips of violent convulsions. They might have placed a wooden spatula between her teeth to keep her from biting her tongue, or tried to ensure her airway remained open.

"The fact that they both just kind of turned their backs and said 'Oh, well, there's nothing we can do' ... really didn't ring true, particularly for Clarkson," he says.

"They should have done something ... to help her breathe. It was an odd ending, in a sense."