Doctors have historically advised against pregnancy in women with complex heart defects. But pregnancy may be possible for some of them, according to new guidance based on emerging data.

The American Heart Association published the new advice Thursday in its journal Circulation.

Pregnancy is a serious decision for women with complex congenital heart defects, said Mary M. Canobbio, R.N., chair of the statement writing group and a lecturer at UCLA School of Nursing in Los Angeles.

“Before, they were told categorically they could not get pregnant. We give them hope that it’s possible, but it’s not an easy road and it needs to be managed extremely carefully by a whole team of people who know how to manage them,” she said.

Changes from pregnancy tax the heart and circulatory system. Blood volume increases by about 40 percent, cardiac output increases 30 percent to 50 percent, and heart rate rises 10 to 20 beats per minute.

“These changes can dramatically affect the woman’s heart function,” Canobbio said.

This is especially risky for women with complex congenital heart defects, which are abnormalities that require immediate medical care at birth and care for the rest of their lives. Previously, women with such conditions were advised not to become pregnant, Canobbio said.

One of the first questions a doctor needs to ask is whether the patient’s heart can manage the additional blood volume associated with pregnancy, Canobbio said. Another factor, she said, is whether the defect and any associated complications, such as abnormal rhythms, are under control.

Among the most common complex defects are single ventricle defects, which may include weakness or absence of one of the heart’s two pumping chambers; reversal of the two main arteries leaving the heart; high blood pressure in the arteries of the lungs; and critical narrowing of a valve on the left side of the heart.

Depending on their particular complex heart defect, pregnant women have a higher risk of irregular heart rhythms and heart failure, Canobbio said. They may also face life-threatening rupture of the aorta or stiffening or blockage of a mechanical heart valve. Some may also be at risk for miscarriages, stillborn birth or premature birth, the statement says.

Some types of medications used to treat congenital heart defects, such as blood thinners, are known to harm the fetus, and may have to be switched during the early part of the pregnancy, Canobbio said. Other drugs may need to be stopped during pregnancy.

Authors recommend pre-pregnancy medical counseling that describes the risks that a pregnancy and delivery poses for women with complex heart defects and their unborn baby. This includes genetic counseling to determine the chance their child could inherit a heart defect.

A delivery plan specific to the woman’s condition is also essential to prepare the medical team to treat problems during or after delivery, Canobbio said. Such a plan could include inducing labor so experts are available during labor and delivery and using an epidural combined with narcotics to limit the pregnant woman’s urge to “push,” which helps minimize increased demands on the heart.

The authors recommend that women with complex defects get expert care during their pregnancy and give birth in centers that can provide a cardiologist experienced in treating their condition, an obstetrician trained in high-risk maternal-fetal medicine, heart anesthesiologists and heart surgeons. Monitoring of the mother needs to continue after delivery because the effects of pregnancy can linger for six weeks to six months.