A new study suggests that smokers who develop preeclampsia during pregnancy are at particular risk of suffering the complications associated with the disorder - including preterm delivery, low birth weight and stillbirth.

The findings may not sound surprising. But they actually present something of a paradox, as past studies have linked smoking to a reduced risk of developing preeclampsia in the first place.

Preeclampsia is a syndrome marked by a sudden increase in blood pressure after the 20th week of pregnancy and a buildup of protein in the urine due to stress on the kidneys. Most women with preeclampsia deliver a healthy baby, but the condition can develop into a life-threatening condition called eclampsia, which can cause seizures or coma.

Preeclampsia can also slow the growth of the fetus and increase the risks of preterm birth, placental abruption - where the placenta separates from the uterine wall before delivery of the newborn, potentially leading to heavy bleeding that can be life-threatening to mother and child.

A number of studies have found that pregnant smokers are less likely than non-smokers to develop preeclampsia, for reasons that are not yet clear.

But these latest findings, reported in the American Journal of Obstetrics & Gynecology, suggest that once preeclampsia develops, smoking exacerbates the risk of complications, according to Elizabeth Miller and colleagues at the Ottawa Hospital in Canada.

Using a database with information on more than 300,000 births between 2004 and 2006, the researchers found that women who smoked during pregnancy had a slightly lower rate of preeclampsia - 1.2 percent, versus 1.5 percent among non-smokers.

But among women with preeclampsia, smokers were more likely to have serious complications.

For example, 9 percent of smokers had a baby who was small for gestational age - smaller than the norm for the baby’s sex and the week of pregnancy during which he or she was born. That compared with 5 percent of non-smokers who had preeclampsia.

Similarly, just over 3 percent of smokers with preeclampsia suffered placental abruption, versus 0.7 percent of non-smokers with the condition. The rates of stillbirth were 1.8 percent and 0.9 percent, respectively.

When Miller’s team factored in variables like the mother’s age and how many pregnancies she’d had (preeclampsia is more common in first-time mothers), smokers with preeclampsia were three to six times more likely to have a stillbirth, preterm delivery, placental abruption or undersized newborn than non-smokers without preeclampsia.

Non-smokers with preeclampsia also had elevated risks, but not of the magnitude seen among smokers.

Despite the fact that smoking is linked to a relatively lower risk of developing preeclampsia, experts have always stressed that the risks of smoking during pregnancy - including miscarriage, poor fetal growth and preterm delivery - far outweigh the potential benefit.

The current findings reinforce the importance of quitting smoking during, or preferably before, pregnancy, according to Miller’s team. They also indicate that smokers with preeclampsia should have their pregnancies closely monitored, and be offered particularly intensive smoking-cessation counseling.

It is hard to explain why smoking might protect against preeclampsia development yet worsen its outcome, according to the researchers.

But, they note, cigarettes do contain substances that inhibit blood vessels from constricting, which may help protect against preeclampsia. On the other hand, smoking also reduces levels of hormone-like substances called prostacyclins, which help dilate blood vessels. In women who have preeclampsia, Miller’s team speculates, smoking may exacerbate any reduction in oxygen getting to the fetus.

SOURCE: American Journal of Obstetrics & Gynecology, online July 1, 2010.